Progenitors Of Type Ia Supernovae Research Articles

The Appearance of Type Ia Supernova Progenitors: If Not SSSs, then What Do They Look Like?

Abstract“What do the progenitors of Type Ia supernovae (SNe Ia) look like? How can we hope to find them?” We focus on the epoch during which mass is incident on a white dwarf (WD) at high rates (> 10−7M⊙ yr−1). Such epochs are expected in single-degenerate (SD) progenitors, double-degenerate (DD) progenitors, and in a wide range of binaries with WDs that will not achieve the Chandrasekhar mass, MCh. High-rate accretion onto a WD produces high luminosities through accretion alone; in addition, most calculations show that quasisteady or episodic nuclear burning can occur, increasing the luminosity by more than an order of magnitude. If the photosphere is not much larger than the WD, the emission will have values of kT in the range of tens of eV, and the source will appear as a luminous supersoft x-ray source (SSS). Studies of local SSSs that are good candidates for nuclear-burning WDs (NBWDs) suggest that many have low duty cycles of SSS activity. This is consistent with the fact that binary WD models predict about 100 times as many SSSs in external galaxies of all types as are actually detected. Interstellar absorption does not appear to be the problem. Instead, it is likely that the ~1037−1038 erg s−1 emitted by NBWDs emerges in other wavebands. The challenge we face is to search for highly luminous systems within the Milky Way and nearby galaxies that have unusual properties consistent with NBWDs, and inconsistent with other physical models. Model tests can then be conducted for individual candidates, allowing us to identify large numbers of progenitors years before explosion.

Binary Paths to Type Ia Supernovae Explosions: the Highlights

AbstractThis symposium was focused on the hunt for the progenitors of Type Ia supernovae (SNe Ia). Is there a main channel for the production of SNe Ia? If so, are these elusive progenitors single degenerate or double degenerate systems? Although most participants seemed to favor the single degenerate channel, there was no general agreement on the type of binary system at play. An observational puzzle that was highlighted was the apparent paucity of supersoft sources in our Galaxy and also in external galaxies. The single degenerate channel (and as it was pointed out, quite possibly also the double degenerate channel) requires the binary system to pass through a phase of steady nuclear burning. However, the observed number of supersoft sources falls short by a factor of up to 100 in explaining the estimated birth rates of SNe Ia. Thus, are these supersoft sources somehow hidden away and radiating at different wavelengths, or are we missing some important pieces of this puzzle that may lead to the elimination of a certain class of progenitor? Another unanswered question concerns the dependence of SNe Ia luminosities on the age of their host galaxy. Several hypotheses were put forward, but none was singled out as the most likely explanation.It is fair to say that at the end of the symposium the definitive answer to the vexed progenitor question remained well and truly wide open.

The Mass of the White Dwarf in the Recurrent Nova CI Aquilae

AbstractIf recurrent novae (RNe) are progenitors of Type Ia Supernovae (SNe Ia), their white dwarfs (WD) must have masses close to the Chandrasekhar limit. The most reliable means of determining WD masses in RNe is dynamically, via radial-velocity and rotational-broadening measurements of the companion star. Such measurements require the system to be both eclipsing and to show absorption features from the secondary star. The only other reliable RNe mass estimate is for U Sco, which has a WD mass of 1.55 ± 0.24 M⊙ (Thoroughgood et al. 2001).We present new time-resolved, intermediate-resolution spectroscopy of the eclipsing RN CI Aql during quiescence. We find the mass of the WD to be 1.02 ± 0.08 M⊙ and the mass of the secondary star to be 2.41 ± 0.2 M⊙. We estimate the radius of the secondary to be 2.10 ± 0.07 R⊙. The high mass ratio of q=2.37 ± 0.15 and the high secondary star mass suggests that mass transfer occurs on a thermal timescale. We suggest that CI Aql is evolving into a supersoft x-ray source, and ultimately will explode as an SN Ia.

A Single Degenerate Model for Ultra Super-Chandrasekhar Mass Progenitors of Type Ia Supernovae – Young and Low Metallicity Environments –

AbstractSome Type Ia supernovae (SNe Ia) are suggested to have progenitor white dwarfs (WDs) with mass of up to 2.4–2.8 M⊙, highly exceeding the Chandrasekhar mass limit. We present a new single degenerate (SD) model for SNe Ia progenitors, in which the WD mass can increase by accretion up to 2.3 (2.7) M⊙ from the initial value of 1.1 (1.2) M⊙. The results are consistent with high luminosity SNe Ia such as SN 2003fg, SN 2006gz, SN 2007if, and SN 2009dc. There are three characteristic mass ranges of exploding WDs. In an extreme massive case, differentially rotating WDs explode as a SNe Ia soon after the WD mass exceeds 2.4 M⊙ because of a secular instability at T/|W|~0.14. For a mid mass range of MWD=1.5–2.4 M⊙, which is supported by differential rotation, it takes some spinning-down time until carbon is ignited to induce an SN Ia explosion. For a lower mass range of MWD=1.38–1.5 M⊙, they can be supported by rigid rotation until the angular momentum is lost. We also suggest the ultra super-Chandrasekhar mass SNe Ia are born in young and low metallicity environments.

Two Distributions Shedding Light on Type Ia Supernovae Progenitors: Delay Times and G-Dwarf Metallicities

AbstractUsing a population number synthesis code with detailed binary evolution, we calculate the distribution of the number of Type Ia supernovae as a function of time after starburst. This is done for both main progenitor scenarios (single degenerate and double degenerate), but also with various evolutionary assumptions (such as mass transfer efficiency, angular momentum loss, and common envelope description). The comparison of these theoretically predicted delay time distributions with observations in elliptical galaxies then allows us to constrain the evolutionary scenarios and parameters. From the morphological shape of the distributions, we conclude that all supernovae Ia cannot be produced through the single degenerate scenario alone, with the best match being obtained when both scenarios contribute. Within the double degenerate scenario, most systems go through a phase of quasi-conservative, stable Roche lobe overflow. We propose stellar rotation as a possible solution for the underestimation of the absolute numbers of events predicted by many theoretical population synthesis studies. A brief comparison with these other studies is made, showing good correspondence under the nontrivial condition of equivalent assumptions. We also investigate the influence of different supernova Ia progenitors and evolutionary parameters on the theoretical distribution of the iron abundance of G-type dwarfs in the Galactic disk. These stars are good indicators of the entire chemical history of the Galaxy, and their predicted metallicity distribution can also be compared to the observed ones. This again limits the number of acceptable combinations of assumptions. Supporting previous results, the best correspondence is found in the case where both the single and double degenerate scenario contribute.

A Search for Type Ia Supernova Progenitors: the Central Stars of the Planetary Nebulae NGC 2392 and NGC 6026

AbstractWe use photoionization modeling to assess the binary nature of the central stars of NGC 2392 and NGC 6026. If they are close binaries, they are potential Type Ia supernovae (SNe Ia) progenitors if the total mass exceeds the Chandrasekhar limit. We show that the nucleus of NGC 2392 likely has a hot, massive (≃1 M⊙) white dwarf companion, and a total mass of ~1.6 M⊙, making it an especially interesting system. The binary mass in NGC 6026 is less, ~1.1 M⊙. Even though its orbital period is short, it is not considered to be an SNe~Ia progenitor.

Why are Supersoft X-ray Fluxes So Weak in Early-type Galaxies? – A Clue to Type Ia SNe Progenitors –

AbstractSupersoft X-ray fluxes in early-type galaxies provide an excellent test for Type Ia supernovae (SNe Ia) progenitors: the double degenerate (DD) scenario is believed to produce no supersoft sources (SSSs) except just before the SN Ia explosion, while the single degenerate (SD) scenario produces SSSs in some phases of the symbiotic channel. Recent observations of the supersoft X-ray flux of early-type galaxies show a remarkable agreement with theoretical predictions of the SD scenario, which thus turns out to be a strong support for the SD scenario, despite the original observations aimed at the opposite conclusion. Here I explain why X-ray fluxes are so weak in early-type galaxies. (1) Candidate binaries in the SD scenario become SSSs only during a short time on their way to SNe Ia explosions, because they spend a large part of their lifetime in a wind phase. (2) During the SSS phase, symbiotic stars emit very weak supersoft X-ray fluxes even if the WD is very massive. It should be emphasized that supersoft X-ray symbiotic stars are very rare and we need more observations to understand their nature.

The Changing Nature of QU Carinae: SN Ia Progenitor or a Hoax?

AbstractThe race to the elusive Type Ia supernovae (SNe Ia) progenitors is at its zenith, with numerous clues from SNe Ia ejecta and a dearth of observational candidates. Still, the single degenerate channel is a viable route of mass accumulation onto a white dwarf to the Chandrasekhar limit. I present long-term high resolution spectroscopy of QU Carinae, one of the most promising single degenerate SNe Ia progenitors. I discuss its highly variable nature and compare it to current scenarios for mass accumulation onto high-mass white dwarfs, eventually leading to WD detonation and to a supernova explosion.

Theoretical Delay Time Distributions

AbstractWe briefly discuss the method of population synthesis to calculate theoretical delay time distributions of Type Ia supernova progenitors. We also compare the results of different research groups and conclude that, although one of the main differences in the results for single degenerate progenitors is the retention efficiency with which accreted hydrogen is added to the white dwarf core, this alone cannot explain all the differences.

Searching for Type Ia Supernovae in Globular Clusters

AbstractGlobular clusters are known to have enhanced populations of binaries with compact objects, owing to the dynamical interactions taking place in their dense stellar environments. Both the single degenerate and double degenerate Type Ia supernova progenitors are expected to be similarly enhanced with respect to the field, but the theoretical predictions are highly uncertain. Here we discuss an archival survey aimed at constraining the fraction of Type Ia supernovae that explode inside globular clusters.

Spin-Up/Spin-Down Models

AbstractAngular momentum transport plays an important role in mass transfer systems, and can significantly spin up an accreting star. When the accretor is a white dwarf (WD) on its way to becoming a Type Ia supernova (SN Ia), the spin up of the WD can have significant consequences for the appearance of the progenitor, the characteristics of the explosion and its aftermath, the geometry of the supernova remnant, and for single-degenerate models, the appearance of the donor star post-explosion. These consequences can be “game changers”, altering results that have long been taken for granted. We discuss key features of our spin-up/spin-down models and their implications. We relate our models to work still needed to address the difficult physical issues related to angular momentum transport and its effects on the properties and appearance of Type Ia supernova progenitors.

Connecting RS Ophiuchi to [some] type Ia supernovae

Aims: Recurrent nova systems like RS Oph have been proposed as a possible channel to Type Ia Supernova explosions, based on the high mass of the accreting white dwarf. Additional support to this hypothesis has been recently provided by the detection of circumstellar material around SN2006X and SN2007le, showing a structure compatible with that expected for recurrent nova outbursts.In this paper we investigate the circumstellar environment of RS Oph and its structure, with the aim of establishing a firmer and independent link between this class of objects and Type Ia SN progenitors. Methods: We study the time evolution of CaII, NaI and KI absorption features in RS Oph, before, during, and after the last outburst, using multi-epoch, high-resolution spectroscopy, and applying the same method adopted for SN2006X and SN2007le. Results: A number of components, blue-shifted with respect to the systemic velocity of RS Oph, are detected. In particular, one feature strongly weakens in the first two weeks after the outburst, simultaneously with the disappearance of very narrow P-Cyg profiles overimposed on the much wider nova emission lines of H, He, FeII and other elements. Conclusions: We interpret these facts as the signature of density enhancements in the circumstellar material, suggesting that the recurrent eruptions might indeed create complex structures within the material lost by the donor star. This establishes a strong link between RS Oph and the progenitor system of the Type Ia SN2006X, for which similar features have been detected.

Double white dwarfs and LISA

Close pairs of white dwarfs are potential progenitors of type Ia supernovae and they are common, with the order of 100–300 million in the Galaxy. As such they will be significant, probably dominant, sources of the gravitational waves detectable by LISA. In the context of LISA's goals for fundamental physics, double white dwarfs are a source of noise, but from an astrophysical perspective, they are of considerable interest in their own right. In this paper I discuss our current knowledge of double white dwarfs and their close relatives (and possible descendants) the AM CVn stars. LISA will add to our knowledge of these systems by providing the following unique constraints: (i) an almost direct measurement of the galactic merger rate of DWDs from the detection of short period systems and their period evolution, (ii) an accurate and precise normalization of binary evolution models at shortest periods, (iii) a determination of the evolutionary pathways to the formation of AM CVn stars, (iv) measurements of the influence of tidal coupling in white dwarfs and its significance for stabilizing mass transfer, and (v) discovery of numerous examples of eclipsing white dwarfs with the potential for optical follow-up to test models of white dwarfs.

Violent mergers of nearly equal-mass white dwarf as progenitors of subluminous Type Ia supernovae

Context. The origin of subluminous Type Ia supernovae (SNe Ia) has long eluded any explanation, because all Chandrasekhar-mass models have severe problems reproducing them. Recently, it has been proposed that violent mergers of two white dwarfs of 0.9 M⊙ could lead to subluminous SNe Ia events that resemble 1991bg-like SNe Ia.

ASYMMETRIES IN THE EXPANSION AND EMISSION FROM YOUNG SUPERNOVA REMNANTS

We present two-dimensional and three-dimensional numerical simulations of asymmetric young supernova remnants (SNRs) carried out with the hydrodynamical code YGUAZU, aiming to quantitatively assess the role of different factors that may give origin to such asymmetries in their expansion. In particular, we are interested in modeling the morphology of Tycho's SNR to address whether the companion star of a Type Ia supernova progenitor has played a role in the subsequent evolution of the remnant. With the results from the numerical simulations, we can not only study the morphology of the SNR but also compute the emission of the remnant in different spectral bands. In particular, we simulate X-ray maps, which can be directly compared to recent and previous observations of Tycho's SNR. Our results suggest that the most likely explanation for Tycho's morphology is that after the supernova (SN) explosion the shock front stripped the envelope of its companion. We represent this effect by adding a conical region with an enhanced density into the initial sphere immediately after the explosion. Assuming that Tycho's companion was a massive red giant star, we explore different values of the angle of aperture and mass excess of the conical region. A good agreement with observational data was found for the model with a mass excess of 0.3 M ☉ and an aperture of 90°. After the collision with the SN shock wave, the companion would become an He-rich star. This scenario would gain observational support if a star with these characteristics is found in the vicinity of the center of the SN explosion.

Soft-band X/K luminosity ratios in late-type galaxies and constraints on the population of supersoft X-ray sources

We study X-ray to K-band luminosity ratios (L_X/L_K) of late-type galaxies in the 0.3-0.7 keV energy range. From the Chandra archive, we selected nine spiral and three irregular galaxies with point source detection sensitivity better than 5 x 10^36 erg/s in order to minimize the contribution of unresolved X-ray binaries. In late-type galaxies cold gas and dust may cause significant interstellar absorption, therefore we also demanded the existence of publicly available HI maps. The obtained L_X/L_K ratios vary between (5.4-68) x 10^27 erg/s/L_K,sun exceeding by factor of 2-20 the values obtained for gas-poor early-type galaxies. Based on these results we constrain the role of supersoft X-ray sources as progenitors of type Ia supernovae (SNe Ia). For majority of galaxies the upper limits range from ~3% to ~15% of the SN Ia frequency inferred from K-band luminosity, but for a few of them no meaningful constraints can be placed. On a more detailed level, we study individual structural components of spiral galaxies: bulge and disk, and, for grand design spiral galaxies, arm and interarm regions.

SPECTRA OF TYPE IA SUPERNOVAE FROM DOUBLE DEGENERATE MERGERS

The merger of two white dwarfs (a.k.a. double degenerate merger) has often been cited as a potential progenitor of type Ia supernovae. Here we combine population synthesis, merger and explosion models with radiation-hydrodynamics light-curve models to study the implications of such a progenitor scenario on the observed type Ia supernova population. Our standard model, assuming double degenerate mergers do produce thermonuclear explosions, produces supernova light-curves that are broader than the observed type Ia sample. In addition, we discuss how the shock breakout and spectral features of these double degenerate progenitors will differ from the canonical bare Chandrasekhar-massed explosion models. We conclude with a discussion of how one might reconcile these differences with current observations.

SUPERSOFT X-RAY PHASE OF SINGLE DEGENERATE TYPE Ia SUPERNOVA PROGENITORS IN EARLY-TYPE GALAXIES

In the single degenerate (SD) scenario for Type Ia supernova (SN Ia) progenitors, an accreting white dwarf (WD) is expected to undergo a supersoft X-ray source (SSS) phase. Recently, Gilfanov & Bogdan (2010, hereafter GB10) claimed that observed X-ray fluxes of early type galaxies would be too low to be consistent with the prediction of the SD scenario based on rather simple assumptions. We present realistic evolutionary models of SD systems and calculate durations of SSS phases. In most cases, accreting WDs spend a large fraction of time in the optically thick wind phase and the recurrent nova phase rather than the SSS phase. Thus the SSS phase lasts only for a few hundred thousand years. This is by a factor of ~10 shorter than those adopted by GB10 where the SN~Ia progenitor WD was assumed to spend most of its life as a SSS. The theoretical X-ray luminosity of the SSS has a large uncertainty because of the uncertain atmospheric model of mass-accreting WDs and absorption of soft X-rays by the companion star's cool wind material. We thus adopt an average of the observed fluxes of existing symbiotic SSSs, i.e., ~0.4 x 10^{36} erg s^{-1} for 0.3--0.7 keV. Using these SSS duration and soft X-ray luminosity, we show that the observed X-ray flux obtained by GB10 is rather consistent with our estimated flux in early type galaxies based on the SD scenario. This is a strong support for the SD scenario as a main-contributor of SNe Ia in early type galaxies.

SIMPLIFIED HYDROSTATIC CARBON BURNING IN WHITE DWARF INTERIORS

We introduce two simplified nuclear networks that can be used in hydrostatic carbon burning reactions occurring in white dwarf interiors. They model the relevant nuclear reactions in carbon-oxygen white dwarfs (COWDs) approaching ignition in Type Ia supernova (SN Ia) progenitors, including the effects of the main e-captures and \beta-decays that drive the convective Urca process. They are based on studies of a detailed nuclear network compiled by the authors and are defined by approximate sets of differential equations whose derivations are included in the text. The first network, N1, provides a good first order estimation of the distribution of ashes and it also provides a simple picture of the main reactions occurring during this phase of evolution. The second network, N2, is a more refined version of N1 and can reproduce the evolution of the main physical properties of the full network to the 5% level. We compare the evolution of the mole fraction of the relevant nuclei, the neutron excess, the photon energy generation and the neutrino losses between both simplified networks and the detailed reaction network in a fixed temperature and density parcel of gas.

The orbital period of V458 Vulpeculae, a post-double common-envelope nova

Abstract We present time-resolved optical spectroscopy of V458 Vulpeculae (Nova Vul 2007 No. 1) spread over a period of 15 months starting 301 d after its discovery. Our data reveal radial-velocity variations in the He iiλ5412 and He iiλ4686 emission lines. A period analysis of the radial-velocity curves resulted in a period of 98.096 47 ± 0.000 25 min (0.068 122 55 ± 0.000 000 17 d) which we identify with the orbital period of the binary system. V458 Vul is therefore the planetary nebula central binary star with the shortest period known. We explore the possibility of the system being composed of a relatively massive white dwarf (M1 ≳ 1.0 M⊙) accreting matter from a post-asymptotic giant branch star which produced the planetary nebula observed. In this scenario, the central binary system therefore underwent two common-envelope episodes. A combination of previous photoionization modelling of the nebular spectra, post-asymptotic giant branch evolutionary tracks and the orbital period favour a mass of M2 ∼ 0.6 M⊙ for the donor star. Therefore, the total mass of the system may exceed the Chandrasekhar mass, which makes V458 Vul a Type Ia supernova progenitor candidate.