Width-luminosity Relation Research Articles

Underluminous 1991bg-like Type Ia supernovae are standardizable candles

ABSTRACT It is widely accepted that the width–luminosity relation used to standardize normal Type Ia supernovae (SNe Ia) breaks down in underluminous, 1991bg-like SNe Ia. This breakdown may be due to the choice of parameter used as a stand-in for the width of the SN Ia light curve. Using the colour stretch parameter sBV instead of older parameters resolves this issue. Here, I assemble a sample of 14 nearby 1991bg-like SNe Ia from the literature, all of which have independent host-galaxy distance moduli and little to no reddening. I use Gaussian process regression to fit the light curves of these SNe in U/u, B, V, g, R/r, I/i, and H, and measure their peak absolute magnitudes. I find statistically significant (>5σ confidence level in the optical and >4σ in the near-infrared) correlations between the peak absolute magnitudes of the 1991bg-like SNe Ia and their sBV values in the range 0.2 < sBV < 0.6. These correlations are broadly consistent with fits to sBV < 0.7 SNe Ia with preliminary B- and V-band peak absolute magnitudes from the Carnegie Supernova Project and significantly inconsistent with similar fits to normal and transitional SNe Ia (with 0.7 < sBV < 1.1). The underluminous width–luminosity relation shown here needs to be properly calibrated with a homogeneous sample of 1991bg-like SNe Ia, after which it could be used as a rung on a new cosmological distance ladder. With surface-brightness fluctuations (or another non-Cepheid method) used to calibrate distances to nearby 1991bg-like SNe Ia, such a ladder could produce an independent measurement of the Hubble–Lemaître constant, H0.

The Progenitors of Superluminous Type Ia Supernovae

Recent observations of type Ia supernovae (SNe Ia) have discovered a subclass of “super-Chandrasekhar” SNe Ia (SC SNe Ia) whose high luminosities and low ejecta velocities suggest that they originate from the explosions of white dwarfs (WDs) with masses that exceed the Chandrasekhar mass limit. Different models have been proposed to explain the progenitors of these explosions, including a “magnetized WD” model and a “WD merger” model. To test the robustness of these models, we conduct a 1D numerical parameter survey of WD explosions using these models as initial conditions. We follow the explosions using the hydrodynamics code Castro and then use the radiation transport code SuperNu to create light curves and spectra for the models. We find that while both classes of models fall within the range of SC SNe Ia observations on the light-curve width–luminosity relation, only the WD merger models reproduce the observed low ejecta velocities. The light curves of our merger models are more similar photometrically to observations than our magnetized models. Given this, we discuss possible explanations for the brightest SC SNe Ia observations that cannot be reproduced with our WD merger models. This study provides the basis for future SC SNe Ia observations and higher-dimensional numerical models.

Double detonations: variations in Type Ia supernovae due to different core and He shell masses – II. Synthetic observables

ABSTRACT Double detonations of sub-Chandrasekhar mass white dwarfs are a promising explosion scenario for Type Ia supernovae, whereby a detonation in a surface helium shell triggers a secondary detonation in a carbon-oxygen core. Recent work has shown that low-mass helium shell models reproduce observations of normal SNe Ia. We present 3D radiative transfer simulations for a suite of 3D simulations of the double detonation explosion scenario for a range of shell and core masses. We find light curves broadly able to reproduce the faint end of the width–luminosity relation shown by SNe Ia, however, we find that all of our models show extremely red colours, not observed in normal SNe Ia. This includes our lowest mass helium shell model. We find clear Ti ii absorption features in the model spectra, which would lead to classification as peculiar SNe Ia, as well as line blanketing in some lines of sight by singly ionized Cr and Fe-peak elements. Our radiative transfer simulations show that these explosion models remain promising to explain peculiar SNe Ia. Future full non-LTE simulations may improve the agreement of these explosion models with observations of normal SNe Ia.

Evidence for strong progenitor age dependence of type Ia supernova luminosity standardization process

ABSTRACT Supernova (SN) cosmology is based on the assumption that the width–luminosity relation (WLR) and the colour–luminosity relation (CLR) in the type Ia SN luminosity standardization would not show absolute magnitude differences with progenitor age. Unlike this expectation, recent age datings of stellar populations in host galaxies have shown significant correlations between progenitor age and Hubble residual (HR). Here, we show that this correlation originates from a strong progenitor age dependence of the zero-points of the WLR and the CLR, in the sense that SNe from younger progenitors are fainter each at given light-curve parameters x1 and c. This 4.6σ result is reminiscent of Baade’s discovery of the zero-point variation of the Cepheid period–luminosity relation with age, and, as such, causes a serious systematic bias with redshift in SN cosmology. Other host properties show substantially smaller and insignificant offsets in the WLR and CLR for the same data set. We illustrate that the differences between the high-$z$ and low-$z$ SNe in the WLR and CLR, and in HR after the standardization, are fully comparable to those between the correspondingly young and old SNe at intermediate redshift, indicating that the observed dimming of SNe with redshift may well be an artefact of overcorrection in the luminosity standardization. When this systematic bias with redshift is properly taken into account, there is little evidence left for an accelerating universe, in discordance with other probes, urging the follow-up investigations with larger samples at different redshift bins.

Double detonations of sub-MCh CO white dwarfs: variations in Type Ia supernovae due to different core and He shell masses

Sub-Chandrasekhar mass carbon-oxygen white dwarfs with a surface helium shell have been proposed as progenitors of Type Ia supernovae (SNe Ia). If true, the resulting thermonuclear explosions should be able to account for at least some of the range of SNe Ia observables. To study this, we conducted a parameter study based on three-dimensional simulations of double detonations in carbon-oxygen white dwarfs with a helium shell, assuming different core and shell masses. An admixture of carbon to the shell and solar metallicity are included in the models. The hydrodynamic simulations were carried out using the AREPO code. This allowed us to follow the helium shell detonation with high numerical resolution, and this improves the reliability of predicted nucleosynthetic shell detonation yields. The addition of carbon to the shell leads to a lower production of 56Ni, while including solar metallicity increases the production of intermediate mass elements. The production of higher mass elements is further shifted to stable isotopes at solar metallicity. Moreover, we find different core detonation ignition mechanisms depending on the core and shell mass configuration. This has an influence on the ejecta structure. We present the bolometric light curves predicted from our explosion simulations using the Monte Carlo radiative transfer code ARTIS and make comparisons with bolometric SNe Ia data. The bolometric light curves of our models show a range of brightnesses, which is able to account for subluminous to normal brightness SNe Ia. We show the model bolometric width-luminosity relation compared to data for a range of model viewing angles. We find that, on average, our brighter models lie within the observed data. The ejecta asymmetries produce a wide distribution of observables, which might account for outliers in the data. However, the models overestimate the extent of this compared to data. We also find that the bolometric decline rate over 40 days, Δm40(bol), appears systematically faster than data.

ASASSN-15pz: Revealing Significant Photometric Diversity among 2009dc-like, Peculiar SNe Ia∗

Abstract We report comprehensive multi-wavelength observations of a peculiar Type Ia-like supernova (“SN Ia-pec”) ASASSN-15pz. ASASSN-15pz is a spectroscopic “twin” of SN 2009dc, a so-called “Super-Chandrasekhar-mass” SN, throughout its evolution, but it has a peak luminosity that is dimmer and comparable to the SN 1991T sub-class of SNe Ia at the luminous end of the normal width-luminosity relation. The synthesized 56Ni mass of is also substantially less than that found for several 2009dc-like SNe. Previous well-studied 2009dc-like SNe have generally suffered from large and uncertain amounts of host-galaxy extinction, which is negligible for ASASSN-15pz. Based on the color of ASASSN-15pz, we estimate a host extinction for SN 2009dc of and confirm its high luminosity ( ). The 2009dc-like SN population, which represents ∼1% of SNe Ia, exhibits a range of peak luminosities, and do not fit onto the tight width-luminosity relation. Their optical light curves also show significant diversity of late-time (≳50 days) decline rates. The nebular-phase spectra provide powerful diagnostics to identify the 2009dc-like events as a distinct class of SNe Ia. We suggest referring to these sources using the phenomenology-based “2009dc-like SN Ia-pec” instead of “Super-Chandrasekhar SN Ia,” which is based on an uncertain theoretical interpretation.

Type Ia supernovae have two physical width–luminosity relations and they favour sub-Chandrasekhar and direct collision models - II. Colour evolution

While the width-luminosity relation (WLR) among type Ia supernovae (slower is brighter) is one of the best studied properties of this type of events, its physical basis has not been identified convincingly. The 'luminosity' is known to be related to a clear physical quantity -- the amount of $^{56}$Ni synthesized, but the 'width' has not been quantitatively linked yet to a physical time scale. We show that the recombination time of $^{56}$Fe and $^{56}$Co from doubly to singly ionized states causes the typical observed break in the color curve B-V due to a cliff in the mean opacities, and is a robust width measure of the light curve, which is insensitive to radiation transfer uncertainties. A simple photospheric model is shown to predict the recombination time to an accuracy of $\sim5$ days, allowing a quantitative understanding of the color WLR. Two physical times scales of the width luminosity relation are shown to be set by two column densities -- the total column density which sets the gamma-ray escape time $t_0$ (previous Paper I) and the $^{56}$Ni column density which sets the recombination time (this Paper II). Central detonations of sub-$\rm M_{ch}$ WDs and direct WD collision models have gamma-ray escape times and recombination times which are consistent with observations across the luminosity range of type Ia's. Delayed detonation Chandrasekhar mass models have recombination times that are broadly consistent with observations, with tension at the bright end of the luminosity range and inconsistent gamma-ray escape times at the faint end.

Type Ia supernovae have two physical width–luminosity relations and they favour sub-Chandrasekhar and direct collision models – I. Bolometric

While the width-luminosity relation (WLR) among type Ia supernovae (slower is brighter) has been extensively studied, its physical basis has not been convincingly identified. In particular, the 'width' has not been quantitatively linked yet to a physical time scale. We demonstrate that there are two robust fundamental time scales that 1. can be calculated based on integral quantities of the ejecta, with little dependence on radiation transfer modeling and 2. can be inferred from observations. The first is the gamma-ray escape time $t_0$, which determines the long-term evolution of the bolometric light curve and is studied in this Paper I. The second is the recombination time of $^{56}$Fe and $^{56}$Co, which sets the long-term color evolution of the emitted light and is studied in Paper II. Here we show that the gamma-ray escape time $t_0$ can be derived with $\sim 15\%$ accuracy from bolometric observations based on first principles. When applied to a sample of supernovae, the observed values of $t_0$ span a narrow range of $30-45$ days for the wide range of observed $^{56}\rm Ni$ masses $0.1M_{\odot}\lesssim M_{^{56}\rm Ni}\lesssim 1 M_{\odot}$. %This (trivial) bolometric WLR This narrow range of the gamma-ray escape time across the range of luminosities (a trivial WLR) is consistent with central detonations and direct collisions of sub-Chandrasekhar mass white dwarfs (WDs) but not with delayed detonation models for explosions of Chandrasekhar mass WDs, which are therefore disfavored as the primary channel for the population of type Ia supernovae. Computer codes for extracting $t_0$ from observations and models and for calculating gamma-ray transfer in 1D-3D are provided.

Probing type Ia supernova properties using bolometric light curves from the Carnegie Supernova Project and the CfA Supernova Group

We present bolometric light curves constructed from multi-wavelength photometry of Type Ia supernovae (SNe Ia) from the Carnegie Supernova Project and the CfA Supernova Group, using near-infrared observations to provide robust constraints on host galaxy dust extinction. This set of light curves form a well-measured reference set for comparison with theoretical models. Ejected mass and synthesized $^{56}$Ni mass are inferred for each SN Ia from its bolometric light curve using a semi-analytic Bayesian light curve model, and fitting formulae provided in terms of light curve width parameters from the SALT2 and SNooPy light curve fitters. A weak bolometric width-luminosity relation is confirmed, along with a correlation between ejected mass and the bolometric light curve width. SNe Ia likely to have sub-Chandrasekhar ejected masses belong preferentially to the broad-line and cool-photosphere spectroscopic subtypes, and have higher photospheric velocities and populate older, higher-mass host galaxies than SNe Ia consistent with Chandrasekhar-mass explosions. Two peculiar events, SN 2006bt and SN 2006ot, have normal peak luminosities but appear to have super-Chandrasekhar ejected masses.

Carnegie Supernova Project-II: The Near-infrared Spectroscopy Program*∗This paper includes data gathered with the 6.5-m Magellan telescopes at Las Campanas Observatory, Chile.

Shifting the focus of Type Ia supernova (SN Ia) cosmology to the near infrared (NIR) is a promising way to significantly reduce the systematic errors, as the strategy minimizes our reliance on the empirical width-luminosity relation and uncertain dust laws. Observations in the NIR are also crucial for our understanding of the origins and evolution of these events, further improving their cosmological utility. Any future experiments in the rest-frame NIR will require knowledge of the SN Ia NIR spectroscopic diversity, which is currently based on a small sample of observed spectra. Along with the accompanying paper, Phillips et al., we introduce the Carnegie Supernova Project-II (CSP-II), to follow-up nearby SNe Ia in both the optical and the NIR. In particular, this paper focuses on the CSP-II NIR spectroscopy program, describing the survey strategy, instrumental setups, data reduction, sample characteristics, and future analyses on the data set. In collaboration with the Harvard-Smithsonian Center for Astrophysics (CfA) Supernova Group, we obtained 661 NIR spectra of 157 SNe Ia. Within this sample, 451 NIR spectra of 90 SNe Ia have corresponding CSP-II follow-up light curves. Such a sample will allow detailed studies of the NIR spectroscopic properties of SNe Ia, providing a different perspective on the properties of the unburned material; the radioactive and stable nickel produced; progenitor magnetic fields; and searches for possible signatures of companion stars.

Evidence for Sub-Chandrasekhar Mass Type Ia Supernovae from an Extensive Survey of Radiative Transfer Models

Abstract There are two classes of viable progenitors for normal Type Ia supernovae (SNe Ia): systems in which a white dwarf explodes at the Chandrasekhar mass ( ), and systems in which a white dwarf explodes below the Chandrasekhar mass (sub- ). It is not clear which of these channels is dominant; observations and light-curve modeling have provided evidence for both. Here we use an extensive grid of 4500 time-dependent, multiwavelength radiation transport simulations to show that the sub- model can reproduce the entirety of the width–luminosity relation, while the model can only produce the brighter events , implying that fast-declining SNe Ia come from sub- explosions. We do not assume a particular theoretical paradigm for the progenitor or explosion mechanism, but instead construct parameterized models that vary the mass, kinetic energy, and compositional structure of the ejecta, thereby realizing a broad range of possible outcomes of white dwarf explosions. We provide fitting functions based on our large grid of detailed simulations that map observable properties of SNe Ia, such as peak brightness and light-curve width, to physical parameters such as and total ejected mass. These can be used to estimate the physical properties of observed SNe Ia.

Evidence for sub-Chandrasekhar-mass progenitors of Type Ia supernovae at the faint end of the width–luminosity relation

The faster light-curve evolution of low-luminosity Type Ia supernovae (SNe Ia) suggests that they could result from the explosion of white dwarf (WD) progenitors below the Chandrasekhar mass ($M_{\rm Ch}$). Here we present 1D non-LTE time-dependent radiative transfer simulations of pure central detonations of carbon-oxygen WDs with a mass ($M_\rm{tot}$) between 0.88 M$_{\odot}$ and 1.15 M$_{\odot}$, and a $^{56}\rm{Ni}$ yield between 0.08 M$_{\odot}$ and 0.84 M$_{\odot}$. Their lower ejecta density compared to $M_{\rm Ch}$ models results in a more rapid increase of the luminosity at early times and an enhanced $\gamma$-ray escape fraction past maximum light. Consequently, their bolometric light curves display shorter rise times and larger post-maximum decline rates. Moreover, the higher $M(^{56}\rm{Ni})/M_\rm{tot}$ ratio at a given $^{56}\rm{Ni}$ mass enhances the temperature and ionization level in the spectrum-formation region for the less luminous models, giving rise to bluer colours at maximum light and a faster post-maximum evolution of the $B-V$ colour. For sub-$M_{\rm Ch}$ models fainter than $M_B\approx -18.5$ mag at peak, the greater bolometric decline and faster colour evolution lead to a larger $B$-band post-maximum decline rate, $\Delta M_{15}(B)$. In particular, all of our previously-published $M_{\rm Ch}$ models (standard and pulsational delayed detonations) are confined to $\Delta M_{15}(B) < 1.4$ mag, while the sub-$M_{\rm Ch}$ models with $M_\rm{tot}\lesssim 1$ M$_{\odot}$ extend beyond this limit to $\Delta M_{15}(B)\approx 1.65$ mag for a peak $M_B\approx -17$ mag, in better agreement with the observed width-luminosity relation (WLR). Regardless of the precise ignition mechanism, these simulations suggest that fast-declining SNe Ia at the faint end of the WLR could result from the explosion of WDs whose mass is significantly below the Chandrasekhar limit.

Luminosity distributions of Type Ia supernovae

We have assembled a dataset of 165 low redshift, $z<$0.06, publicly available type Ia supernovae (SNe Ia). We produce maximum light magnitude ($M_{B}$ and $M_{V}$) distributions of SNe Ia to explore the diversity of parameter space that they can fill. Before correction for host galaxy extinction we find that the mean $M_{B}$ and $M_{V}$ of SNe Ia are $-18.58\pm0.07$mag and $-18.72\pm0.05$mag respectively. Host galaxy extinction is corrected using a new method based on the SN spectrum. After correction, the mean values of $M_{B}$ and $M_{V}$ of SNe Ia are $-19.10\pm0.06$ and $-19.10\pm0.05$mag respectively. After correction for host galaxy extinction, `normal' SNeIa ($\Delta m_{15}(B)<1.6$mag) fill a larger parameter space in the Width-Luminosity Relation (WLR) than previously suggested, and there is evidence for luminous SNe Ia with large $\Delta m_{15}(B)$. We find a bimodal distribution in $\Delta m_{15}(B)$, with a pronounced lack of transitional events at $\Delta m_{15}(B)$=1.6 mag. We confirm that faster, low-luminosity SNe tend to come from passive galaxies. Dividing the sample by host galaxy type, SNe Ia from star-forming (S-F) galaxies have a mean $M_{B}=-19.20 \pm 0.05$ mag, while SNe Ia from passive galaxies have a mean $M_{B}=-18.57 \pm 0.24$ mag. Even excluding fast declining SNe, `normal' ($M_{B}<-18$ mag) SNe Ia from S-F and passive galaxies are distinct. In the $V$-band, there is a difference of 0.4$ \pm $0.13 mag between the median ($M_{V}$) values of the `normal' SN Ia population from passive and S-F galaxies. This is consistent with ($\sim 15 \pm $10)% of `normal' SNe Ia from S-F galaxies coming from an old stellar population.

Strong near-infrared carbon in the Type Ia supernova iPTF13ebh

We present near-infrared (NIR) time-series spectroscopy, as well as complementary ultraviolet (UV), optical, and NIR data, of the Type Ia supernova (SN Ia) iPTF13ebh, which was discovered within two days from the estimated time of explosion. The first NIR spectrum was taken merely 2.3 days after explosion and may be the earliest NIR spectrum yet obtained of a SN Ia. The most striking features in the spectrum are several NIR C I lines, and the C I {\lambda}1.0693 {\mu}m line is the strongest ever observed in a SN Ia. Interestingly, no strong optical C II counterparts were found, even though the optical spectroscopic time series began early and is densely-cadenced. Except at the very early epochs, within a few days from the time of explosion, we show that the strong NIR C I compared to the weaker optical C II appears to be general in SNe Ia. iPTF13ebh is a fast decliner with {\Delta}m15(B) = 1.79 $\pm$ 0.01, and its absolute magnitude obeys the linear part of the width-luminosity relation. It is therefore categorized as a "transitional" event, on the fast-declining end of normal SNe Ia as opposed to subluminous/91bg-like objects. iPTF13ebh shows NIR spectroscopic properties that are distinct from both the normal and subluminous/91bg-like classes, bridging the observed characteristics of the two classes. These NIR observations suggest composition and density of the inner core similar to that of 91bg-like events, and a deep reaching carbon burning layer not observed in slower declining SNe Ia. There is also a substantial difference between the explosion times inferred from the early-time light curve and the velocity evolution of the Si II {\lambda}0.6355 {\mu}m line, implying a long dark phase of ~ 4 days.

The white dwarf’s carbon fraction as a secondary parameter of Type Ia supernovae

Binary stellar evolution calculations predict that Chandrasekhar-mass carbon/oxygen white dwarfs (WDs) show a radially varying profile for the composition with a carbon depleted core. Many recent multi-dimensional simulations of Type Ia supernovae (SNe Ia), however, assume the progenitor WD has a homogeneous chemical composition. In this work, we explore the impact of different initial carbon profiles of the progenitor WD on the explosion phase and on synthetic observables in the Chandrasekhar-mass delayed detonation model. Spectra and light curves are compared to observations to judge the validity of the model. The explosion phase is simulated using the finite volume supernova code LEAFS, which is extended to treat different compositions of the progenitor WD. The synthetic observables are computed with the Monte Carlo radiative transfer code ARTIS. Differences in binding energies of carbon and oxygen lead to a lower nuclear energy release for carbon depleted material; thus, the burning fronts that develop are weaker and the total nuclear energy release is smaller. For otherwise identical conditions, carbon depleted models produce less Ni-56. Comparing different models with similar Ni-56 yields shows lower kinetic energies in the ejecta for carbon depleted models, but only small differences in velocity distributions and line velocities in spectra. The light curve width-luminosity relation (WLR) obtained for models with differing carbon depletion is roughly perpendicular to the observed WLR, hence the carbon mass fraction is probably only a secondary parameter in the family of SNe Ia.

The ejected mass distribution of Type Ia supernovae: a significant rate of non-Chandrasekhar-mass progenitors

The ejected mass distribution of type Ia supernovae directly probes progenitor evolutionary history and explosion mechanisms, with implications for their use as cosmological probes. Although the Chandrasekhar mass is a natural mass scale for the explosion of white dwarfs as type Ia supernovae, models allowing type Ia supernovae to explode at other masses have attracted much recent attention. Using an empirical relation between the ejected mass and the light curve width, we derive ejected masses $M_\mathrm{ej}$ and $^{56}$Ni masses $M_\mathrm{Ni}$ for a sample of 337 type Ia supernovae with redshifts $z < 0.7$ used in recent cosmological analyses. We use hierarchical Bayesian inference to reconstruct the joint $M_\mathrm{ej}$-$M_\mathrm{Ni}$ distribution, accounting for measurement errors. The inferred marginal distribution of $M_\mathrm{ej}$ has a long tail towards sub-Chandrasekhar masses, but cuts off sharply above 1.4 $M_\odot$. Our results imply that 25\%-50\% of normal type Ia supernovae are inconsistent with Chandrasekhar-mass explosions, with almost all of these being sub-Chandrasekhar-mass; super-Chandrasekhar-mass explosions make up no more than 1\% of all spectroscopically normal type Ia supernovae. We interpret the type Ia supernova width-luminosity relation as an underlying relation between $M_\mathrm{ej}$ and $M_\mathrm{Ni}$, and show that the inferred relation is not naturally explained by the predictions of any single known explosion mechanism.

TYPE Ia SUPERNOVAE FROM MERGING WHITE DWARFS. I. PROMPT DETONATIONS

Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). While it is not entirely clear if and when an explosion is triggered in such systems, numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such "peri-merger" detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase. Synthetic light curves and spectra are generated for comparison with observations. Three models are considered with primary masses 0.96 Msun, 1.06 Msun, and 1.20 Msun. Of these, the 0.96 Msun dwarf merging with an 0.81 Msun companion, with a Ni56 yield of 0.58 Msun, is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to "super-Chandrasekhar" mass SNe Ia. While the synthetic light curves and spectra of some of the models resemble observed SNe Ia, the significant asymmetry of the ejecta leads to large orientation effects. The peak bolometric luminosity varies by more than a factor of 2 with the viewing angle, and the velocities of the spectral absorption features are lower when observed from angles where the light curve is brightest. The largest orientation effects are seen in the ultraviolet, where the flux varies by more than an order of magnitude. Despite the large variation with viewing angle, the set of three models roughly obeys a width-luminosity relation, with the brighter light curves declining more slowly in the B-band. Spectral features due to unburned carbon from the secondary star are also seen in some cases.

Synthetic light curves and spectra for three-dimensional delayed-detonation models of Type Ia supernovae

In a companion paper, Seitenzahl et al. (2013) presented a set of three-dimensional delayed detonation models for thermonuclear explosions of near-Chandrasekhar mass white dwarfs (WDs). Here, we present multi-dimensional radiative transfer simulations that provide synthetic light curves and spectra for those models. The model sequence explores both changes in the strength of the deflagration phase (controlled by the ignition configuration) and the WD central density. In agreement with previous studies, we find that the strength of the deflagration significantly affects the explosion and the observables. Variations in the central density also have an influence on both brightness and colour, but overall it is a secondary parameter in our set of models. In many respects, the models yield a good match to normal Type Ia supernovae (SNe Ia): peak brightness, rise/decline time scales and synthetic spectra are all in reasonable agreement. There are, however, several differences. In particular, the models are too red around maximum light, manifest spectral line velocities that are a little too high and yield I-band light curves that do not match observations. Although some of these discrepancies may simply relate to approximations made in the modelling, some pose real challenges to the models. If viewed as a complete sequence, our models do not reproduce the observed light-curve width-luminosity relation (WLR) of SNe Ia: all our models show similar B-band decline rates, irrespective of peak brightness. This suggests that simple variations in the strength of the deflagration phase in Chandrasekhar-mass deflagration-to-detonation models do not readily explain the observed diversity of normal SNe Ia. This may imply that some other parameter within the Chandrasekhar-mass paradigm is key to the WLR, or that a substantial fraction of normal SNe Ia arise from an alternative explosion scenario.

THE SPECTROSCOPIC DIVERSITY OF TYPE Ia SUPERNOVAE

We present 2603 spectra of 462 nearby Type Ia supernovae (SN Ia) obtained during 1993-2008 through the Center for Astrophysics Supernova Program. Most of the spectra were obtained with the FAST spectrograph at the FLWO 1.5m telescope and reduced in a consistent manner, making data set well suited for studies of SN Ia spectroscopic diversity. We study the spectroscopic and photometric properties of SN Ia as a function of spectroscopic class using the classification schemes of Branch et al. and Wang et al. The width-luminosity relation appears to be steeper for SN Ia with broader lines. Based on the evolution of the characteristic Si II 6355 line, we propose improved methods for measuring velocity gradients, revealing a larger range than previously suspected, from ~0 to ~400 km/s/day considering the instantaneous velocity decline rate at maximum light. We find a weaker and less significant correlation between Si II velocity and intrinsic B-V color at maximum light than reported by Foley et al., owing to a more comprehensive treatment of uncertainties and host galaxy dust. We study the extent of nuclear burning and report new detections of C II 6580 in 23 early-time spectra. The frequency of C II detections is not higher in SN Ia with bluer colors or narrower light curves, in conflict with the recent results of Thomas et al. Based on nebular spectra of 27 SN Ia, we find no relation between the FWHM of the iron emission feature at ~4700 A and Dm15(B) after removing the two low-luminosity SN 1986G and SN 1991bg, suggesting that the peak luminosity is not strongly dependent on the kinetic energy of the explosion for most SN Ia. Finally, we confirm the correlation of velocity shifts in some nebular lines with the intrinsic B-V color of SN Ia at maximum light, although several outliers suggest a possible non-monotonic behavior for the largest blueshifts.

Confronting 2D delayed-detonation models with light curves and spectra of Type Ia supernovae

We compare models for Type Ia supernova (SN Ia) light curves and spectra with an extensive set of observations. The models come from a recent survey of 44 two-dimensional delayed-detonation models computed by Kasen, Roepke & Woosley (2009), each viewed from multiple directions. The data include optical light curves of 251 SNe Ia and 2231 low-dispersion spectra from the Center for Astrophysics, plus data from the literature. The analysis uses standard techniques employed by observers, including MLCS2k2, SALT2, and SNooPy for light-curve analysis, and the Supernova Identification (SNID) code of Blondin & Tonry for spectroscopic comparisons to assess how well the models match the data. We show that the models that match observed spectra best lie systematically on the observed width-luminosity relation. Conversely, we reject six models with highly asymmetric ignition conditions and a large amount (>1 M_sun) of synthesized 56Ni that yield poor matches to observed SN Ia spectra. More subtle features of the comparison include the general difficulty of the models to match the U-band flux at early times, caused by a hot ionized ejecta that affect the subsequent redistribution of flux at longer wavelengths. We examine ways in which the asymptotic kinetic energy of the explosion affects both the predicted velocity and velocity gradient in the Si II and Ca II lines. Models with an asymmetric distribution of 56Ni are found to result in a larger variation of photometric and spectroscopic properties with viewing angle, regardless of the initial ignition setup. We discuss more generally whether highly anisotropic ignition conditions are ruled out by observations, and how detailed comparisons between models and observations involving both light curves and spectra can lead to a better understanding of SN Ia explosion mechanisms.