ABSTRACTS
Vela Jr.: Then and There
Estimates of the age and distance of the supernova remnant Vela Jr. (G266.2-1.2, RX J0852.0-4622) continue to evolve. We highlight the recently published Chandra measurement of the expansion rate of a thin filament on the northwestern rim. If these results are representative of the remnant as a whole, then the expansion rate and the hydrodynamic models of Truelove and McKee suggest that the remnant is between 2.4 and 5.1 kyr old. We find that the remnant can be no closer than 0.5 kpc. A variety of other distance estimates and constraints suggest that it is no further than about 1 kpc. This range is consistent with the nearer portion of the Vela Molecular Ridge and with the Vela OB1 association of which the progenitor may have been a member. Furthermore, this range of distances suggests that it is highly unlikely that the gamma-ray emission is dominated by neutral pion decay.
The multiwavelength analysis of SNR MSH 11-61A
The shock front of a supernova remnant (SNR) is expected to be able to accelerate cosmic rays efficiently, producing non-thermal X-ray and gamma-ray emission. The gamma-rays can arise from inverse Compton scattering, non-thermal bremsstrahlung or from the decay of a neutral pion into two photons. Due to this degeneracy, a means of distinguishing between these mechanisms is crucial for our understanding of the origin of this observed emission. SNRs known to be interacting with nearby molecular clouds are effective targets for detecting and studying the production of gamma-rays from the decay of a neutral pion. Using 70 months of data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, we detect gamma-ray emission coincident with mixed morphology SNR MSH 11- 61A which is known to be interacting with a molecular cloud toward the north and south west. We investigate the origin of this emission by performing broadband modeling of its non thermal emission using both leptonic and hadronic processes, and conclude that the emission is most likely hadronic in nature. In addition, we also present our analysis of an 111 ks archival Suzaku observation of this remnant. Our study shows that the X-ray emission from MSH 11-61A arises from shock-heated ejecta with the bulk of the X-ray emission arising from a recombining plasma, while the emission towards the east arises from an ionising plasma.
The Supernova Remnant View of Type Ia SN Progenitors
Despite decades of continuing observational and theoretical efforts, the identity of the progen- itor systems of Type Ia Supernovae (SN Ia) remains obscure. Recent results have added to the controversy about the nature of the binary companion of the exploding white dwarf, which must be either another white dwarf (double degenerate systems, DD) or a non-degenerate star (single degenerate systems, SD). On the one hand, there are no clear signs of dynamical interaction between SN ejecta and circumstellar material, which seems to favor DD systems. On the other hand, there is mounting evidence that at least some exploding SN Ia have ejecta masses very close to the Chandrasekhar limit, which is more naturally explained by the SD scenario. I will describe recent X-ray observations of Type Ia Supernova Remnants that can shed light on the properties of SN Ia progenitors and the SD vs. DD debate.
Detection of C III in the Type Ia SN 2010kg
In homologously expanding Type Ia supernova ejecta the velocities of the spectral features can help us to map the spatial abundance and density profiles. Although strongly overlapping broad lines prevent unique line identifications, for high S/N spectra it is possible to set reasonable constraints on the contributing chemical elements. These cases give us the opportunity to detect ions that occur infrequently in supernova ejecta. I present synthetic spectra, calculated with Syn++, that fit high S/N spectra of the Type Ia SN 2010kg at eleven epochs between -11 and +4 days with respect to B-maximum. The minimum velocities of the line forming regions for most of the ions agree well with previous findings. The well-known high-velocity features of the Ca IR triplet, Ca H&K and Si 6355 are detected. Some other ions, like Fe II and Mg II, also form features at ~2000 - 5000 km/s above the photosphere. We identify a single absorption feature at ~4400 Angstrom as due to C III. This feature has been usually identified as a Si III line in the literature. However, we show that the assumption of Si III results in an inferior fit to not only this feature but also to the whole spectral range. On the other hand, neither C I nor C II can be identified in the spectra, which may not support the real presence of carbon. Since C III is found at ~5000 km/s above the photosphere, if true, it may give interesting implications about the physical conditions in the ejecta.
Radio rebrightening of the GRB afterglow by the accompanying supernova
The gamma-ray burst (GRB) jet powers the afterglow emission by shocking the surrounding medium, and radio afterglow can now be routinely observed to almost a year after the explosion. Long-duration GRBs are accompanied by supernovae (SN) that typically contain much more energy than the GRB jet. Here we consider the fact that the supernova blast wave will also interact with the external medium and produce radio emission, which will peak at much later time (since it is non-relativistic), when the SN blast wave transitions from a coasting phase to a decelerating Sedov-Taylor phase. We predict that this component will peak generally a few tens of years after the explosion and it will outshine the GRB powered afterglow well-before its peak emission. In the case of GRB 030329, where the external density is constrained by the ~10-year coverage of the radio GRB afterglow, the radio emission is predicted to start rising over the next decade and to continue to increase for the following decades up to a level of ~0.5 mJy. Detection of the SN-powered radio emission will greatly advance our knowledge of particle acceleration in ~0.1c shocks.
Dust formation and processing in the clumpy supernova remnant Cassiopeia A
Dust and molecules are observed in various supernovae and their remnants, but their formation and evolution in these hostile, shocked environments are still unclear. In some supernova remnants, such as the 330 years-old remnant Cas A, the reverse shock is currently reprocessing the material formed in the supernova ejecta. The aim of the present study is to assess whether supernovae are important contributors to the dust budget of galaxies. The Cas A remnant results from the explosion of a 19 Msun star as a Type IIb supernova characterised by a low-density ejecta. We first model the formation of molecules, dust clusters, and dust grains in the supernova ejecta to derive dust grain size distributions. We then model the impact of the reverse shock on oxygen- and carbon-rich ejecta clumps, and investigate the post-shock chemistry by considering various reverse shock velocities. We study the destruction of molecules and dust clusters by the shock, and their reformation using a chemical kinetic model, and investigate the effect of thermal and non-thermal sputtering on the dust within the clumps. Our results show that only medium- and large-sized grains survive non-thermal sputtering in dense clumps, and that small dust grains do not efficiently reform in the shocked gas. This result indicates that the dust formed in the supernova ejecta and destroyed by the reverse shock is unable to reform from the gas phase in the remnant. We then investigate the effect of thermal sputtering on the clump dust injected into the high-temperature interclump medium over several clump-crushing times. We consider a range of possible interclump temperatures, and determine the final dust mass initially formed in the supernova that survives the remnant phase and is injected into the ISM. Oxide grains are almost completely destroyed, pointing to the inability of SN Type IIb to contribute significantly to the galactic dust budget. Large grains, with radius ~ 1 micron (such as formed in Type IIP SNe) are required to survive the remnant phase. Carbon and silicon carbide grains are more resistant, and survive even at smaller radii. SNRs with dense clumps and dust grain size distributions including large grains can be significant contributors to the dust budget in the early as well as in the local universe.
The Effects of Novel Semi-Convection and Overshoot Prescriptions on Presupernova Stars
Using a prescription for semi-convection from Wood et al. 2013 and an overshoot prescription from Brummell et al. 2002, we are able to analyze the possible effects of these two processes on presupernova models. In a preliminary study of 15 solar mass non-rotating models, we categorize 90 models according to their envelope structure and find general trends in the structure and nucleosynthesis. We find that varying the overshoot strength over the range of physically reasonable values can cause the helium and carbon/oxygen core masses to vary by about a solar mass. We also find that blue loops exist only in models with low overshoot and low semi-convection strength, and that, of these, the stars with lowest overshoot strength have high carbon to oxygen ratios in their cores compared to the rest of the models in the sample. These specific models also display exceptionally low compactness compared to the rest of the sample but we see no other notable trends in compactness with overshoot or semi-convection. We are currently looking into finding any clear observational metrics of these models to identify possible ways to constrain the strength of either process.
The Ultraviolet Diversity of Type Ia Supernovae
Type Ia supernovae exhibit a much larger dispersion in the ultraviolet than in the optical. This diversity is revealed through Hubble Space Telescope spectroscopy and Swift Ultra-Violet/Optical Telescope photometry and includes normal as well as spectroscopically peculiar objects. I will show the expected differences predicted in theoretical models with varying metallicity, density gradients, and viewing angles. The observed scatter in UV colors is greater than predicted from metallicity variations alone. I will show searches for correlations amongst other observed parameters. I will also discuss cosmological implications, including improving the standardization of type Ia SNe in the optical and testing for evolution of SN properties with redshift.
The physics of stellar angular momentum transport in massive stars
TBA
Convection-dominated vs. SASI-dominated Supernova Explosions
We explore the similarities and differences between the behavior of convection-dominated and SASI-dominated models of the post-core bounce supernova environment near the explosion luminosity threshold. Using our code GenASiS, we employ a simplified, parametrized model that allows us not only to control the nature of the evolution (explosion vs. non-explosion, convection-dominated vs. SASI-dominated), but to efficiently run dozens of three-dimensional models in order to statistically evaluate the simulation outcomes.
Powering Superluminous Supernovae: Simulations and model comparisons
The onset of fast-cadence automated wide-field transient search projects such as the Texas Supernova Search (TSS), the PTF and PanSTARRS but also archival data research (SDSS-II) allowed for the detection and study of a new class of Superluminous Supernovae (SLSNe). With a few tens of well-studied SLSNe available to date a tremendous degree of diversity is revealed, in terms of the observed light curves and spectra. Three competing classes of models have been proposed to account for some or all SLSNe: Pair Instability Supernova (PISN) explosions, magneto-rotational energy injection from the spin down of newly-born magnetars and massive SN ejecta - circumstellar matter interaction. We present results from numerical hydrodynamics simulations performed with the FLASH code of both the PISN and the CSM interaction processes and discuss their applicability to the case of SLSNe.
Novae with Giant Companions: Circumstellar Interaction in our Galactic Backyard
In the past five years, the number of well-studied novae with red giant companions has grown four-fold. These systems represent precious opportunities to study the details of blastwave interaction with circumstellar material and the distribution of circumbinary material in symbiotic systems. I will present our exquisite multi-wavelength coverage (in radio, optical, X-ray, and gamma-ray bands) of these novae, and discuss implications for circumstellar interaction in supernovae and Type Ia progenitors.
All in the Family: Discovering the Natures of Type Ia Progenitors by Identifying Close Cousins
It has proved to be remarkably difficult to determine the nature(s) of the astrophysical systems that produce Type Ia supernovae. Fortunately, the binary pathways that produce Type Ia supernovae may be viewed as a family tree that produces many other interesting and distinctive progeny. By studying these more recognizable progeny and conducting multiple censuses, we can uncover some of the family secrets have kept the Type Ia progenitors from our view. We will consider some well known cousins, such as cataclysmic variables and symbiotic binaries, and also some more distant cousins, including accreting neutron stars and black holes, main-sequence binaries containing B-stars, and white dwarfs with sub-stellar companions.
Probing the Extremes of Pre-SN Mass-Loss
A non-negligible fraction of massive stars undergo enhanced (possibly violent/eruptive) mass-loss in the final decades before core collapse. Theoretical models of such mass-loss are challenging and observational probes are necessary to help constrain the full diversity of pre-SN mass-loss (e.g. density profile, physical extent), the mechanism by which this mass is ejected, and the progenitors of various sub-classes of events. In this talk I will present new results from PS1 observations of rapidly-evolving SN and super-luminous Type IIn SN (SLSN-II), which probe two different extremes of pre-SN mass-loss. In particular, I will highlight the discovery of a new class of transients along with their rates and implications for models of stellar evolution, as well as new constraints on the progenitors of SLSN-II based on a joint analysis of their explosion/CSM properties and host galaxy environments.
Carbon-Atmosphere White Dwarfs as Failed Type Ia Supernovae
Some or all type Ia supernovae may result from the merger of two C/O-core white dwarf stars. Such mergers are also expected to have other outcomes, such as massive white dwarfs, though none have been convincingly identified. Because they are forged in the same fire, these unexploded remnants would be imprinted with information about the same physical process that potentially leads to type Ia supernovae. We will show evidence that the carbon-dominated atmosphere white dwarf stars (the hot DQs) are a population of double degenerate merger products. As hot, massive white dwarfs, they appear to be young, but we will present kinematic evidence that they are, in fact, much older. This apparent contradiction is resolved if they were recently reheated in a merger. The merger hypothesis also naturally explains all the other peculiarities of the class: a large fraction are magnetic, their atmospheres are dominated by carbon and oxygen, they are massive, and many display monoperiodic brightness variations with periods from 5-20 minutes. All of this is consistent with a merger than spins up the remnant and generates a magnetic field. As failed type Ia supernovae, the hot DQs provide the best observational endpoints against which simulations of double degenerate mergers can be tested. We will also discuss their formation rate, which has direct consequences for the number of double degenerate mergers available to become type Ia supernovae.
Direct Identification of Core-Collapse SN Progenitors
To fully understand the universal influence of supernovae (SNe), to enable their use as probes of stellar evolution throughout cosmic time, and to connect to theoretical models of stellar explosions, it is absolutely essential to observationally determine their progenitors or progenitor systems. The most definitive insight is derived by directly identifying the star(s) prior to explosion. Although several progenitors have been identified in high-quality ground-based images for the nearest SNe, this is primarily accomplished using Hubble Space Telescope (HST) archival image data. Confirmation is obtained either by post-explosion HST or ground-based adaptive optics (AO) images. I will briefly present recent examples of core-collapse SN progenitors and provide inferences, based on the cumulative knowledge to date, about the progenitors, effectively, as a function of initial mass.
A two-parameter criterion for classifying the explodability of massive stars by the neutrino-driven mechanism
Thus far, judging the fate of a massive star (either a neutron star (NS) or a black hole) solely by its structure prior to core collapse has been ambiguous. Our work and previous attempts find a non-monotonic variation of successful and failed supernovae with zero-age main-sequence mass, for which no single structural parameter can serve as a good predictive measure. However, we identify two parameters computed from the pre-collapse structure of the progenitor, which in combination allow for a clear separation of exploding and non-exploding cases with only few exceptions (~1--2.5%) in our set of 621 investigated stellar models. One parameter is M4, defining the enclosed mass for a dimensionless entropy per nucleon of s = 4, and the other is mu4 = dm/dr|_{s=4}, being the mass-derivative at this location. The two parameters mu4 and M4*mu4 can be directly linked to the mass-infall rate, Mdot, of the collapsing star and the electron-type neutrino luminosity of the accreting proto-NS, L_nue ~ M_ns*Mdot, which play a crucial role in the "critical luminosity" concept for the theoretical description of neutrino-driven explosions as runaway phenomenon of the stalled accretion shock. All models were evolved employing the approach of Ugliano et al. for simulating neutrino-driven explosions in spherical symmetry. The neutrino emission of the accretion layer is approximated by a gray transport solver, while the uncertain neutrino emission of the 1.1 Msun proto-NS core is parametrized by an analytic model. The free parameters connected to the core-boundary prescription are calibrated to reproduce the observables of Supernova 1987A for five different progenitor models.
SASI- and Convection-Dominated Explosions in 3D
The neutrino mechanism of core-collapse supernovae requires the supporting action of hydrodynamic instabilities to drive a successful explosion. Two instabilities can break the spherical symmetry of the stalled shock: neutrino-driven convection and the Standing Accretion Shock Instability (SASI). Depending on the progenitor properties, either one of these instabilities can dominate the explosion. I'll present results from 3D simulations of parameterized core-collapse supernovae which demonstrate that these two regimes can occur in 3D. SASI-dominated explosions can succeed in 3D with up to ~20% lower neutrino luminosity than in 2D, due to the additional kinetic energy provided by spiral modes. Increasing the spatial resolution decreases the magnitude of this effect, but does not eliminate it. In contrast, convection-dominated explosions show a much smaller difference between 2D and 3D, in agreement with previous studies. I'll discuss the implications for the diversity of explosion paths in a realistic supernova environment.
Single-Degenerate Type Ia Supernovae are Preferentially Overluminous
Recent observational and theoretical progress has favored merging and helium-accreting sub-Chandrasekhar mass white dwarfs in the double-degenerate and the double-detonation channels, respectively, as the most promising progenitors of normal Type Ia supernovae (SNe Ia). Thus the fate of rapidly-accreting Chandrasekhar mass white dwarfs in the single-degenerate channel remains more mysterious than ever. In this talk, I will clarify the nature of ignition in Chandrasekhar-mass single-degenerate SNe Ia by providing a new, elementary derivation of the existence of a characteristic length scale which establishes a transition from central ignitions to buoyancy-driven ignitions. Using this criterion, combined with data from three-dimensional simulations of convection and ignition, I will demonstrate that the overwhelming majority of ignition events within Chandrasekhar-mass white dwarfs in the single-degenerate channel are buoyancy-driven, and consequently lack a vigorous deflagration phase. Thus, single-degenerate SNe Ia are generally expected to lead to overluminous 1991T-like SNe Ia events. I will also discuss the rates predicted from both the population of supersoft X-ray sources and binary population synthesis models of the single-degenerate channel, and demonstrate these are broadly consistent with the observed rates of overluminous SNe Ia. I will further demonstrate that the single-degenerate channel contribution to the normal and failed 2002cx-like rates is not likely to exceed 1% of the total SNe Ia rate. I will conclude with a range of observational tests of overluminous SNe Ia which will either support or strongly constrain the single-degenerate scenario.
Hunting the rst generations of stars and galaxies
The new Australian SkyMapper 1.3m telescope is carrying out a photometric survey of the entire Southern Sky. From using ugriz filter plus an additionally narrow filter placed at the Ca K line at 3933A, stellar parameters can be obtained for all stars observed. This allows for an efficient selection of a variety of stellar types, including metal-poor stars. Recent efforts to search for the most metal-poor stars have indeed delivered a new record holder for the most iron-poor star: no iron lines were detected in the high-resolution follow-up Magellan spectrum and only an upper limit of [Fe/H]ˇ-7.1 could be determined. Contrary to its iron deficiency, the star has a significant amount of carbon. This abundance pattern can be explained with the star being a second-generation star in the universe which formed from a gas cloud enriched by only one PopIII first star. What was the environment in which these early stellar generations formed? A spectroscopic study of the faintest dwarf galaxy Segue 1 has shed light on this question. Given the chemical abundance patterns of some of its only few stars) with metallicities ranging from -4 ˇ [Fe/H] ˇ -1) suggest that this tiny galaxy may be a surviving first galaxy from the early universe. This suggestion is in line with recent age measurements for similar ultra-faint dwarfs which showed these galaxies to be single-age stellar systems that are about as old as the universe itself.
Supernova Shrapnel: Interpreting the Pieces of a Nearby Star
Supernovae (SNe) occur at a rate of ~1-3 per century in the Milky Way. It is reasonable to expect that in the lifetime of Earth, at least one (if not several) SNe have occurred within 100 pc of Earth, presenting an opportunity for a SN to produce measurable effects on Earth. I'll discuss terrestrial iron-60 evidence for such a nearby event and examine possible sources for the iron-60 signal to include not only Core-Collapse SNe, but also Electron-Capture SNe, Thermonuclear/Type Ia SNe, Kilonovae/Neutron-Star Mergers, and Super-Asymptotic Giant Branch stars. I'll also present a number of factors that influence the signal from an extra-solar source to include uptake and dust filtering and identify constraints to apply to each possible source in order to identify the most likely progenitor of the iron-60 signal.
The spin rate of pre-collapse stellar cores: wave driven angular momentum transport in massive stars
The core rotation rates of massive stars have a substantial impact on the nature of core collapse supernovae and their compact remnants. We demonstrate that internal gravity waves (IGW), excited via envelope convection during a red supergiant phase or during vigorous late time burning phases, can have a significant impact on the rotation rate of the pre-SN core. In typical (10 Msun < M < 20 Msun) supernova progenitors, IGW may substantially spin down the core, leading to iron core rotation periods greater than roughly 50 s. Angular momentum (AM) conservation during the supernova would entail minimum NS rotation periods of roughly 3 ms. In most cases, the combined effects of magnetic torques and IGW AM transport likely lead to substantially longer rotation periods. However, the stochastic influx of AM delivered by IGW during shell burning phases inevitably spin up a slowly rotating stellar core, leading to a maximum possible core rotation period. We estimate maximum iron core rotation periods of roughly 10^4 s in typical core collapse supernova progenitors, and corresponding spin periods less than 400 ms for newborn neutron stars. This is comparable to the typical birth spin periods of most radio pulsars. Stochastic spin-up via IGW during shell O/Si burning may thus determine the initial rotation rate of most neutron stars. For a given progenitor, this theory predicts a Maxwellian distribution in pre-collapse core rotation frequency that is uncorrelated with the spin of the overlying envelope.
Spontaneous Detonation Initiation in White Dwarfs
Despite over forty years of active research, the nature of the white dwarf progenitors of Type Ia supernovae remains unclear. However, in the last decade, various progenitor scenarios have highlighted the need for detonations to be the primary mechanism by which these white dwarfs are consumed. How these detonations are triggered is an open question and remains an active area of research. In this paper we study how detonations are spontaneously initiated due to temperature inhomogeneities, e.g., hotspots, in burning nuclear fuel in a simplified physical scenario. We model and evolve these hotspots numerically and show that the minimum spontaneous wave speed, the speed at which a thermal runaway travels down a temperature gradient, gives a criterion for the formation of detonations. We show that for spontaneous wave speeds less than the Chapman-Jouguet speed, detonations can form, which corresponds to both steep and moderate temperature gradients. We also determine an analytic criterion for the size of a hotspot region for which detonations in burning carbon-oxygen material can occur. Our results suggest that spontaneous detonations may easily form under a diverse range of conditions, likely allowing any number of progenitor scenarios to initiate detonations that burn up the star.
Hot subdwarf stars and their connection to thermonuclear supernovae
Hot subdwarfs are compact helium stars formed by stripping a red giant from its hydrogen envelope by close binary interactions. Hot subdwarfs in close binaries with massive white dwarf companions are candidates for the progenitors of thermonuclear supernovae. As soon as the white dwarf explodes, the surviving hot subdwarf might the ejected from the bi- nary and accelerated to velocities high enough to leave our Galaxy. Such hypervelocity hot subdwarfs might therefore become important tools to study thermonuclear supernovae.
Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging
We present deep Hubble Space Telescope (HST) imaging at the locations of four, potentially hostless, long-faded Type Ia supernovae (SNe Ia) in low-redshift, rich galaxy clusters that were identified in our Multi-Epoch Nearby Cluster Survey. The depth of our data, assuming a steep faint-end slope for the galaxy cluster luminosity function (alpha_d=-1.5), includes all but ~0.2% of the stellar mass in galaxies expected in each cluster (~0.005% with alpha_d=-1.0). This depth is a factor of 10 better than our ground-based imaging limits. Two of the four SNe Ia still have no possible host galaxy associated with them (M_R>-9.2), confirming that their progenitors belong to the intracluster stellar population of stripped stars. The third SNe Ia appears near a faint disk galaxy (M_V=-12.2), but has a relatively high probability of being a chance alignment. A faint, red, point source is coincident with the fourth SN Ia's explosion position (M_V=-8.4), and it may be either a globular cluster (GC) or faint dwarf galaxy. We estimate the surface densities of cluster dwarf galaxies and GCs at the location of this SN Ia and show that a GC is more likely due to the proximity of an elliptical galaxy, but neither object type can be ruled out. The SN Ia rate is theoretically expected to be elevated in GC systems due to their stellar density, and if verified this would be the first confirmed instance of such an event. We find that the faint, red, point source host implies that the SN Ia rate in dwarfs or GCs may be enhanced, but within previous observational constraints. We also consider and reject the possibility that we could be detecting extremely late-time emission from the SNe Ia or its shocked companion. We demonstrate that finding 1 in 4 apparently hostless cluster SN Ia to be hosted by a dwarf galaxy or GC does not preclude the use of SNe Ia as bright tracers of the of intracluster light at higher redshifts, but that it will be necessary to refine our constraints on the rate in dwarfs and GCs with extremely deep imaging for a larger sample of low-redshift, apparently hostless SNe Ia.
Late-Time Photometry of Type Ia Supernova SN2012cg Reveals the Radioactive Decay of 57Co
Seitenzahl et al. (2009) have predicted that ~3 years after its explosion, the light we receive from a Type Ia supernova will come mostly from reprocessing of electrons and X-rays emitted by the radioactive decay chain 57Co --> 57Fe, instead of the decay chain 56Ni --> 56Co --> 56Fe that dominates the supernova light at earlier times. Using the Hubble Space Telescope, we followed the light curve of the Type Ia supernova SN2012cg out to ~1050 days after it reached maximum light. Our measurements are consistent with the light curves predicted by the contribution of energy from the reprocessing of internal-conversion and Auger electrons, as well as X-rays, emitted by the decay of 57Co. This provides conclusive evidence that 57Co is produced in Type Ia supernova explosions.
X-ray and gamma-ray observations of supernova explosions and remnants
Understanding the origin of supernova explosions remains one of the outstanding problems in astrophysics. In core collapse explosions significant departures from spherical symmetry are required in order to re-energize the shock wave that must propagate throughout the entire star in a successful supernova. Whether these asymmetries are driven by low-mode convective modes or extreme jet-driven instabilities is a matter of debate - both may be relevant in different progenitors. The recent ability to spatially and spectrally resolve nuclear decay lines from 44Ti has led to important new constraints in the two most famous supernova remnants, Cassiopeia A and SN1987A. In addition, the recent detection of gamma-rays from the nearby Type Ia explosion 2014J has directly confirmed that a significant fraction of the progenitor white dwarf mass is converted into radioactive 56Ni. Hard X-ray observations can now place strong constraints on the level of mixing in the explosion. I will review these recent results and place them in the context of theoretical models.
A very luminous magnetar-powered supernova associated with an ultra-long Gamma-Ray Burst
A new class of ultra-long duration (>10 ks) gamma-ray bursts has recently been suggested. They may originate in the explosion of stars with much larger radii than the normal long gamma-ray bursts. Hitherto no supernova has been associated with an ultra-long gamma-ray burst. Here we report that a supernova was associated with the ultra-long duration burst 111209A, at z=0.677. This supernova is a factor of >3x more luminous and its spectrum distinctly different from other type Ic supernovae associated with long gamma-ray bursts. The continuum slope resembles those of super-luminous supernovae, but the light curve evolves much faster. The combination of high luminosity and low metal-line opacity cannot be reconciled with typical type Ic supernovae, but can be reproduced by a model where extra energy is injected by a strongly magnetized neutron star (a magnetar), which has also been proposed as the explanation for the super-luminous supernovae.
Self-magnetized Core-Collapse Supernovae
The core-collapse supernova mechanism remains one of the unsolved problems in theoretical astrophysics. In particular, a combination of nuclear physics, hydrodynamic instabilities, and neutrino physics have been unable to produce robust explosions for all but the lowest-mass progenitors. This calls for extending the physics contributing to core-collapse supernova models, such as non-standard equations of state, magnetic fields, and potential systematic effects from progenitor models. In this work, we study core-collapse supernova explosions with a new supernova code, Proteus, that solves the equations of magnetohydrodynamics assuming the single fluid approximation. We take into account self-generation and diffusion of magnetic fields, include the effects of rotation, and account for the effects of neutrino-matter interactions using the Isotropic Diffusion Source Approximation. Proteus uses a block-structured adaptive mesh for solving the MHD equations; an efficient multigrid algorithm is used to solve the Poisson equation for self-gravity; the neutrino transport solver uses a separate, nonuniformly-spaced radial mesh. We discuss the components of the new approach, the computational performance of the code, and the results of its application to the explosion of 15 and 20 solar mass progenitors.
Predicting nucleosynthesis observables in CCSNe with self-consistent simulations
Gamma-ray observations of nuclear abundances in core-collapse supernova (CCSN) ejecta, highlighted by recent NuSTAR observations of the Ti-44 spatial distribution in Cassiopeia A, potentially allows nucleosynthesis calculations to place powerful constraints on conditions deep in the interiors of supernovae and their progenitors. This ability to probe where direct observations cannot make such calculations an invaluable tool for understanding the CCSN mechanism. Unfortunately, despite knowing for two decades that supernovae are intrinsically multi-dimensional events, discussions of CCSN nucleosynthesis have been predominantly based on 1D models which use a contrived energy source to launch an explosion and often ignore important neutrino effects. We investigate CCSN nucleosynthesis with self-consistent, axisymmetric simulations using a multidimensional radiation hydrodynamics code. These models represent a necessary improvement over their parameterized counterparts in char- acterizing the impact of the hydrodynamically unstable, neutrino-driven explosion on the supernova ejecta. I will discuss the current state of our effort to bridge the gap between these first-principles nucleosynthesis simulations and observations of supernova remnants.
Ejecta-Companion Interaction in iPTF 13bvn
iPTF 13bvn is a type Ib supernova that has most likely exploded from a binary progenitor. Previous works have placed strong constraints on the progenitor system that can reproduce the light curve and pre-supernova photometry by evolutionary and hydrodynamical simulations. According to their evolutionary calculations, the secondary star is most likely to be an overluminous 18-45 Msun OB type star. But in their calculations they do not include the effect of ejecta-companion interaction. Here I report results of hydrodynamical simulations of the supernova ejecta hitting the companion star in iPTF 13bvn. Although we find no mass stripping or "kick" velocity given to the star, we found that the star is likely to expand due to the shock heating. We also conducted long term post-impact evolution calculations to estimate the appearance of the companion when it becomes visible in the supernova ejecta. Our results reveal that the companion may be significantly puffed-up, affecting the appearance when it becomes visible. We expect the companion to be rather red and will be found earlier than was previously estimated.
Multidimensional Simulations of Core-Collapse Supernovae and the Implications for Nucleosynthesis
Core-collapse supernovae (CCSNe), the culmination of massive stellar evolution, are the principle actors in the story of our elemental origins. Though brought back to life by neutrino heating, the development of the supernova is inextricably linked to three-dimensional fluid flows, with large scale hydrodynamic instabilities allowing successful explosions that spherical symmetry would prevent. The importance of the neutrino interactions and the three-dimensional fluid flows that they drive have often been ignored when the nucleosynthesis that occurs in these explosions, and their resulting impact on galactic chemical evo- lution, is discussed. I will present results from simulations of successful explosions using our CHIMERA code, and discuss how the multidimensional character of the explosions directly impacts the development of the explosion as well as the nucleosynthesis and other observables of core-collapse supernovae.
Microscopic equation of state and neutrino response from chiral effective field theory
The equation of state, transport and linear response properties of neutron-rich matter from sub- to supra-nuclear densities directly affect neutron star structure, the dynamics of core-collapse supernovae and r-process nucleosynthesis. In this talk I will describe recent progress in constructing a thermodynamic equation of state of nuclear matter based on the low-energy realization of QCD, chiral effective field theory, which incorporates realistic microphysics such as multi-pion exchange processes and three-body forces. Bulk properties of zero-temperature symmetric nuclear matter around saturation density are shown to be well described without additional fine tuning, as are selected thermodynamic observables. Consistent response functions are needed to understand neutrino propagation in a newly-born proto-neutron star, and I will present first calculations of charged-current weak reactions from chiral effective field theory that determine the neutrino mean free path in warm and dilute neutron-rich matter.
Simulating the supernova neutrinosphere with heavy ion collisions
Much of the "action" in core collapse supernovae happens near the neutrinosphere in warm low-density neutron rich matter. These conditions can be reproduced on earth with heavy ion collisions at relatively low energies. Neutrinosphere temperatures are easily reproduced, while low sub-nuclear densities are reached as the collision system expands. Finally, very neutron rich conditions can be extrapolated by comparing results with neutron rich and proton rich radioactive beams. Heavy ion collisions can provide information on the equation of state, composition of light clusters, the symmetry energy, and nuclear response functions for neutrino interactions. This information is important in determining the spectrum of both electron neutrinos and electron anti-neutrinos.
Modeling Spectropolarimetric Interaction Between Type IIn SNe and Distributed Emission Sources
The multi-component emission line profiles in the spectra of SNe Type IIn arise from strong interaction between the SN ejecta and pre-existing dense circumstellar material (CSM). Radiation arising from the shocked inner CSM and scattering outward through the extended, ionized, asymmetric outer regions produces complex polarization signatures across strong hydrogen and helium emission lines. In these objects, the SN photosphere is completely obscured by the CSM, making it unusually difficult to extract information about the nature of the explosion or the properties of the progenitor star. Modeling the polarization, however, provides a forensic link between the properties of the radiating and scattering regions and the characteristics of the progenitor. My work explores the effects of inclination, geometry, luminosity, temperature, and optical depth on the polarization of the H alpha line using a three-dimensional Monte Carlo radiative transfer code called SLIP. A key advantage of SLIP is that it can treat the contribution of a distributed emission source (the shock) to the overall polarization signature, an effect that has not been investigated previously. I present comparisons of the model results with spectropolarimetric observations of the Type IIn SNe 1997eg and 2010jl and speculate about the nature of their progenitors.
GRB 060218: Towards a Unified Model for the Weakest Engine-Driven Explosions
We consider a model for the low-luminosity gamma-ray burst GRB 060218 that plausibly accounts for the prompt X-ray/gamma-ray emission during the first 3000 s, the bright optical optical emission from 10^4 to 10^5 s, the X-ray afterglow out to 10 days, and the radio emission from 2 days onward. The key components of our model are: (1) a long-lived (t_j ~ 3000 s) central engine and accompanying low-luminosity (L_j ~ 10^{45} erg s-1), mildly relativistic jet; (2) a dense mass-loss envelope containing ~ 4 × 10^{-3} Msun of material immediately surrounding the progenitor star; and (3) a moderate amount of dust (A_V ~ 0.1) in the circumstellar environment. We demonstrate that blackbody emission from the transparency radius in a relatively wide (theta0 ~ 10°), low-power jet can fit the prompt thermal X-ray emission. The later spherical phase of the same outflow can produce the radio emission via synchrotron radiation from external shocks. Meanwhile, interaction of the associated supernova 2006aj with the dense circumstellar envelope extending to ~10^{12} cm can fit the early optical emission. The X-ray afterglow can be interpreted as a light echo of the prompt emission from dust at ~100 pc, which explains the steep spectrum and lack of a jet break. We constrain the central engine and circumstellar environment based on fits to observations. Our results indicate that long-duration, low-luminosity GRBs such as GRB 060218 and GRB 100316D originate from unusual progenitors that undergo strong pre-explosion mass-loss and produce intrinsically long-duration jets. This picture predicts the detection of Type Ib/c supernovae accompanied by transrelativistic radio afterglows, but without associated prompt X-ray/gamma-ray emission, when GRB 060218-like events are observed off-axis.
Bulk Properties and Ignition in Simple Models of Double Detonation Type Ia Progenitors
The low-Mach hydrodynamics code Maestro is used to model the nuclear burning and convection in thin helium shells on sub-Chandrasekhar mass carbon/oxygen white dwarfs. The dynamics are modeled in full 3D with a realistic equation of state and a simple nuclear reaction network. We model many different configurations at the cost of using simple models. We find we are able to reproduce runaway found in 1D with low-mass shells, configurations that are in short-term equilibrium, and a model that has a nova-like convective runaway. These outcomes and their characterization will be discussed.
Three-dimensional Core-Collapse Simulations by the Garching Group: The Route to Explosions
The talk will present a status report of three-dimensional stellar core-collapse simulations by the Garching group. The physics of successful explosion models will be discussed.
The progenitors of electron-capture supernovae
Electron-capture supernovae (ECSNe) are the explosions resulting from the collapse of a degenerate oxygen-neon (ONe) core during an oxygen de agration. The evolution of ECSN progenitor stars is in many aspects still poorly understood. I will review recent research that has attempted to determine whether or not ECSNe could even occur and if so, to quantify their relative frequency. The answers to these questions are strongly sensitive to the uncertainties in mass loss rates, nuclear physics and convective boundary mixing. Some of these uncertainties are also important for the accretion-induced collapse (AIC) of ONe white dwarf stars. The ECSN problem must, along with many other outstanding problems in stellar physics, be carried forward into the maturing discipline of 3D simulation.
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Spiral Disk Instability in Binary White Dwarf Mergers as a Viable Pathway To Type Ia Supernovae
Type Ia supernovae (SNe Ia) are among the most-studied high-energy astrophysical events because of their importance to cosmology, and the study of dark energy. However, the stellar progenitors which give rise to SNe Ia remain mysterious. A leading mechanism for normal SNe Ia is the merger of two white dwarfs in the double-degenerate (DD) channel. Despite promising observational evidence in their support, until recently it was not clear how detonation conditions could be achieved in a self-consistent fashion during the merger of carbon-oxygen white dwarf binaries. In collaboration with European colleagues, in high-resolution three-dimensional numerical simulations, we have recently found for the first time that gravitational instability in a merging white dwarf binary leads to a self-consistent detonation of a primary WD on a dynamical time scale. Further implications of this mechanism will be explored in the solution of the SN Ia progenitor problem.
Are pulsars spun up or down by the spiral modes of SASI
The spin of a neutron star at birth may be impacted by the asymmetric character of the explosion of its massive progenitor. During the first second after bounce, the Standing Accretion Shock Instability (SASI) is able to redistribute angular momentum and spin-up a neutron star born from a non-rotating progenitor through the nonlinear interaction of spiral modes. Focusing on a non-rotating progenitor, we perform 2D numerical simulations of a simplified setup in cylindrical geometry to investigate the symmetry breaking between counter-rotating spiral modes. We show that this event occurs only if the ratio of the initial shock radius to the neutron star radius exceeds a critical value. The amount of angular momentum redistributed by a spiral mode is compared to analytical estimates in the linear regime. Numerical simulations of the non-linear regime reveals significant stochastic variations including a reversal of the direction of rotation of the accretion shock. The impact of the initial rotation of the progenitor on the angular momentum budget is also discussed. Our study aims at estimating the potential role of SASI on the pulsar spin at birth.
Companions of Supernovae
When a Type Ia supernova (SN Ia) occurs in a binary system, the companion will be blasted by the supernova ejecta. Such interactions have been simulated and used to argue that for some Type Ia supernovae there should be a surviving companion with unusual features. The absence of observational evidence (e.g. in SN1006, SN1572, SN1604, SNR 0519-69.0, and SNR 0509-67.5) for such energized companions has been argued to be evidence for the absence of former companions in SN Ia. However, this is based on inhomogeneous and small datasets (so far five remnants have been studied to different quality and quantity) and might thus be flawed. To alleviate this problem we assembled a panchromatic dataset of supernova remnants in the Magellanic Clouds. This was then supplemented with optical Gemini spectra of over a hundred companion candidates, as well as more than 700 spectra for stars that are close to the supernova which survive as comparison objects and two study the stellar environment. The selected remnants are of both thermonuclear and core-collapse origin. This choice was deliberate, as to answer the questions: What are the companions of supernovae? What features does a companion exhibit after being blasted by a supernova? In this talk, I will introduce our survey, discuss the lack of companions in SN Ia remnants, the findings for Core collapse remnants and our combined conclusions for companions of supernovae.
Old Stars as Chemical Tracer Particles
As a galaxy evolves, the relative contribution of different nucleosynthetic sources changes. For example, core collapse supernovae produce a lot of alpha elements early in a galaxy's evolution. As Type Ia supernovae ramp up, they produce a great deal of iron, lowering the alpha/Fe ratio. Because stars remember the elemental composition of the gas from which they formed, they encode the relative contribution of core collapse and Type Ia supernovae to the galaxy's enrichment. Dwarf galaxies are near enough to permit this resolved stellar spectroscopy, and they are simple enough to allow straightforward interpretations of their chemical enrichment histories. I will show Keck/DEIMOS measurements of carbon, alpha elements, and iron in dwarf galaxies, and I will offer interpretations of those measurements in terms of the star formation and gas flow histories.
Discovery of a Failed Supernova Candidate with the Large Binocular Telescope
We review the observational and theoretical grounds to expect that 10-30% of core collapses lead to formation of a black hole in a failed supernova without a dramatic external signature. Next we discuss our ongoing survey with the Large Binocular Telescope to discover examples of failed supernova. At present, we have one very good candidate, which lies in the most likely progenitor mass range and implies a rate consistent with expectations.
Multi-D Core-Collapse Supernova Models and the Multi-Messenger Signatures
After we briefly summarize a status of core-collapse supernova models, we report our re- cent results based on two-(2D) and three-dimensional (3D) radiation-hydrodynamics sim- ulations. We also talk about signals of neutrinos and gravitational waves expected from the self-consistent models, which would be important to extract the information of the yet uncertain mechanism of explosion from the multi-messenger observables. We also report our code development where 3D general-relativistic hydrodynamics is now meeting with spectral neutrino transport.
Can we really see pair-instability supernovae?
It is well-known that very massive stars (100--300 Msun) are observed. These stars form massive oxygen core (above 60 Msun), which eventually explodes due to pair-creation instability mechanism. In theory, explosions of these very massive stars should be detected among other supernova explosions. We base the post-explosion supernova simulations on two pair-instability supernova (PISN) models calculated with the evolutionary code BEC (N. Langer's evolutionary code). These are 150 Msun and 250 Msun models at metallicity of 1/20th solar metallicity. We produce our simulations with the radiation hydrodynamics code STELLA (Blinnikov et al. 2006). The 150 Msun model explodes as extended red supergiant and produces event similar to bright type II supernova. We analyse its synthetic light curve and the dependence on nickel amount and degree of stripping. We compare the 150 Msun explosion to different observed supernovae. Among others are bright SN 1992am, linear SN 1979C, SN 2009kf and others. The 250 Msun model explodes as yellow supergiant which produces a huge amount of radioactive nickel (19 Msun). We analyse the synthetic properties of this supernova and compare to so-called superluminous supernovae. We conclude that (1) low-mass PISNe might be relevant for explanation of bright type II supernovae (SNIIP and SNIIL), those which require high explosion energy and high amount of radioactive nickel, and (2) high-mass PISNe might explain observed properties of a few slowly evolving superluminous supernovae.
Dissecting Supernova Remnants to Probe Progenitors and Explosion Mechanisms
Although supernovae (SNe) are detected routinely through dedicated surveys, these events are often too distant to resolve the SN ejecta and the immediate surroundings of the ex- ploded stars. Fortunately, supernova remnants (SNRs) offer an up-close view to investigate explosions and environments on sub-pc scales. Nearly 400 SNRs have now been identified in the Milky Way and nearby galaxies, offering the necessary basis for systematic study and comparison between sources. In this talk, I will highlight investigations of SNR asymmetry and nucleosynthesis, and I will discuss the implications of this work regarding the nature and mechanisms of the originating explosions.
Constraining Type Ia supernova progenitors and diversity using spectroscopic observations
There is increasing observational evidence that there is more than one way to make a Type Ia supernova. Detailed spectroscopic measurements of Type Ia supernovae, ranging from as soon after explosion as possible to many years later are vital for distinguishing between proposed progenitor scenarios. Uncovering direct signatures of their progenitor systems (e.g. circumstellar material, material stripped from a potential companion star) in observed spectra is also an active area of research. In this talk, I will present an overview of the links between the latest spectroscopic observations of Type Ia supernovae and their explo- sion mechanisms. The impact of recent results on our understanding of Type Ia supernova diversity will also be highlighted.
Fully successful, failed or barely failed: the fate of jets trying to break through their stellar progenitors
Recent observations are painting a complex picture of stellar explosions. In this talk I review how data obtained across the electromagnetic spectrum are helping us constraining the presence and properties of jets in supernova explosions.
Core collapse supernovae from blue supergiants : The evolutionary history of SN 1987A
SN 1987A is historically one of the most remarkable supernova explosions to be seen from Earth. Due to the proximity of its location in the LMC, it remains the most well-studied object outside the solar system. It was also the only supernova whose progenitor was observed prior to its explosion. SN 1987A however, was a unique and enigmatic core collapse supernova. It was the first Type II supernova to have been observed to have exploded while its progenitor was a blue supergiant (BSG). Until then Type II supernovae were expected to originate from explosions of red supergiants (RSGs). A spectacular triple-ring nebula structure, rich in helium and nitrogen, was observed around the remnant, indicating a recent RSG phase before becoming a BSG. Even today it is not entirely understood what the evolutionary history may have been to cause a BSG to explode. The most commonly accepted hypothesis for its origin is the merger of a massive binary star system. An evolutionary scenario for such a binary system, was proposed by Podsiadlowski (1992) (P92). Through SPH simulations of the merger and the stellar evolution of the post-merger remnant, Ivanova & Podsiadlowski (2002) and (2003) (I&M) could successfully obtain the RSG to BSG transition of the progenitor. The aim of the present work is to produce the evolutionary history of the progenitor of SN 1987A and its explosion. We construct our models based on the results of P92 and I&M. Here, the secondary (less massive) star is accreted on the primary, while being simultaneously mixed in its envelope over a period of 100 years. The merged star is evolved until the onset of core collapse. For this work we use the 1-dimensional, implicit, hydrodynamical stellar evolution code, KEPLER. A large parameter space is explored, consisting of primary (16-20 Ms) and secondary masses (5-8 Ms), mixing boundaries, and accreting timescales. Those models whose end states match the observed properties of the progenitor of SN 1987A are exploded. The nuclear yields and light curve of the explosion are then compared with the observed data of SN 1987A.
Electromagnetic Signatures of Neutron Star Mergers
Coalescing stellar mass compact objects (binary neutron stars and black holes) are promising sources for the detection of gravitational waves by Advanced LIGO and Virgo in the next few years. Maximizing the scientific return from such a discovery will require identifying a coincident electromagnetic counterpart. One possible counterpart is a short gamma ray burst, powered by the accretion of a centrifugally supported torus onto the central black hole. Neutron star mergers are also accompanied by a thermal optical/IR transient, powered by the radioactive decay of neutron-rich elements synthesized in the merger ejecta (a kilonova). In addition to providing a beacon to the gravitational wave chirp, kilonovae provide direct probes of an astrophysical site for the r-process. I will describe recent work showing how the delay until black hole formation following the merger may be imprinted in the kilonova light curves and colors. I will also describe how free neutrons in the outermost layers of the ejecta power a bright precursors to the main kilonova emission, which could enhance the prospects for its detection.
Dependence of Light-curves and Spectra on Metallicity in Type Ia Supernova Models
Based on the quasi-equilibrium that occurs during incomplete silicon burning, it is expected that the neutron excess in the composition of the exploding white dwarf (WD) in a Type Ia Supernovae (SN Ia) will determine the relative abundances of intermediate mass elements (IME), including Si, S, and Ca, in a predictable way. This may provide a way to infer, or at least constrain, the composition of the progenitor WD from abundances in the explosion itself determined from spectra near maximum light. In order to explore possible dependencies on the model and intrinsic yield of the SNIa explosion, we have post-processed embedded tracer particles capturing density and temperature histories from two cases with differing yields of radioactive nickel taken from the set of 2D explosion models presented in Krueger et al. (2012.) Post-processing was done with the TORCH software instrument using a 225 specie nuclear network for each model over a set of six assumed metallicities: 0.1, 0.5, 1.33, 2.0, 3.0, and 4.0 Z/Zsolar. We find that while the overall yields of IMEs differ between the two explosion models, the trends in the yields with metallicity are similar. With increasing metallicity, Si28 yields remain near constant, Ca40 yields decline, and Fe54 yields increase. Artificial light curves and spectra were created using the PHOENIX-REB radiative transfer software instrument for both cases at all metallicities. Both cases produced a higher than average peak brightness consistent with their radioactive Ni masses, and increasing metallicity produced dimmer, faster declining explosions. Spectra produced two blended features in the 3000 and 4000 Angstrom range which are promising candidates for use with the relatively unaffected Si P-Cygni feature at around 6000 Angstroms to infer progenitor metallicity.
Extended shock breakout signals from inflated stellar envelopes
Stars close to the Eddington luminosity can have large low-density inflated envelopes. We suggest that the rise times of shock breakout signals from supernovae can be extended significantly if supernova progenitors have an inflated envelope. If the shock breakout occurs in such inflated envelopes, the shock breakout signals diffuse in them, and their rise time can be significantly extended. Then, the rise times of the shock breakout signals are dominated by the diffusion time in the inflated envelope rather than the light-crossing time of the progenitor. We show that our inflated Wolf-Rayet star models whose radii are of the order of the solar radius can have shock breakout signals longer than ~100 sec. The existence of inflated envelopes in Wolf-Rayet supernova progenitors may be related to the long shock breakout signal observed in Type Ib SN 2008D.
The Supernova-GRB Connection
A wide range of GRB-like events are now associated with supernova, indicating that jets are a relatively common feature of stellar explosions. I will review recent theoretical advances in understanding the connection between supernova, GRBs, and the range of engine-driven events.
Multidimensional Instabilities in Core Collapse Supernovae Revisited
The most popular scenario for core-collapse supernova explosions relies on the joint action of neutrino heating from the young neutron star and hydrodynamic instabilities like convection in the post-shock convection. In this talk, we shall outline a quantitative theory of how the neutrino heating and the instabilities interact with each other, and demonstrate how simple analytic arguments can be used to derive the 20-30% reduction in the critical luminosity required for explosion in multiple dimensions compared to 1D. We also discuss the role of pre-collapse initial perturbations on the instabilities in the post-shock region and adumbrate how our findings show a path towards to a simple phenomenological approach to connect supernova progenitor and explosion properties.
Conditions for Explosion
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Multidimensional Core-Collapse Simulations in FLASH
Core-collapse marks the death of a massive star and the birth of either a neutron star or black hole. Predicting the ability of the supernova shock to successfully unbind the stellar mantle through computational models of the core-collapse supernova central engine has historically been difficult. However, in the recent years our understanding has progressed significantly due to multidimensional simulations with sophisticated neutrino transport. In this presentation, I will show new multidimensional results using the open-source FLASH hydrodynamics code and a recently implemented, two moment neutrino transport scheme. With these simulations, we investigate aspects of the explosion mechanism in multiple progenitors and explore the impact of collapse-phase physics on the post-bounce multidimensional dynamics.
Multi-dimensional simulations with the isotropic diffusion source approximation for neutrino transport
The neutrino mechanism of core-collapse supernova is investigated via non-relativistic, two- dimensional (2D), neutrino radiation-hydrodynamic simulations. For the transport of electron flavor neutrinos, we use the isotropic diffusion source approximation (IDSA) scheme, which decomposes the transported particles into a trapped particle and a streaming particle components. Heavy neutrinos are described by a leakage scheme. Unlike the ``ray-by-ray'' approach by other multi- dimensional IDSA implementations in spherical coordinates, we use cylindrical coordinates and solve the trapped particle component in multiple dimensions, improving the proto-neutron star resolution and the neutrino transport in angular and temporal directions. We perform 1D and 2D ab initio simulations from prebounce core collapse to several hundred milliseconds postbounce with 11, 15, 21, and 27 $M_\odot$ progenitors from Woosley et al.~(2002) with the HS(DD2) equation of state. We obtain robust explosions with diagnostic energy $E_{\rm dig} \gtrsim 0.1- 0.5$~B for all considered 2D models within approximately $100-300$ milliseconds after bounce and find that explosions are mostly dominated by the neutron-driven convection, although standing accretion shock instabilities are observed as well. We also find that the level of electron deleptonization during collapse may dramatically affect the postbounce evolution,e.g.~the ignorance of neutrino-electron scattering during collapse will leads a stronger explosion.
The response of a helium white dwarf to an exploding type Ia supernova
We present results from numerical simulations of the interacting ejecta from an exploding CO white dwarf with a He WD donor in the double-detonation scenario for Type Ia supernovae and show the possibility of exploding the companion WD. We also present the long time imprint of the collision on the supernova remnant and use it to constrain the double detonation scenario for Type Ia supernovae.
Evolution of massive single and binary stars - their fate and remnants
The final fate of massive stars, the type of explosion and the remnant they leave behind, is mostly governed by the masses of their helium cores and hydrogen envelopes in the latest stages of evolution. While for single stars wind mass loss is the only channel to reduce their mass, stars that are a member of a binary system are also assumed to lose their hydrogen envelope due to Roche lobe overflow or a common envelope phase after core hydrogen burning. We aim at assigning the ZAMS masses of massive stars, both single or members of a binary system, to their remnant masses and in quantifying their compactness predict their most likely remnants, neutron stars or black holes.
Ultraviolet Properties of Type II-P Supernovae
The Swift Ultra-Violet/Optical Telescope (UVOT) has revolutionized the understanding of supernova (SN) behavior in the ultraviolet (UV). Type II SNe have hydrogen envelopes which are shock heated and radiate a large fraction of their luminosity in the UV. Swift/UVOT has observed over one hundred SNe II, mostly at very low redshifts. These give a great sampling of the temperature evolution and the changing fraction of the luminosity coming from the UV. We have been studying the photometric properties of Type II-P SNe in the Swift Optical/Ultraviolet Supernova Archive, comparing the absolute magnitudes and color evolution. Early-time physical properties are obtained through comparison to a perfect blackbody. From these studies, we hope to get a better understanding of the SNe II-P progenitors and energetics.
Superluminous Supernovae
Through the year 2000, the brightest absolute magnitude published for a supernova was about -20. Today, modern surveys routinely turn out discoveries 0.5 to almost 3 magnitudes brighter than this. These so-called "superluminous supernovae" were not expected, and the physical conditions necessary for some supernovae to shine 10-100 times brighter than normal remain a mystery. Several scores of superluminous supernovae have been cataloged, and a basic taxonomy has been established: some superluminous supernovae show spectroscopic evidence for the presence of hydrogen, and others do not. There are hints that these two groups have a tendency to be found in different host environments, which could suggest distinct physical origins. Some studies, however, find a possible link between these groups. Potential power sources for superluminous supernovae include heating from an unusually large mass of radioactive matter, interaction of the supernova ejecta with material cast off prior to the explosion, and the injection of energy from a nascent magnetar. I will review the current state of the field with a focus on the observed sample and some potentially interesting sources of contamination.
An Astronomical Time Machine: Light Echoes from Historic Supernovae and Stellar Eruptions
Tycho Brahe's observations of a supernova in 1572 challenged the dogma that the celestial realm was unchanging. Now, 440 years later we have once again seen the light that Tycho saw as simple reflections from walls of Galactic dust. These light echoes, as well as ones detected from other historical events such as Cas A and Eta Carinae's Great Eruption, give us a rare opportunity in astronomy: the direct observation of the cause (the explosion/eruption) and the effect (the remnant) of the same astronomical event. But we can do more: the light echoes let us look at the explosion from different angles, and permit us to map the asymmetries in the explosion. I will discuss how the unprecedented three-dimensional view of these exciting events allows us to unravel some of their secrets.
Cold Dust and Ejecta in Young Supernova Remnants with Herschel
Whether supernovae (SNe) are a significant source of dust has been a long-standing debate. I will review infrared observations of the young supernova remnants (YSNRs) -- Cas A, SNR 1E102.2-7219 (E0102), N132D, and the Crab Nebula using Spitzer and Herschel data. These SNRs reveal evidence of dust formation and show that SNe are important sites of dust formation, and produce dust on short timescales. Herschel observations unambiguously strengthen the presence of cold dust in SN 1987A, Cas A and the Crab Nebula. We present detection of cold dust from the YSNR G54.1+0.3 and a few additional SNRs using rich archival data sets of Herschel PACS and SPIRE imaging from the infrared Galactic Plane Survey (HIGAL) program and Planck data. We serendipitously discovered a dust feature peaking at 21 micron from G54.1+0.3, and the 21-micron dust is remarkably similar to that of Cas A from Rho et al. (2008). The IRS spectrum from the western shell shows the 21-micron dust feature and strong [Ar II] and weak [Ne II], [S III] and [Si II] lines. We detected submm emission from G54.1+0.3 using CSO SHARCII (at 350 micron) and LABOCA (at 870 micron). We present dust fitting using continuous distributions of ellipsoidal (CDE) grain models. Spectral fitting requires a combination of dust composition including SiO2, SiC, and Al2O3 which are responsible for 21-micron, 11 micron dust features and long-wavelength continuum, respectively. The estimated mass of G54.1+0.3 is comparable to that of Crab Nebula. Herschel observations of G21.5-0.9, N132D and 3C358 and SN 2004et will be presented with their spectral energy distributions. I will discuss presence of cold dust, dust features and composition observed in YSNRs and origin of the dust, and will compare them with pre-solar grains in meteorites. Finally, I will present the correlation of dust composition and elemental abundance of ejecta with their types of SNe, and discuss and significance of supernovae dust in the early Universe and galaxies.
Hydrodynamical Biases in Chemical Enrichment by Supernovae
The Swift Ultra-Violet/Optical Telescope (UVOT) has revolutionized the understanding of supernova (SN) behavior in the ultraviolet (UV). Type II SNe have hydrogen envelopes which are shock heated and radiate a large fraction of their luminosity in the UV. Swift/UVOT has observed over one hundred SNe II, mostly at very low redshifts. These give a great sampling of the temperature evolution and the changing fraction of the luminosity coming from the UV. We have been studying the photometric properties of Type II-P SNe in the Swift Optical/Ultraviolet Supernova Archive, comparing the absolute magnitudes and color evolution. Early-time physical properties are obtained through comparison to a perfect blackbody. From these studies, we hope to get a better understanding of the SNe II-P progenitors and energetics.
A metric space for SNIa spectra: a new method to assess explosion scenarios
Over the past years calibrated light curves of type Ia SNe became a major tool to determine the expansion history of the Universe, and considerable attention has been given to both observations and models of these events. However, until now, their progenitors are not known. The observed diversity of light curves and spectra seems to point at different progenitor channels and explosion mechanisms. We present a new way to compare model predictions with observations in a systematic way. Our method is based on the construction of a metric space for type Ia supernova spectra by means of Expectation Maximization Principle Component Analysis (EMPCA), taking care of missing and/or noisy data, and making use of Partial Least Square regression (PLS) to find correlations between spectral properties and photometric data (Sasdelli et al. 2015). We investigate realizations of the three major classes of explosion models which are presently discussed: delayed-detonation Chandrasekhar-mass explosions, sub-Chandrasekhar mass detonations, and double-degenerate mergers, and compare them with data. We show that in the PC space all model classes have observed counterparts, supporting the idea that different progenitors are likely. However, all scenarios face problems if they attempt to fit the observed correlations between spectral properties and light curves and colors. Possible reasons are briefly discussed.
Impacts of magnetorotational instability on core-collapse supernovae
The effects of magnetic field and rotation on core-collapse supernovae have been studied as possible agents to drive the explosion other than neutrino heating. Previous simulations showed that magnetic fields wound by differential rotation trigger a powerful explosion when they are strong prior to collapse, say ~10^12 - 10^13 G, and the cores are rotating rapidly. I will present results of our recent numerical studies (Sawai & Yamada 2014, ApJ, 784, L10; Sawai & Yamada 2015, arXiv:1504.03035) which shows that magnetic fields boost the explosion even when they are weak initially. In our 2D-axisymmetric high-resolution simulations, the magnetic fields are amplified drastically by the magnetorotational instability (MRI). The angular momentum transfer induced by the MRI causes the expansion of the heating region, by which the accreting matter gains an additional time to be heated by neutrinos. The MRI also drifts low-Y_p matter from the deep inside of the core to the heating region, which makes the net neutrino heating rate larger by the reduction of the cooling due to the electron capture. These two effects enhance the efficiency of the neutrino heating, which is found to be the key to the boost of explosion.
Supernova Neutrino Detection
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Do double degenerate double detonations drive destructive dwarf death?
The nature of the progenitors of Type Ia supernovae remains a central mystery in supernova astrophysics. In this talk, I will focus on the promising "double degenerate double detonation" channel, in which helium transferred from a companion white dwarf ignites a surface detonation on a primary white dwarf. The surface detonation in turn ignites a core detonation and subsequent supernova. I will describe our recent results on the ignition of these detonations and the effect of the binary's pre-supernova evolution on the surrounding environment. I will also show ongoing work characterizing the interaction of the supernova ejecta with the surviving companion white dwarf in order to match late-time observations of Type Ia supernovae and their remnants.
Modelling thermonuclear supernovae from different progenitor systems - explosion simulations, nucleosynthesis, observables
In 2011 the Nobel Prize in Physics was awarded to Perlmutter, Riess, and Schmidt "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae". These Type Ia supernovae (SNe Ia) are also the dominant contributor to iron-group nucleosynthesis and leading candidates for the elusive sites of high energy cosmic rays, the p-process isotopes, and perhaps Galactic positrons. SNe Ia are thought to be thermonuclear explosions of white dwarf stars. Despite their ubiquitous importance to cosmology and astronomy, their progenitor systems and the explosion mechanism(s) are still largely unknown. This "SN Ia progenitor and explosion mechanism problem" is one of the great unsolved problems in astrophysics, complicated by the fact that in recent years several unique sub-classes of SNe Ia have been described. I will discuss leading explosion models for SNe Ia, present results from multi-dimensional hydrodynamical simulations of the final seconds of the star's life, and highlight our comprehensive approach to create a unified model for the progenitors and explosion mechanisms of SNe Ia.
Observations of Type Ia Supernovae Strongly Interacting with Their Circumstellar Medium
In this talk I will summarize the published observations of Ia-CSM objects, which are Type Ia supernova (SN Ia) with indications of significant interaction with their circumstellar medium (CSM), most obviously in the form of relatively narrow H-alpha emission in their spectra. Approximately 15 such objects have been described in the literature, including PTF11kx, which was initially classified as a somewhat overluminous SN Ia, but developed peculiari- ties (including strong H-alpha emission) after CSM-interaction began about 2 months after explosion. In addition to strong H-alpha emission, the spectra of Ia-CSM objects show an almost complete lack of He I emission and large Balmer decrements (perhaps due to colli- sional excitation of hydrogen via the SN ejecta overtaking slower-moving CSM shells). They also exhibit evidence of dust formation through a decrease in the red wing of the H-alpha emission 75-100 days past maximum brightness, and half of the Ia-CSM objects show strong Na I D absorption from their host galaxy. The absolute magnitudes (uncorrected for extinc- tion) of Ia-CSM objects are found to be -21 ˇ MR ˇ -19 mag, and they also seem to show UV emission at early times and strong IR emission at late times (but no detected radio or X-ray emission).
Ultraviolet-Optical Spectra and Photometry of the 1999aa-like Supernova iPTF14bdn
We present photometric and spectral series in the ultraviolet and optical of the 1999aa-like SN iPTF14bdn. Pre-maximum Swift UVOT photometry of iPTF14bdn shows bluer than normal colors and color evolution towards redder values, in contrast to normal SNe Ia. Combined optical and UVOT grism spectra from 10 days prior to B-band maximum allow us to identify the source of this feature to be a low opacity 'window' between 2800-3200A resulting from the presence of doubly ionized iron group ions. The high temperatures necessary for this behavior are consistent with Ni mixing into the outer layers of the SN at early times.
Novae as Mini-Supernovae: Particle Acceleration and Gamma-Ray Production in Nova Shocks
The surprising recent identification of classical novae as a new class of gamma-ray sources revealed a strong physical connection between novae and supernovae (SNe). Observations from radio to X-ray wavelengths in pursuit of the origin of gamma-rays from novae indicate that novae produce multiple, distinct outflows that collide with each other. The resulting shocks lead to particle acceleration, GeV gamma-rays, shaping of the ejecta, and possibly even the creation of dust. Moreover, theoretical studies of gamma-ray emission from novae are likely to place new constraints on the nature of particle acceleration in shocks. I will explore what the SNe community might learn from considering particle acceleration in a different context - that of novae.
Another case of time-variable sodium features in a nearby Type Ia supernova
Different Type Ia supernova progenitor models predict different mass-loss processes at different phases of the systems' evolution. This should lead to a difference in the properties of the circumstellar environment about the exploding white-dwarf star of different progenitor systems. Therefore, studying the circumstellar environment of Type Ia supernovae can help reveal the nature of their progenitor systems. At the time of explosion nearby circumstellar material, if present, will be ionized and start undergoing recombination around maximum light and after, depending on the properties of the material. This should be observed as time-variable absorption features in the spectra, if the material lies on the line-of-sight to the event. High-resolution-spectroscopy is needed to observe narrow and/or weak features and provides a high sensitivity to changes in these lines. To date, this behavior has been observed in a relative small fraction of Type Ia supernovae. I will present new high-spectral-resolution observation of a few Type Ia events, one of which exhibits slight but monotonous time-variability in the sodium features. This variability is not observed in other lines. I will discuss our analysis of the data and our conclusion on the likelihood of the variability having a circumstellar origin.
Unraveling the non-monotonicity of pre-supernova evolution and its implications
The systematics of pre-supernova core compactness as a function of initial mass and input physics is explored using a large number of fine-meshed surveys of full hydrogenic stars and bare CO core models. The variation of the core compactness as a function of mass is found to be robustly non-monotonic and is heavily dependent on the complex interplay of shell burning episodes during the advanced stages of evolution. The full star grid is exploded by a piston model based on an actual neutrino-transport calculation, which was calibrated by the observed energy of SN1987A. Many progenitors do not explode and those do explode do not form a simple connected set, as the final kinetic energy of the SN is correlated with the core compactness. The resulting nucleosynthesis (excluding r-process, including a Type-Ia contribution) and neutron star distribution agree reasonably well with the observables. But the overproduction of carbon and deficient production of s-process species argues for a lower mass loss rate than assumed and perhaps a larger rate for 22Ne(a,n)25Mg as well.
Progenitors, Supernovae, and Neutron Stars
Core-collapse supernovae are dynamical phase transitions of massive stellar cores to neutron stars. The key process is how the gravitational energy is transferred to ejecta. Although the exact mechanism is still under a thick veil, recent multi-dimensional simulations have partially succeeded to produce explosions. In this talk, I will show our recent results of neutrino-radiation hydrodynamic simulations, especially focusing on progenitor dependences, i.e. which kind of progenitor models are likely to produce supernova explosions. In addition, I will show results of very long-term simulation until a minutes after the onset of explosion, with which we can probe the formation of neutron stars that are the final outcomes of core-collapse supernovae.
Core-Collapse Supernova Science with Advanced LIGO and Virgo
The upcoming era of advanced-generation gravitational wave observatories will offer unprece- dented sensitivity to gravitational waves from galactic and nearby extragalactic core-collapse supernovae. I present the current status of, and preliminary results from, LIGO/Virgo searches for gravitational waves from core-collapse supernovae. I discuss ongoing efforts to extract waveform details, estimate physical parameters, and learn about core-collapse su- pernova dynamics from gravitational waves observed from the next nearby core collapse event.
Remarkable Photometric and Spectroscopic Features in Recent Type Ib Supernovae
Type Ib supernovae have been considered to be core-collapse explosions at a state lacking hydrogen layer but remaining helium layer in the end of evolution of massive stars. Recent dramatic discovery is that the progenitor of a typical Type Ib SN iPTF13bvn had been directly detected in pre-explosion image as a blue star. However, it is still under debate that the progenitor was either a helium star or a Wolf-Rayet star. Thus, we poorly understand the detail of the evolutionary path as well as the explosion mechanism of massive progenitors of Type Ib supernovae. In this talk, we introduce remarkable features found in some recent Type Ib supernovae, which would bring a hint on these questions. SN 2012au is bright (-18.7 mag at maximum in R band) among Type Ib supernovae and its explosion energy reaches as high as 10^52 erg being comparable with those of energetic Type Ic supernovae (hypernovae). In the late-phase light curve, SN 2012au shows a significant break at ~300 days after explosion. Similar feature has been also found in a nearby Type Ib/Ic SN 2012h (which was unfortunately missed during its early phase) by us. Another Type Ib SN 2014C showed a strong H-alpha emission line having a width of 1000-2000 km/s in a spectrum taken at ~200 days after explosion. This velocity is apparently slower than those of other emission lines originated in the supernova ejecta (e.g., [O I] 6300, 6364) but higher than those of H II regions in the host galaxy. The tail of its light curve is so slow. These facts would be related with the ejecta interacting with the CSM. We will discuss the cause of these unique features from a viewpoint of mass-loss activity in the progenitor stars just before the explosions.
Recent developments on hydrodynamical instabilities and neutrinos in 3D
According to the delayed explosion scenario of core-collapse supernovae, neutrinos play a fundamental role reviving the explosion and carrying imprints of the supernova hydrody- namics. I will present a novel and intriguing neutrino-driven hydrodynamical instability, termed LESA (Lepton-number Emission Self-sustained Asymmetry). The most conspicu- ous manifestation of LESA is the lepton-number flux emission primarily in one hemisphere. LESA may have important implications for neutrino-flavor oscillations, nucleosynthesis and neutron-star kicks.
Evolution of the Composite Supernova Remnant G327.1-1.1
I will present deep Chandra X-ray observations and hydrodynamic modeling of an evolved composite supernova remnant (SNR) G327.1-1.1 that consists of a supernova (SN) blast wave expanding into the ambient medium, and a central pulsar wind nebula (PWN) produced by a rapidly rotating pulsar. Previous X-ray studies of this SNR showed a highly complex morphology; a compact X-ray core embedded in a cometary structure, prong-like features extending into large arcs in the SNR interior, and thermal emission from the SNR shell. The overall structure suggests that the PWN has undergone a recent asymmetric interaction with the SN reverse shock that completely disrupted the nebula. Such an asymmetric interaction can occur as a result of a density gradient in the ambient medium and/or a moving pulsar that displaces the PWN from the center of the SNR. The pulsar in G327.1-1.1 appears to be regenerating a new PWN, which may be deforming into a bow shock due to the pulsar's rapid motion through the SNR. The study presented here reveals new information about the SNR properties, and provides new insight into the nature of the system's complex morphology and the late-phase evolution of composite SNRs in general.
KEGS - The Kepler Extra-Galactic Survey
Kepler's unique technical capabilities are not only well suited for finding and studying exoplanets, but also supernovae and extra-galactic transients. I will give an overview of the Kepler Extra-Galactic survey - a program using Kepler to search for supernovae, active galactic nuclei, and other transients in galaxies. To date we have found three type Ia supernovae and three core-collapse supernovae. The 30-minute cadence of Kepler has revealed subtle features in the light-curves of these supernova not detectable with any other survey. With a high-cadence, high precision survey, shock break-out in a large number of SN can be found, improving our understanding of supernova progenitors. We can also search in nearby galaxies for very fast and faint transients, filling in a previously unaccessible parameter space. Lastly, the precision data of any discovered type Ia supernova combined with ground based data can dramatically improve our use of type Ia for determining distances and measuring the properties of dark energy.
Nucleosynthesis with multi-dimensional supernova models
Understanding of supernova nucleosynthesis for iron-group and beyond has been hampered by the still unknown physical conditions of innermost ejecta from collapsing cores. In this talk we present our latest result of nucleosynthesis studies making use of multi-dimensional, self-consistently exploding, neutrino transport models of supernovae for various progenitor masses.
Sterile neutrino dark matter and core-collapse supernovae
The nature of dark matter and the explosion mechanism of core-collapse supernovae remain two of the biggest open questions in astrophysics. A heavy sterile neutrino species may provide a solution to both of these problems. Recent observations of galaxies and galaxy clusters indicate that dark matter may be a ~keV mass sterile neutrino. In core-collapse supernovae, sterile neutrinos can efficiently transport energy from the protoneutron star core to the stalled shock via oscillations between electron neutrinos and sterile neutrinos. We have performed self-consistent simulations of core-collapse supernovae including a sterile neutrino with mass and mixing angle of a dark matter candidate. We have found that some choices of mass and mixing angle result in enhanced neutrino reheating and result in successful explosions, even in models that would not otherwise explode. Sterile neutrino dark matter and core-collapse supernovae
SNR G284.3-1.8 and the High-Mass Gamma-Ray Binary 1FGL J1018.6-5856
It is extremely rare when a high-mass X-ray binary is found to be associated with supernova remnant. It is also rare when a high-mass binary is bright in gamma-rays. SNR G284.3-1.8 appears to be a unique remnant hosting such a binary. We present results from Chandra and XMM-Newton observations of the SNR and the binary. G284.3-1.8 has asymmetrically distributed Mg-rich ejecta, which has only been observed in one other remnant. It also appears to be interacting with a molecular cloud in the north. It is clear that the SN progenitor was a massive star, but the unknown nature of the compact object complicates the binary evolution modeling. We will report the variability and spectral analysis for the binary X-ray emission and discuss the constraints our results place on the SNR progenitor system and the high-mass binary evolution scenarios.
Probing the Three Dimensional Nature of Supernovae; Results from the Supernova Spectropolarimetry Project (SNSPOL)
The Supernova Spectropolarimetry (SNSPOL) project aims to complete a long term comprehensive spectropolarimetric survey of all types of supernovae. The principal goal of this effort is to improve our understanding of the predominance and characteristics of asymmetries in and around supernova explosions. This is achieved by monitoring the polarimetric evolution of our targets using the 61" Kuiper, the 90" Bok, and the 6.5-m MMT telescopes together with the CCD Imaging/Spectropolarimeter (SPOL). During the past five years we've observed more than 55 supernovae with approximately 80% of those being observed during multiple epochs. Here we present selected results from multi-epoch spectropolarimetry of several supernovae including SN 2009ip, SN 2010jl, SN 2011dh, SN 2011fe, SN 2012ab, SN 2013ej, and SN 2014J.
Modeling the Early Phases of Type Ia Supernovae
Both the Chandrasekhar-mass single degenerate and sub-Chandra mass double detonation models for SNe Ia begin with a slowly convecting region gradually building up to the localized ignition of a burning front. Modeling these phases allows us to understand the spatial and temporal distribution of the hot spots that seed these burning fronts, and in turn, allows for a more consistent investigation of the subsequent explosion. We discuss a series of simulations exploring the ignition process in both models and the algorithmic challenges they pose.