Chandrasekhar-mass White Dwarf Research Articles

Including a luminous central remnant in radiative transfer simulations for type iax supernovae

Abstract Type Iax supernovae (SNe Iax) are proposed to arise from deflagrations of Chandrasekhar mass white dwarfs. Previous deflagration simulations have achieved good agreement with the light curves and spectra of intermediate-luminosity and bright SNe Iax. However, the model light curves decline too quickly after peak, particularly in red optical and NIR bands. Deflagration models with a variety of ignition configurations do not fully unbind the white dwarf, leaving a remnant polluted with 56Ni. Emission from such a remnant may contribute to the luminosity of SNe Iax. Here we investigate the impact of adding a central energy source, assuming instantaneous powering by 56Ni decay in the remnant, in radiative transfer calculations of deflagration models. Including the remnant contribution improves agreement with the light curves of SNe Iax, particularly due to the slower post-maximum decline of the models. Spectroscopic agreement is also improved, with intermediate-luminosity and faint models showing greatest improvement. We adopt the full remnant 56Ni mass predicted for bright models, but good agreement with intermediate-luminosity and faint SNe Iax is only possible for remnant 56Ni masses significantly lower than those predicted. This may indicate that some of the 56Ni decay energy in the remnant does not contribute to the radiative luminosity but instead drives mass ejection, or that escape of energy from the remnant is significantly delayed. Future work should investigate the structure of remnants predicted by deflagration models and the potential roles of winds and delayed energy escape, as well as extend radiative transfer simulations to late times.

Type Ia supernovae from chemically segregated white dwarfs

Type Ia supernovae are the outcome of the explosion of a carbon–oxygen white dwarf in a close binary system. They are thought to be the main contributors to the galactic nucleosynthesis of iron-peak elements, with important contributions to the yields of intermediate-mass elements. Recent analyses of the phase diagram of carbon and oxygen containing impurities such as 22Ne and 56Fe in conditions relevant to white dwarf interiors suggest that both isotopes can partially separate when the temperature of the star is low enough to start solidifying. The purpose of the present paper is to examine the impact of this separation on the yields of the different chemical species synthesized during explosions. We used a one-dimensional supernova code to evaluate the impact of the sedimentation assuming different degrees of chemical separation. We find that the main properties of the ejecta, the kinetic energy, and the ejected mass of 56Ni only vary slightly when the separation is taken into account. However, the yields of important isotopes that are used as diagnostic tools, such as manganese, can be strongly modified. Furthermore, the chemical separation studied here is able to change several indicators related to the metallicity of the progenitor (such as the mass ratio of calcium to sulphur in the ejecta or the UV flux of the supernova) and to its mass, whether it is a Chandrasekhar-mass white dwarf or a substantially lighter one (such as the imprint of stable nickel on late-time infrared spectra or that related to the presence of radioactive nickel at the center of the ejecta).

Early-time spectroscopic modelling of the transitional Type Ia Supernova 2021rhu with tardis

ABSTRACT An open question in SN Ia research is where the boundary lies between ‘normal’ Type Ia supernovae (SNe Ia) that are used in cosmological measurements and those that sit off the Phillips relation. We present the spectroscopic modelling of one such ‘86G-like’ transitional SN Ia, SN 2021rhu, that has recently been employed as a local Hubble Constant calibrator using a tip of the red-giant branch measurement. We detail its modelling from −12 d until maximum brightness using the radiative-transfer spectral-synthesis code tardis. Please check and correct this paper accordingly. We base our modelling on literature delayed-detonation and deflagration models of Chandrasekhar mass white dwarfs, as well as the double-detonation models of sub-Chandrasekhar mass white dwarfs. We present a new method for ‘projecting’ abundance profiles to different density profiles for ease of computation. Due to the small velocity extent and low outer densities of the W7 profile, we find it inadequate to reproduce the evolution of SN 2021rhu as it fails to match the high-velocity calcium components. The host extinction of SN 2021rhu is uncertain but we use modelling with and without an extinction correction to set lower and upper limits on the abundances of individual species. Comparing these limits to literature models we conclude that the spectral evolution of SN 2021rhu is also incompatible with double-detonation scenarios, lying more in line with those resulting from the delayed-detonation mechanism (although there are some discrepancies, in particular a larger titanium abundance in SN 2021rhu compared to the literature). This suggests that SN 2021rhu is likely a lower luminosity, and hence lower temperature, version of a normal SN Ia.

On the nature of Tycho Brahe’s supernova

At the 450 years anniversary of its observation, the supernova named after Tycho Brahe, SN 1572, can be explained in the terms used nowadays to characterize Type Ia supernovae (SNe Ia). By assembling the records of the observations made in 1572–74 and evaluating their uncertainties, it is possible to recover the light curve and the color evolution of this supernova. It is found that, within the SNe Ia family, the event should have been a SN Ia with a normal rate of decline. Concerning the color evolution of SNe Ia, the most recently recovered records reaffirm previous findings of its being a normal SN Ia. The abundance studies from X–ray spectroscopy of the whole remnant point to a nuclear burning of the kind of a delayed detonation explosion of a Chandrasekhar–mass white dwarf. A tentative single degenerate path to explosion was suggested from the exploration of the stars in the field of SN 1572. Though, the origin in a double degenerate is being considered as well. Tycho Brahe’s supernova, being the first supernova studied by astronomers, is still the subject of very intensive debates nowadays.

Chandrasekhar-mass white dwarfs are the progenitors of a small fraction of Type Ia supernovae according to nucleosythesis constraints

ABSTRACT The precise progenitor system of type Ia supernovae (SNe Ia), whether it is a white dwarf (WD) close to the Chandrasekhar limit or substantially less massive, has been a matter of debate for decades. Recent research by our group on the accretion and simmering phases preceding the explosion of a massive WD has shown that the central density at thermal runaway lies in the range 3.6 − 6.3 × 109 g cm−3 for reasonable choices of accretion rate on to the WD and progenitor metallicity. In this work, we have computed one-dimensional simulations of the explosion of such WDs, with special emphasis on the chemical composition of the ejecta, which in all cases is extremely rich in neutronized isotopes of chromium (54Cr) and titanium (50Ti). We show that, in order to reconcile such a nucleosynthesis with the isotopic abundances of the Solar system, Chandrasekhar-mass WDs can account for at most 26 per cent of normal-luminosity SNe Ia, or at most 20 per cent of all SNe Ia.

On the Rotation Properties of a Post-explosion Helium-star Companion in Type Iax Supernovae

Recent studies have suggested that type Iax supernovae (SNe Iax) are likely to result from a weak deflagration explosion of a Chandrasekhar-mass white dwarf in a binary system with a helium (He)-star companion. Assuming that most SNe Iax are produced from this scenario, in this work we extend our previous work on the three-dimensional hydrodynamical simulation of ejecta-companion interaction by taking the orbital and spin velocities of the progenitor system into account. We then follow the post-impact evolution of a surviving He-star companion by using the one-dimensional stellar evolution code MESA. We aim to investigate the post-explosion rotation properties of a He-star companion in SNe Iax. We find that the He-star companion spins down after the impact due to the angular-momentum loss and expansion caused by the mass-stripping and shock heating during the interaction. This leads to the situation where the surface rotational speed of the surviving companion can drop to one-third of its pre-explosion value when it expands to a maximum radius a few years after the impact. Subsequently, the star shrinks and spins up again once the deposited energy is released. This spin-switching feature of the surviving He-star companions of SNe Iax may be useful for the identification of such objects in future observations.

Type Iax supernovae from deflagrations in Chandrasekhar mass white dwarfs

Context. Due to the ever increasing number of observations during the past decades, Type Ia supernovae are nowadays regarded as a heterogeneous class of optical transients consisting of several subtypes. One of the largest of these subclasses is the class of Type Iax supernovae. They have been suggested to originate from pure deflagrations in carbon-oxygen Chandrasekhar mass white dwarfs because the outcome of this explosion scenario is in general agreement with their subluminous nature. Aims. Although a few deflagration studies have already been carried out, the full diversity of the class has not been captured yet. This, in particular, holds for the faint end of the subclass. We therefore present a parameter study of single-spot ignited deflagrations in Chandrasekhar mass white dwarfs varying the location of the ignition spark, the central density, the metallicity, and the composition of the white dwarf. We also explore a rigidly rotating progenitor to investigate whether the effect of rotation can spawn additional trends. Methods. We carried out three-dimensional hydrodynamic simulations employing the LEAFS code. Subsequently, detailed nucleosynthesis results were obtained with the nuclear network code YANN. In order to compare our results to observations, we calculated synthetic spectra and light curves with the ARTIS code. Results. The new set of models extends the range in brightness covered by previous studies to the lower end. Our single-spot ignited explosions produce 56Ni masses from 5.8 × 10−3 to 9.2 × 10−2 M⊙. In spite of the wide exploration of the parameter space, the main characteristics of the models are primarily driven by the mass of 56Ni and form a one-dimensional sequence. Secondary parameters seem to have too little impact to explain the observed trend in the faint part of the Type Iax supernova class. We report kick velocities of the gravitationally bound explosion remnants from 6.9 to 369.8 km s−1. The magnitude as well as the direction of the natal kick is found to depend on the strength of the deflagration. Conclusions. This work corroborates the results of previous studies of deflagrations in Chandrasekhar mass white dwarfs. The wide exploration of the parameter space in initial conditions and viewing angle effects in the radiative transfer lead to a significant spread in the synthetic observables. The trends in observational properties toward the faint end of the class are, however, not reproduced. This motivates a quantification of the systematic uncertainties in the modeling procedure and the influence of the 56Ni-rich bound remnant to get to the bottom of these discrepancies. Moreover, while the pure deflagration scenario remains a favorable explanation for bright and intermediate luminosity Type Iax supernovae, our results suggest that other mechanisms also contribute to this class of events.

SN 2020kyg and the rates of faint Iax supernovae from ATLAS

ABSTRACT We present multiwavelength follow-up observations of the ATLAS discovered faint Iax supernova SN 2020kyg that peaked at an absolute magnitude of Mg ≈ −14.9 ± 0.2, making it another member of the faint Iax supernova population. The bolometric light curve requires only ≈7 × 10−3 M⊙ of radioactive 56Ni, with an ejected mass of Mej ∼ 0.4 M⊙ and a low kinetic energy of E ≈ 0.05 ± 0.02 × 1051 erg. We construct a homogeneous volume-limited sample of 902 transients observed by ATLAS within 100 Mpc during a 3.5 yr span. Using this sample, we constrain the rates of faint Iax (Mr ≳ −16) events within 60 Mpc at $12^{+14}_{-8}{{\ \rm per\ cent}}$ of the SN Ia rate. The overall Iax rate, at $15^{+17}_{-9}{{\ \rm per\ cent}}$ of the Ia rate, is dominated by the low-luminosity events, with luminous SNe Iax (Mr ≲ −17.5) like 2002cx and 2005hk, accounting for only $0.9^{+1.1}_{-0.5}{{\ \rm per\ cent}}$ of the Ia rate (a 2σ upper limit of approximately 3 per cent). We favour the hybrid CONe WD + He star progenitor channel involving a failed deflagration of a near Chandrasekhar mass white dwarf, expected to leave a bound remnant and a surviving secondary companion, as a candidate explanation for faint Iax explosions. This scenario requires short delay times, consistent with the observed environments of SNe Iax. Furthermore, binary population synthesis calculations have suggested rates of $1\!-\!18{{\ \rm per\ cent}}$ of the SN Ia rate for this channel, consistent with our rate estimates.

Metallicity-dependent nucleosynthetic yields of Type Ia supernovae originating from double detonations of sub-MCh white dwarfs

Double detonations in sub-Chandrasekhar mass carbon-oxygen white dwarfs (WD) with helium shells ares potential explosion mechanisms for Type Ia supernovae. The mechanism consists of a shell detonation and subsequent core detonation. The focus of our study is the effect of the progenitor metallicity on the nucleosynthetic yields. For this, we computed and analyzed a set of 11 different models with varying core and shell masses at four different metallicities each. This results in a total of 44 models at metallicities between 0.01 Z⊙ and 3 Z⊙. Our models show a strong impact of the metallicity in the high-density regime. The presence of 22Ne causes a neutron-excess that shifts the production from 56Ni to stable isotopes such as 54Fe and 58Ni in the α-rich freeze-out regime. The isotopes of the metallicity implementation further serve as seed nuclei for additional reactions in the shell detonation. The production of 55Mn increases with metallicity, confirming the results of previous work. A comparison of elemental ratios relative to iron shows a good match to solar values for some models. Super-solar values are reached for Mn at 3 Z⊙ and solar values in some models at Z⊙. This indicates that the required contribution of Type Ia supernovae originating from Chandrasekhar-mass WDs can be lower than estimated in previous work to reach solar values of [Mn/Fe] at [Fe/H] = 0. Our galactic chemical evolution models suggest that Type Ia supernovae from sub-Chandrasekhar mass white dwarfs, along with core-collapse supernovae, could account for more than 80% of the solar Mn abundance. Using metallicity-dependent Type Ia supernova yields helps to reproduce the upward trend of [Mn/Fe] as a function of metallicity for the solar neighborhood. These chemical evolution predictions, however, depend on the massive star yields adopted in the calculations.

Measuring an Off-center Detonation through Infrared Line Profiles: The Peculiar Type Ia Supernova SN 2020qxp/ASASSN-20jq

Abstract We present and analyze a near-infrared (NIR) spectrum of the underluminous Type Ia supernova SN 2020qxp/ASASSN-20jq obtained with NIRES at the Keck Observatory, 191 days after B-band maximum. The spectrum is dominated by a number of broad emission features, including the [Fe ii] at 1.644 μm, which is highly asymmetric with a tilted top and a peak redshifted by ≈2000 km s−1. In comparison with 2D non-LTE synthetic spectra computed from 3D simulations of off-center delayed-detonation Chandrasekhar-mass (M ch) white dwarf (WD) models, we find good agreement between the observed lines and the synthetic profiles, and are able to unravel the structure of the progenitor’s envelope. We find that the size and tilt of the [Fe ii] 1.644 μm profile (in velocity space) is an effective way to determine the location of an off-center delayed-detonation transition (DDT) and the viewing angle, and it requires a WD with a high central density of ∼4 × 109 g cm−3. We also tentatively identify a stable Ni feature around 1.9 μm characterized by a “pot-belly” profile that is slightly offset with respect to the kinematic center. In the case of SN 2020qxp/ASASSN-20jq, we estimate that the location of the DDT is ∼0.3M WD off center, which gives rise to an asymmetric distribution of the underlying ejecta. We also demonstrate that low-luminosity and high-density WD SN Ia progenitors exhibit a very strong overlap of Ca and 56Ni in physical space. This results in the formation of a prevalent [Ca ii] 0.73 μm emission feature that is sensitive to asymmetry effects. Our findings are discussed within the context of alternative scenarios, including off-center C/O detonations in He-triggered sub-M Ch WDs and the direct collision of two WDs. Snapshot programs with Gemini/Keck/Very Large Telescope (VLT)/ELT-class instruments and our spectropolarimetry program are complementary to mid-IR spectra by the James Webb Space Telescope (JWST).

Chemical Abundances in Sgr A East: Evidence for a Type Iax Supernova Remnant

Abstract Recent observations have shown a remarkable diversity of observational behaviors and explosion mechanisms in thermonuclear supernovae (SNe). An emerging class of peculiar thermonuclear SNe, called Type Iax, show photometric and spectroscopic behaviors distinct from normal Type Ia. Their origin remains highly controversial, but pure turbulent deflagration of white dwarfs (WDs) has been regarded as the leading formation theory. The large population of Type Iax indicates the existence of unidentified Galactic Type Iax supernova remnants (SNRs). We report evidence that SNR Sgr A East in the Galactic center resulted from a pure turbulent deflagration of a Chandrasekhar-mass carbon–oxygen WD, an explosion mechanism used for Type Iax SNe. Our X-ray spectroscopic study of Sgr A East using 3 Ms of Chandra data shows a low ratio of intermediate-mass elements to Fe and large Mn/Fe and Ni/Fe ratios. This abundance pattern does not accord with the core-collapse or normal Type Ia models. Sgr A East is thus the first Galactic SNR for which a likely Type Iax origin has been proposed and is the nearest target for studying this peculiar class. We compared Sgr A East with the Fe-rich SNRs 3C 397 and W49B, which also have high Mn and Cr abundances and were claimed to result from deflagration-to-detonation explosions of Chandrasekhar-mass WDs (although with disputes). Our study shows that they have distinct abundance patterns. The X-ray spectroscopic studies of thermonuclear SNRs provide observational evidence for the theories that there are diverse explosion channels and various metal outputs for Chandrasekhar-mass WDs.

From Supernova to Supernova Remnant: Comparison of Thermonuclear Explosion Models

Abstract Progress in the three-dimensional modeling of supernovae (SNe) prompts us to revisit the supernova remnant (SNR) phase. We continue our study of the imprint of a thermonuclear explosion on the SNR it produces, which we started with a delayed detonation model of a Chandrasekhar-mass white dwarf. Here we compare two different types of explosion models, each with two variants: two delayed detonation models (N100ddt, N5ddt) and two pure deflagration models (N100def, N5def), where the N number parameterizes the ignition. The output of each SN simulation is used as input to an SNR simulation carried on until 500 yr after the explosion. While all SNR models become more spherical over time and overall display the theoretical structure expected for a young SNR, clear differences are visible among the models, depending on the geometry of the ignition and on the presence or not of detonation fronts. Compared to N100 models, N5 models have a strong dipole component and produce asymmetric remnants. N5def produces a regular-looking, but offset remnant, while N5ddt produces a two-sided remnant. Pure deflagration models exhibit specific traits: a central overdensity, because of the incomplete explosion, and a network of seam lines across the surface, boundaries between burning cells. Signatures from the SN dominate the morphology of the SNR up to 100–300 yr after the explosion, depending on the model, and are still measurable at 500 yr, which may provide a way of testing explosion models.

Type Ia supernovae deflagration-to-detonation transition explosions powered by the Zel’dovich reactivity gradient mechanism

ABSTRACT Our aim in this work is to identify and explain the necessary conditions required for an energetic explosion of a Chandrasekhar-mass white dwarf. We construct and analyse weakly compressible turbulence models with nuclear burning effects for carbon/oxygen plasma at a density expected for the deflagration-to-detonation transition (DDT) to occur. We observe the formation of carbon deflagrations and transient carbon detonations at early times. As turbulence becomes increasingly inhomogeneous, sustained carbon detonations are initiated by the Zel’dovich reactivity gradient mechanism. The fuel is suitably preconditioned by the action of compressive turbulent modes with wavelength comparable to the size of resolved turbulent eddies; no acoustic wave is involved in this process. Oxygen detonations are initiated, aided either by reactivity gradients or by collisions of carbon detonations. The observed evolutionary time-scales are found to be sufficiently short for the above process to occur in the expanding, centrally ignited massive white dwarf. The inhomogeneous conditions produced prior to the DDT might be of consequence for the chemical composition of the outer ejecta regions of Type Ia supernovae from the single degenerate channel, and offer the potential for validation of the proposed model.

Nucleosynthesis imprints from different Type Ia supernova explosion scenarios and implications for galactic chemical evolution

We analyze the nucleosynthesis yields of various Type Ia supernova explosion simulations including pure detonations in sub-Chandrasekhar mass white dwarfs; double detonations and pure helium detonations of sub-Chandrasekhar mass white dwarfs with an accreted helium envelope; a violent merger model of two white dwarfs; and deflagrations and delayed detonations in Chandrasekhar mass white dwarfs. We focus on the iron peak elements Mn, Zn, and Cu. To this end, we also briefly review the different burning regimes and production sites of these elements, as well as the results of abundance measurements and several galactic chemical evolution studies. We find that super-solar values of [Mn/Fe] are not restricted to Chandrasekhar mass explosion models. Scenarios including a helium detonation can significantly contribute to the production of Mn, in particular the models proposed for calcium-rich transients. Although Type Ia supernovae are often not accounted for as production sites of Zn and Cu, our models involving helium shell detonations can produce these elements in super-solar ratios relative to Fe. Our results suggest a re-consideration of Type Ia supernova yields in galactic chemical evolution models. A detailed comparison with observations can provide new insight into the progenitor and explosion channels of these events.

The Interaction of Type Iax Supernova Ejecta with a Helium Companion Star

Abstract Type Iax supernovae (SNe Iax) are an important subluminous class of SNe Ia. However, their progenitors and explosion mechanisms are not yet understood. It has been suggested that SNe Iax may be produced from weak deflagration explosions of Chandrasekhar-mass white dwarfs (WDs) in binary systems with a helium (He) star donor. In such progenitor systems, the stripped He companion material caused by the ejecta-companion interaction is expected to present some He features in their late-time spectra. However, the detection of He lines in late-time spectra of SNe Iax has not yet been successful, which gives an upper limit on the amount of stripped He mass of . In this work, we study the interaction between SN Iax ejecta and a He star companion by performing three-dimensional hydrodynamical simulations with a weak pure deflagration explosion model. We find that about of He material can be stripped off from the companion star by SN explosion, which is very close to (or lower than) the observational upper limit on the total stripped He mass in SNe Iax. We, therefore, conclude that nondetection of He lines in late-time spectra of SNe Iax could be reasonably explained if they indeed resulted from weak pure deflagration explosions of Chandrasekhar-mass WDs in progenitor systems with a He star donor.

White dwarf deflagrations for Type Iax supernovae: polarisation signatures from the explosion and companion interaction

Growing evidence suggests that Type Iax supernovae might be the result of thermonuclear deflagrations of Chandrasekhar-mass white dwarfs in binary systems. We carry out Monte Carlo radiative transfer simulations and predict spectropolarimetric features originating from the supernova explosion and subsequent ejecta interaction with the companion star. Specifically, we calculate viewing-angle dependent flux and polarisation spectra for a 3D model simulating the deflagration of a Chandrasekhar-mass white dwarf and, for a second model, simulating the ejecta interaction with a main-sequence star. We find that the intrinsic signal is weakly polarised and only mildly viewing-angle dependent, owing to the overall spherical symmetry of the explosion and the depolarising contribution of iron-group elements dominating the ejecta composition. The interaction with the companion star carves out a cavity in the ejecta and produces a detectable, but modest signal that is significant only at relatively blue wavelengths (≲5000 Å). In particular, increasingly fainter and redder spectra are predicted for observer orientations further from the cavity, while a modest polarisation signalP~ 0.2 per cent is found at blue wavelengths for orientations 30° and 45° away from the cavity. We find a reasonable agreement between the interaction model viewed from these orientations and spectropolarimetric data of SN 2005hk and interpret the maximum-light polarisation signal seen at blue wavelengths for this event as a possible signature of the ejecta–companion interaction. We encourage further polarimetric observations of SNe Iax to test whether our results can be extended and generalised to the whole SN Iax class.

Type Ia supernova sub-classes and progenitor origin

AbstractThis paper presents a short review on the current state of SN Ia progenitor origin. Type Ia supernova explosions (meaning thermonuclear disruption of a white dwarf) are observed to be widely diverse in peak luminosity, lightcurve width and shape, spectral features, and host stellar population environment. In the last decade alone, theoretical simulations and observational data have come together to seriously challenge the long-standing paradigm that all SNe Ia arise from explosions of Chandrasekhar mass white dwarfs. In this review I highlight some of the major developments (and changing views) of our understanding of the nature of SN Ia progenitor systems. I give a brief overview of binary star configurations and their plausible explosion mechanisms, and infer links between some of the various (observationally-categorized) SN Ia sub-classes and their progenitor origins from a theoretical standpoint.

The Intrinsic Stochasticity of the 56Ni Distribution of Single-degenerate Near-Chandrasekhar-mass SN Ia

Binary Chandrasekhar-mass white dwarfs accreting mass from non-degenerate stellar companions through the single-degenerate channel have reigned for decades as the leading explanation of Type Ia supernovae. Yet, a comprehensive theoretical explanation has not yet emerged to explain the expected properties of the canonical near-Chandrasekhar-mass white dwarf model. A simmering phase within the convective core of the white dwarf leads to the ignition of one or more flame bubbles scattered across the core. Consequently, near-Chandrasekhar-mass single-degenerate SNe Ia are inherently stochastic, and are expected to lead to a range of outcomes, from subluminous SN 2002cx-like events, to overluminous SN 1991T-like events. However, all prior simulations of the single-degenerate channel carried through the detonation phase have set the ignition points as free parameters. In this work, for the first time, we place ignition points as predicted by {\it ab initio} models of the convective phase leading up to ignition, and follow through the detonation phase in fully three-dimensional simulations. Single-degenerates in this framework are characteristically overluminous. Using a statistical approach, we determine the $^{56}$Ni mass distribution arising from stochastic ignition. While there is a total spread of $\gtrsim 0.2 M_{\odot}$ for detonating models, the distribution is strongly left-skewed, and with a narrow standard deviation of $\simeq 0.03 M_{\odot}$. Conversely, if single-degenerates are not overluminous but primarily yield normal or failed events, then the models require fine-tuning of the ignition parameters, or otherwise require revised physics or progenitor models. We discuss implications of our findings for the modeling of single-degenerate SNe Ia.

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.

Balmer-dominated Shocks Exclude Hot Progenitors for Many Type Ia Supernovae

Abstract The evolutionary mechanism underlying Type Ia supernova explosions remains unknown. However, recent efforts to constrain progenitor models based on the influence that their high energy emission would have on the interstellar medium within galaxies have proven successful. For individual remnants, Balmer-dominated shocks reveal the ionization state of hydrogen in the immediately surrounding gas. Here, we report deep upper limits on the temperature and luminosity of the progenitors of four Type Ia remnants with associated Balmer filaments: SN 1006, 0509-67.5, 0519-69.0, and DEM L71. For SN 1006, existing observations of helium-line emission in the diffuse emission ahead of the shock provide an additional constraint on the helium ionization state in the vicinity of the remnant. Using the photoionization code Cloudy, we show that these constraints exclude any hot, luminous progenitor for SN 1006, including stably hydrogen or helium nuclear-burning white dwarfs (WD), as well as any Chandrasekhar-mass WD accreting matter at ≳9.5 × 10−8 M ⊙ yr−1 via a disk. For 0509-67.5, the Balmer emission alone rules out any such WD accreting ≳1.4 × 10−8 M ⊙ yr−1. For 0519-69.0 and DEM L71, the inferred ambient ionization state of hydrogen is only weakly in tension with a recently hot, luminous progenitor, and cannot be distinguished from, e.g., a relatively higher local Lyman continuum background, without additional line measurements. Future deep spectroscopic observations will resolve this ambiguity, and can either detect the influence of any luminous progenitor or rule out the same for all resolved SN Ia remnants.