Progenitors of type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in astrophysics and are crucial for the studies of stellar evolution, galaxy evolution and cosmology. They are generally thought to be thermonuclear explosions of accreting carbon–oxygen white dwarfs (CO WDs) in close binaries, however, the nature of the mass donor star is still unclear. In this article, we review various progenitor models proposed in the past years and summarize many observational results that can be used to put constraints on the nature of their progenitors. We also discuss the origin of SN Ia diversity and the impacts of SN Ia progenitors on some fields. The currently favourable progenitor model is the single-degenerate (SD) model, in which the WD accretes material from a non-degenerate companion star. This model may explain the similarities of most SNe Ia. It has long been argued that the double-degenerate (DD) model, which involves the merger of two CO WDs, may lead to an accretion-induced collapse rather than a thermonuclear explosion. However, recent observations of a few SNe Ia seem to support the DD model, and this model can produce normal SN Ia explosion under certain conditions. Additionally, the sub-luminous SNe Ia may be explained by the sub-Chandrasekhar mass model. At present, it seems likely that more than one progenitor model, including some variants of the SD and DD models, may be required to explain the observed diversity of SNe Ia.     

Mass-accreting white dwarfs and type Ia supernovae

Type Ia supernovae(SNe Ia) play a prominent role in understanding the evolution of the Universe. They are thought to be thermonuclear explosions of mass-accreting carbon-oxygen white dwarfs(CO WDs) in binaries, although the mass donors of the accreting WDs are still not well determined. In this article, I review recent studies on mass-accreting WDs, including H-and He-accreting WDs. I also review currently most studied progenitor models of SNe Ia, i.e., the single-degenerate model(including the WD+MS channel, the WD+RG channel and the WD+He star channel), the doubledegenerate model(including the violent merger scenario) and the sub-Chandrasekhar mass model.Recent progress on these progenitor models is discussed, including the initial parameter space for producing SNe Ia, the binary evolutionary paths to SNe Ia, the progenitor candidates for SNe Ia, the possible surviving companion stars of SNe Ia, some observational constraints, etc. Some other potential progenitor models of SNe Ia are also summarized, including the hybrid CONe WD model, the core-degenerate model, the double WD collision model, the spin-up/spin-down model and the model of WDs near black holes. To date, it seems that two or more progenitor models are needed to explain the observed diversity among SNe Ia.     

Type-Ia supernovae in semidetached binaries

We consider the evolutionary scenarios for close binaries that lead to the formation of semidetached systems in which a white dwarf can accumulate the Chandrasekhar mass through mass accretion from its companion, a main sequence star or a subgiant of mass M ～ 2M_⊙. Such dwarfs probably explode as type-Ia supernovae or collapse to form a neutron star. The population synthesis method is used to analyze the dependence of the model rate of these events in the Galaxy on the common envelope parameter, the mass transfer rate, and the response of a main-sequence star to helium accretion at an intermediate evolutionary stage. The rate of explosions in semidetached systems of this type in the Galaxy was found to be no higher than ?0.2×10^?3 yr^?1, which is less than 10% of the lower level for the empirically estimated SNe Ia rate.     

WD+RG systems as the progenitors of type Ia supernovae

Type Ia supernovae(SNe Ia)play an important role in the study of cosmic evolution,especially in cosmology.There are several progenitor models for SNe Ia proposed in the past years.By considering the effect of accretion disk instability on the evolution of white dwarf(WD)binaries,we performed detailed binary evolution calculations for the WD+red-giant(RG)channel of SNe Ia,in which a carbon-oxygen WD accretes material from a RG star to increase its mass to the Chandrasekhar mass limit.According to these calcu...     

WD + He star systems as the progenitors of Type Ia supernovae and their surviving companion stars

Employing Eggleton’s stellar evolution code with the optically thick wind assumption, we have systematically studied the WD + He star channel of Type Ia supernovae (SNe Ia), in which a carbon–oxygen WD accretes material from a He main-sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. We mapped out the parameter spaces for producing SNe Ia. According to a detailed binary population synthesis approach, we find that the Galactic SN Ia birthrate from this channel is ～0.3×10^?3 yr^?1, and that this channel can produce SNe Ia with short delay times (～45–140 Myr). We also find that the surviving companion stars in this channel have a high spatial velocity (>400 km/s) after the SN explosion, which could be an alternative origin for hypervelocity stars (HVSs), especially for HVSs such as US 708.     

The C/O ratio of He-accreting carbon-oxygen white dwarfs and type Ia supernovae

Type Ia supernovae(SNe Ia) are thermonuclear explosions of carbon-oxygen white dwarfs(CO WDs), and are believed to be excellent cosmological distance indicators due to their high luminosity and remarkable uniformity. However, there exists a diversity among SNe Ia, and a poor understanding of the diversity hampers the improvement of the accuracy of cosmological distance measurements. The variations of the ratios of carbon to oxygen(C/O) of WDs at explosion are suggested to contribute to the diversity. In the canonical model of SNe Ia, a CO WD accretes matter from its companion and increases its mass till the Chandrasekhar mass limit when the WD explodes. In this work, we studied the C/O ratio for accreting CO WDs. Employing the stellar evolution code MESA, we simulated the accretion of He-rich material onto CO WDs with different initial WD masses and different mass accretion rates. We found that the C/O ratio varies for different cases. The C/O ratio of He-accreting CO WDs at explosion increases with a decreasing initial WD mass or a decreasing accretion rate. The various C/O ratios may, therefore, contribute to the diversity of SNe Ia.     

White dwarf models of supernovae and cataclysmic variables

If the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e, composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given tohelium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period X-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model forneon novae.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

UV emission lines in passively evolving galaxies can reveal the progenitors of type Ia supernovae

The question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, the two leading models being the so-called single and double degenerate scenarios. The former implies a large population of accreting, nuclear-burning white dwarfs with photospheric temperatures T～10⁵–10⁶ K during some part of their accretion history. Recently, we demonstrated that a population of accreting white dwarfs large enough to reproduce the observed SN Ia rate would contribute significantly to the ionizing radiation expected from the stellar population in early-type galaxies, now commonly observed to host spatially extended regions of neutral and ionized gas. From our photoionization calculations, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in early-type galaxies from upper limits on the luminosity of a number of emission lines characteristic of ionization by high-temperature sources. Detection (or strong upper limits on) He II 1640 Å and [C II] 1335 Å, expected to be overluminous in these galaxies if the single-degenerate channel holds true, can strongly constrain the total luminosity of nuclear-burning white dwarfs in these populations. In the near-UV, our photoionization calculations demonstrate that the EW of the [O II] 3727 doublet and the [Ne III] 3869/[O II] 3727 ratio can also provide a powerful diagnostic, particularly in post-starburst galaxies. Together with the He II 3203 Å (5 → 3) recombination line, these lines present an excellent opportunity for strongly constraining the population of accreting, nuclear-burning white dwarfs, and in general the available ionizing continuum, at relatively short delay-times (time from initial starburst).     

On evolution of contact eclipsing binaries

It is shown that during contact eclipsing binaries evolution under the influence of stellar wind, magnetic stellar wind and with matter transfer by gas flow, in binary stellar systems there may take place a process of star merger (low mass stars) within 10⁵–10⁷ yr and a fast increase of distance between stars of massive binaries. W UMa-type stars are a finite evolutionary stage of very close and low mass binary pairs. As for contact systems of early spectral types (CE-systems), they are more varied in evolution.     

Modeling the luminosity function of galactic low-mass X-ray binaries

The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities L _x > 10³⁷ erg s^?1 should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ～10^36.5 erg s^?1 due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4–1.9M _⊙ can be born.     

The origin of intergalactic thermonuclear supernovae

The population synthesis method is used to study the possibility of explaining the appreciable fraction of the intergalactic type-Ia supernovae (SN Ia), 20 ₋₁₅ ⁺¹² %, observed in galaxy clusters (Gal-Yam et al. 2003) when close white dwarf binaries merge in the cores of globular clusters. In a typical globular cluster, the number of merging double white dwarfs does not exceed ∼10⁻¹³ per year per average cluster star in the entire evolution time of the cluster, which is a factor of ∼3 higher than that in a Milky-Way-type spiral galaxy. From 5 to 30% of the merging white dwarfs are dynamically expelled from the cluster with barycenter velocities up to 150 km s⁻¹. SN Ia explosions during the mergers of double white dwarfs in dense star clusters may account for ∼1% of the total rate of thermonuclear supernovae in the central parts of galaxy clusters if the baryon mass fraction in such star clusters is ∼0.3%.     

Accreting He-rich material onto carbon-oxygen white dwarfs until explosive carbon ignition

Type Ia supernovae(SNe Ia) play an important role in studies of cosmology and galactic chemical evolution.They are believed to be thermonuclear explosions of carbon-oxygen white dwarfs(CO WDs)when their masses approach the Chandrasekar(Ch) mass limit.However,it is still not completely understood how a CO WD increases its mass to the Ch-mass limit in the classical single-degenerate(SD) model.In this paper,we studied the mass accretion process in the SD model to examine whether the WD can explode as an SN Ia.Employing the stellar evolution code called modules for experiments in stellar astrophysics(MESA),we simulated the He accretion process onto CO WDs.We found that the WD can increase its mass to the Ch-mass limit through the SD model and explosive carbon ignition finally occurs in its center,which will lead to an SN Ia explosion.Our results imply that SNe Ia can be produced from the SD model through steady helium accretion.Moreover,this work can provide initial input parameters for explosion models of SNe Ia.     

Is the X-ray pulsating companion of HD 49798 a possible type Ia supernova progenitor?

HD 49798(a hydrogen depleted subdwarf O6 star) with its massive white dwarf(WD) companion has been suggested to be a progenitor candidate of a type Ia supernova(SN Ia). However, it is still uncertain whether the companion of HD 49798 is a carbon-oxygen(CO) WD or an oxygen-neon(ONe) WD. A CO WD will explode as an SN Ia when its mass grows and approaches the Chandrasekhar limit, but the outcome of an accreting ONe WD is likely to be a neutron star. We generated a series of Monte Carlo calculations that incorperate binary population synthesis to simulate the formation of ONe WD + He star systems. We found that there is almost no orbital period as large as HD 49798 with its WD companion in these ONe WD + He star systems based on our simulations, which means that the companion of HD 49798 might not be an ONe WD. We suggest that the companion of HD 49798 is most likely a CO WD, which can be expected to increase its mass to the Chandrasekhar limit by accreting He-rich material from HD 49798. Thus, HD 49798 and its companion may produce an SN Ia as a result of its future evolution.     

The formation and evolution of low-mass close binaries with compact components

We discuss the formation and evolution of interacting low-mass close binaries with a He-1CO- or ONe-dwarf neutron star or a black hole as a compact component. Mass exchange leads to cataclysmic events in such systems. The rate of semidetached low-mass close binary formation is 5×10^–3 yr^–1 if the accreting component is a He degenerate dwarf, 5×10^–3 yr^–1 if it is a CO-dwarf and 3×10^–8 yr^–1 if it is a neutron star. Systems with compact accretors arise as the result of the common envelope phase of close binary evolution or due to collisions of single neutron stars or dwarfs with low-mass single stars in dense stellar clusters. Evolution of LMCB to the contact phase in semi-detached stages is determined mainly by the angular momentum losses by a magnetic stellar wind and radiation of gravitational waves. Numerical computations of evolution with momentum loss explain observed mass exchange rates in such systems, the absence of cataclysmic variables with orbital periods 2^h–3^h, the low number and the evolutionary status of systems with orbital periods shorter than 80^m. In conclusion we list unsolved problems related to magnetic stellar wind, the distribution of young close binaries over main initial parameters, stability of mass exchange.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Type-Ia supernovae in dense circumstellar gas

We propose a model for the bolometric light curve of a type-Ia supernova (SN Ia) that explodes in a dense circumstellar (CS) envelope. Our modeling of the light curves for SN 2002ic and SN 1997cy shows that the densities of the CS envelopes around both supernovae at a radius of ～7×10¹⁵ cm are similar, while the characteristic ejection time for this envelope around SN 1997cy does not exceed 600 yr. We analyze two possible evolutionary scenarios that could lead to the explosion of a SN Ia inside a dense C S hydrogen envelope: accretion onto a CO white dwarf in a symbiotic binary and the evolution of a single star with an initial mass of about 8M_⊙. If the hypothesis of a SN Ia explosion in a dense CS envelope is correct for SN 2002ic and SN 1997cy, then we must assume that the the rapid loss of the red-supergiant envelope in several hundred years and the subsequent explosion of the CO white dwarf are synchronized by some physical mechanism. This mechanism may be related to the contraction of the white dwarf as it approaches the Chandrasekhar limit. We show that the formation of a (super-)Chandrasekhar mass due to the merger of a CO white dwarf and the CO core of a red supergiant followed by a supernovae explosion is unlikely, since this mechanism does not provide the required synchronization of the rapid mass loss and the explosion.     

Accreting CO material onto ONe white dwarfs towards accretion-induced collapse

The final outcomes of accreting ONe white dwarfs(ONe WDs) have been studied for several decades,but there are still some issues that are not resolved. Recently,some studies suggested that the deflagration of oxygen would occur for accreting ONe WDs with Chandrasekhar masses. In this paper,we aim to investigate whether ONe WDs can experience accretion-induced collapse(AIC) or explosions when their masses approach the Chandrasekhar limit. Employing the stellar evolution code Modules for Experiments in Stellar Astrophysics(MESA),we simulate the longterm evolution of ONe WDs with accreting CO material. The ONe WDs undergo weak multicycle carbon flashes during the mass-accretion process,leading to mass increase of the WDs. We found that different initial WD masses and mass-accretion rates influence the evolution of central density and temperature. However,the central temperature cannot reach the explosive oxygen ignition temperature due to neutrino cooling. This work implies that the final outcome of accreting ONe WDs is electroncapture induced collapse rather than thermonuclear explosion.     

Evolutionary history of the elliptical galaxy NGC 1052

We have obtained Keck spectra for 16 globular clusters (GCs) associated with the merger remnant elliptical NGC 1052, as well as a long-slit spectrum of the galaxy. We derive ages, metallicities and abundance ratios from simple stellar population models using the recently published methods of Proctor & Sansom , applied to extragalactic GCs for the first time. A number of GCs indicate the presence of strong blue horizontal branches that are not fully accounted for in the current stellar population models. We find all of the GCs to be ∼13 Gyr old according to simple stellar populations, with a large range of metallicities. From the galaxy spectrum we find NGC 1052 to have a luminosity-weighted central age of ∼2 Gyr and metallicity of  [Fe/H]∼+0.6  . No strong gradients in either age or metallicity were found to the maximum radius measured  (0.3  r _e≃ 1 kpc)  . However, we do find a strong radial gradient in α-element abundance, which reaches a very high central value. The young central starburst age is consistent with the age inferred from the H  i tidal tails and infalling gas of ∼1 Gyr. Thus, although NGC 1052 shows substantial evidence for a recent merger and an associated starburst, it appears that the merger did not induce the formation of new GCs, perhaps suggesting that little recent star formation occurred. This interpretation is consistent with 'frosting' models for early-type galaxy formation.     

How old are SN Ia progenitor systems? New observational constraints on the distribution of time delays from GALEX

The time delay between the formation of the progenitor systems of Type Ia supernovae (SNe Ia) and their detonation is a vital discriminant between the various progenitor scenarios that have been proposed for them. We use Sloan Digital Sky Survey optical and Galaxy Evolution Explorer ( GALEX ) ultraviolet observations of the early-type host galaxies of 21 nearby SNe Ia and quantify the presence or absence of any young stellar population to constrain the minimum time delay for each supernova. We find that early-type host galaxies lack 'prompt' SNe Ia with time delays of ≲100 Myr and that ∼70 per cent SNe Ia have minimum time delays of 275 Myr–1.25 Gyr, with a median of 650 Myr, while at least 20 per cent SNe Ia have minimum time delays of at least 1 Gyr at 95 per cent confidence and two of these four SNe Ia are likely older than 2 Gyr. The distribution of minimum time delays observed matches most closely the expectation for the single-degenerate channel with a main sequence donor. Furthermore, we do not find any evidence that subluminous SNe Ia are associated with long time delays.