J. C. Wheeler

Type Ia Supernovae: Influence of the Initial Composition on the Nucleosynthesis, Light Curves, and Spectra and Consequences for the Determination of ΩM and Λ

The influence of the initial composition of the exploding white dwarf on the nucleosynthesis, light curves, and spectra of Type Ia supernovae has been studied in order to evaluate the size of evolutionary effects on cosmological timescales, how the effects can be recognized, and how one may be able to correct for them. The calculations are based on a set of delayed detonation models that give a good account of the optical and infrared light curves and of the spectral evolution. The explosions and light curves are calculated using a one-dimensional Lagrangian radiation-hydro code including a nuclear network. Spectra are computed for various epochs using the structure resulting from the light-curve code. Our non-LTE code solves the relativistic radiation transport equations in the comoving frame consistently with the statistical equations and ionization due to γ-radiation for the most important elements (C, O, Ne, Na, Mg, Si, S, Ca, Fe, Co, Ni). About 106 additional lines are included assuming LTE-level populations and an equivalent-two-level approach for the source functions. Changing the initial metallicity Z from Population I to Population II alters the isotopic composition of the outer layers of the ejecta that have undergone explosive O burning. Especially important is the increase of the 54Fe production with metallicity. The influence on the resulting rest-frame visual and blue light curves is found to be small. Detailed analysis of spectral evolution should permit a determination of the progenitor metallicity. Mixing 56Ni into the outer layers during the explosion can produce effects similar to an increased initial metallicity. Mixing can be distinguished from metallicity effects by means of the strong cobalt and nickel lines, by a change of the calcium lines in the optical and IR spectra and, in principle, by γ-ray observations. As the C/O ratio of the white dwarf is decreased, the explosion energy and the 56Ni production are reduced, and the Si-rich layers are more confined in velocity space. A reduction of the C/O ratio by about 60% gives slower rise times by about three days, an increased luminosity at maximum light, a somewhat faster postmaximum decline, and a larger ratio between maximum light and 56Ni tail. A reduction of the C/O ratio has an effect on the colors, light-curve shapes and element distribution similar to a reduction in the deflagration to detonation transition density. However, for the same light-curve shape, the absolute brightness is larger for smaller C/O ratios. An independent determination of the initial C/O ratio and the transition density is possible for local supernovae if detailed analyses of both the spectra and light curves are performed simultaneously. Because the spectra are shifted into different color bands at different redshifts, the effect of metallicity Z on a given observed color is a strong function of redshift. A change of Z by a factor of 3 or of the C/O ratio by 33% alters the peak magnitudes in the optical wavelength range by up to ≈ 0.3 mag for z ≥ 0.2. These variations are comparable to the effect of changes of ΩM and Λ at redshifts of 0.5-1.0. The systematic effects due to changes in composition are expected to remain small up to about z ≈ 0.5 for R-V and up to z ≈ 0.7 for R-I. We discuss how evolution in the progenitor population can be recognized and taken into account. With proper account of evolutionary corrections, supernovae will provide a valuable tool to determine the cosmological parameters of the universe, and they will provide new insight into its chemical evolution.

Jet-induced Explosions of Core Collapse Supernovae

We numerically studied the explosion of a supernova caused by supersonic jets present in its center. The jets are assumed to be generated by a magnetorotational mechanism when a stellar core collapses into a neutron star. We simulated the process of the jet propagation through the star, jet breakthrough, and the ejection of the supernova envelope by the lateral shocks generated during jet propagation. The end result of the interaction is a highly nonspherical supernova explosion with two high-velocity jets of material moving in polar directions and slower moving, oblate, highly distorted ejecta containing most of the supernova material. The jet-induced explosion is entirely due to the action of the jets on the surrounding star and does not depend on neutrino transport or reacceleration of a stalled shock. The jet mechanism can explain the observed high polarization of Types Ib, Ic, and IIsupernovae, pulsar kicks, very high velocity material observed in supernova remnants, indications that radioactive material was carried to the hydrogen-rich layers in SN 1987A, and other observations that are very difficult or impossible to explain by the neutrino energy deposition mechanism. The breakout of the jet from a compact, hydrogen-deficient core may account for the γ-ray burst and radio outburst associated with SN 1998bw/GRB 980425.

SN 2005cs in M51 – II. Complete evolution in the optical and the near-infrared

We present the results of the one-year long observational campaign of the type II plateau SN 2005cs, which exploded in the nearby spiral galaxy M51 (the Whirlpool galaxy). This extensive data set makes SN 2005cs the best observed low-luminosity, ^(56)Ni-poor type II plateau event so far and one of the best core-collapse supernovae ever. The optical and near-infrared spectra show narrow P-Cygni lines characteristic of this SN family, which are indicative of a very low expansion velocity (about 1000 km s^(−1) ) of the ejected material. The optical light curves cover both the plateau phase and the late-time radioactive tail, until about 380 d after core-collapse. Numerous unfiltered observations obtained by amateur astronomers give us the rare opportunity to monitor the fast rise to maximum light, lasting about 2 d. In addition to optical observations, we also present near-infrared light curves that (together with already published ultraviolet observations) allow us to construct for the first time a reliable bolometric light curve for an object of this class. Finally, comparing the observed data with those derived from a semi-analytic model, we infer for SN 2005cs a ^(56)Ni mass of about 3 × 10^(−3) M⊙, a total ejected mass of 8–13 M⊙ and an explosion energy of about 3 × 10^(50) erg .

Deflagration to Detonation Transition in Thermonuclear Supernovae.

We derive the criteria for deflagration to detonation transition (DDT) in a Type Ia supernova. The theory is based on two major assumptions: (1) detonation is triggered via the Zeldovich gradient mechanism inside a region of mixed fuel and products, and (2) the mixed region is produced by a turbulent mixing of fuel and products either inside an active deflagration front or during the global expansion and subsequent contraction of an exploding white dwarf. We determine the critical size of the mixed region required to initiate a detonation in a degenerate carbon-oxygen mixture. This critical length is much larger than the width of the reaction front of a Chapman-Jouguet detonation. However, at densities greater than 5 × 106 g cm-3, it is much smaller than the size of a white dwarf. We derive the critical turbulent intensity required to create the mixed region inside an active deflagration front in which a detonation can form. We conclude that the density ρtr at which a detonation can form in a carbon-oxygen white dwarf is low, less than 2-5 × 107 g cm-3 but greater than 5 × 106 g cm-3.

A PANCHROMATIC VIEW OF THE RESTLESS SN 2009ip REVEALS THE EXPLOSIVE EJECTION OF A MASSIVE STAR ENVELOPE

The double explosion of SN 2009ip in 2012 raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN 2009ip during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from the GeV to the radio band obtained from a variety of ground-based and space facilities (including the Very Large Array, Swift, Fermi, Hubble Space Telescope, and XMM) constrain SN 2009ip to be a low energy (E similar to 1050 erg for an ejecta mass similar to 0.5 M-circle dot) and asymmetric explosion in a complex medium shaped by multiple eruptions of the restless progenitor star. Most of the energy is radiated as a result of the shock breaking out through a dense shell of material located at similar to 5 x 10(14) cm with M similar to 0.1 M-circle dot, ejected by the precursor outburst similar to 40 days before the major explosion. We interpret the NIR excess of emission as signature of material located further out, the origin of which has to be connected with documented mass-loss episodes in the previous years. Our modeling predicts bright neutrino emission associated with the shock break-out if the cosmic-ray energy is comparable to the radiated energy. We connect this phenomenology with the explosive ejection of the outer layers of the massive progenitor star, which later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the massive luminous progenitor star survived. Irrespective of whether the explosion was terminal, SN 2009ip brought to light the existence of new channels for sustained episodic mass loss, the physical origin of which has yet to be identified.

The Effect of Progenitor Age and Metallicity on Luminosity and 56Ni Yield in Type Ia Supernovae

Timmes et al. found that metallicity variations could theoretically account for a 25% variation in the mass of 56Ni synthesized in Type Ia supernovae (SNe Ia), and thus account for a large fraction of the scatter in observed SN Ia luminosities. Higher-metallicity progenitors are more neutron rich, producing more stable burning products relative to radioactive 56Ni. We develop a new method for estimating bolometric luminosity and 56Ni yield in SNe Ia and use it to test the theory with data from the Supernova Legacy Survey. We find that the average 56Ni yield does drop in SNe Ia from high-metallicity environments, but the theory can only account for 7%-10% of the dispersion in SN Ia 56Ni mass, and thus luminosity. This is because the effect is dominant at metallicities significantly above solar, whereas we find that SN hosts have predominantly subsolar or only moderately above-solar metallicities. We also show that allowing for changes in O/Fe with the metallicity [Fe/H] does not have a major effect on the theoretical prediction of Timmes et al., so long as one is using the O/H as the independent variable. Age may have a greater effect than metallicity—we find that the luminosity-weighted age of the host galaxy is correlated with 56Ni yield, and thus more massive progenitors give rise to more luminous explosions. This is hard to understand if most SNe Ia explode when the primaries reach the Chandrasekhar mass. Finally, we test the findings of Gallagher et al. that the residuals of SNe Ia from the Hubble diagram are correlated with host galaxy metallicity, and we find no such correlation.

FIRST-YEAR SLOAN DIGITAL SKY SURVEY-II (SDSS-II) SUPERNOVA RESULTS: CONSTRAINTS ON NONSTANDARD COSMOLOGICAL MODELS

We use the new Type Ia supernovae discovered by the Sloan Digital Sky Survey-II supernova survey, together with additional supernova data sets as well as observations of the cosmic microwave background and baryon acoustic oscillations to constrain cosmological models. This complements the standard cosmology analysis presented by Kessler et al. in that we discuss and rank a number of the most popular nonstandard cosmology scenarios. When this combined data set is analyzed using the MLCS2k2 light-curve fitter, we find that more exotic models for cosmic acceleration provide a better fit to the data than the ΛCDM model. For example, the flat Dvali-Gabadadze-Porrati model is ranked higher by our information-criteria (IC) tests than the standard model with a flat universe and a cosmological constant. When the supernova data set is instead analyzed using the SALT-II light-curve fitter, the standard cosmological-constant model fares best. This investigation of how sensitive cosmological model selection is to assumptions about, and within, the light-curve fitters thereby highlights the need for an improved understanding of these unresolved systematic effects. Our investigation also includes inhomogeneous Lemaitre-Tolman-Bondi (LTB) models. While our LTB models can be made to fit the supernova data as well as any other model, the extra parameters they require are not supported by our IC analysis. Finally, we explore more model-independent ways to investigate the cosmic expansion based on this new data set.

A Spitzer Space Telescope study of SN 2003gd: Still no direct evidence that core-collapse supernovae are major dust factories

We present a new, detailed analysis of late-time mid-infrared observations of the Type II-P supernova (SN) 2003gd. At about 16 months after the explosion, the mid-IR flux is consistent with emission from 4 x 10^(-5) M☉ of newly condensed dust in the ejecta. At 22 months emission from pointlike sources close to the SN position was detected at 8 and 24 μm. By 42 months the 24 μm flux had faded. Considerations of luminosity and source size rule out the ejecta of SN 2003gd as the main origin of the emission at 22 months. A possible alternative explanation for the emission at this later epoch is an IR echo from preexisting circumstellar or interstellar dust. We conclude that, contrary to the claim of Sugerman and coworkers, the mid-IR emission from SN 2003gd does not support the presence of 0.02 M☉ of newly formed dust in the ejecta. There is, as yet, no direct evidence that core-collapse supernovae are major dust factories.

A Wolf–Rayet-like progenitor of SN 2013cu from spectral observations of a stellar wind

The explosive fate of massive Wolf–Rayet stars (WRSs) is a key open question in stellar physics. An appealing option is that hydrogen-deficient WRSs are the progenitors of some hydrogen-poor supernova explosions of types IIb, Ib and Ic (ref. 2). A blue object, having luminosity and colours consistent with those of some WRSs, has recently been identified in pre-explosion images at the location of a supernova of type Ib (ref. 3), but has not yet been conclusively determined to have been the progenitor. Similar work has so far only resulted in non-detections. Comparison of early photometric observations of type Ic supernovae with theoretical models suggests that the progenitor stars had radii of less than 1012 centimetres, as expected for some WRSs. The signature of WRSs, their emission line spectra, cannot be probed by such studies. Here we report the detection of strong emission lines in a spectrum of type IIb supernova 2013cu (iPTF13ast) obtained approximately 15.5 hours after explosion (by ‘flash spectroscopy’, which captures the effects of the supernova explosion shock breakout flash on material surrounding the progenitor star). We identify Wolf–Rayet-like wind signatures, suggesting a progenitor of the WN(h) subclass (those WRSs with winds dominated by helium and nitrogen, with traces of hydrogen). The extent of this dense wind may indicate increased mass loss from the progenitor shortly before its explosion, consistent with recent theoretical predictions.

Discovery of the ultra-bright type II-L supernova 2008es

We report the discovery by the Robotic Optical Transient Search Experiment (ROTSE-IIIb) telescope of SN 2008es, an overluminous supernova (SN) at z = 0.205 with a peak visual magnitude of –22.2. We present multiwavelength follow-up observations with the Swift satellite and several ground-based optical telescopes. The ROTSE-IIIb observations constrain the time of explosion to be 23 ± 1 rest-frame days before maximum. The linear decay of the optical light curve, and the combination of a symmetric, broad Hα emission line profile with broad P Cygni Hβ and Na I λ5892 profiles, are properties reminiscent of the bright Type II-L SNe 1979C and 1980K, although SN 2008es is greater than 10 times more luminous. The host galaxy is undetected in pre-supernova Sloan Digital Sky Survey images, and similar to Type II-L SN 2005ap (the most luminous SN ever observed), the host is most likely a dwarf galaxy with Mr > – 17. Swift Ultraviolet/Optical Telescope observations in combination with Palomar 60 inch photometry measure the spectral energy distribution of the SN from 200 to 800 nm to be a blackbody that cools from 14000 K at the time of the optical peak to 6400 K 65 days later. The inferred blackbody radius is in good agreement with the radius expected for the expansion speed measured from the broad lines (10000 km s^–1). The bolometric luminosity at the optical peak is 2.8 × 10^44 erg s^–1, with a total energy radiated over the next 65 days of 5.6 × 10^50 erg. The exceptional luminosity of SN 2008es requires an efficient conversion of kinetic energy produced from the core-collapse explosion into radiation. We favor a model in which the large peak luminosity is a consequence of the core collapse of a progenitor star with a low-mass extended hydrogen envelope and a stellar wind with a density close to the upper limit on the mass-loss rate measured from the lack of an X-ray detection by the Swift X-Ray Telescope.