Xiangcun Meng

The helium star donor channel for the progenitors of Type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution. Several progenitor models for SNe Ia have been proposed in the past. In this paper we carry out a detailed study of the He star donor channel, in which a carbon-oxygen white dwarf (CO WD) accretes material from a He main-sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. Employing Eggletons stellar evolution code with an optically thick wind assumption, and adopting the prescription of Kato & Hachisu for the mass accumulation efficiency of the He-shell flashes on to the WDs, we performed binary evolution calculations for about 2600 close WD binary systems. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period-secondary mass (log P(i)-M(2)(i)) plane for various WD masses from this channel. The study shows that the He star donor channel is noteworthy for producing SNe Ia (similar to 1.2 x 10(-3) yr(-1) in our Galaxy), and that the progenitors from this channel may appear as supersoft X-ray sources. Importantly, this channel can explain SNe Ia with short delay times (similar to 10(8) yr), which is consistent with the recent observational implications of young populations of SN Ia progenitors.

EVOLVING TO TYPE Ia SUPERNOVAE WITH SHORT DELAY TIMES

The single-degenerate model is currently a favorable progenitor model for Type Ia supernovae (SNe Ia). Recent investigations on the white dwarf (WD) + He star channel of the single-degenerate model imply that this channel is noteworthy for producing SNe Ia. In this paper, we studied SN Ia birthrates and delay times of this channel via a detailed binary population synthesis approach. We found that the Galactic SN Ia birthrate from the WD + He star channel is similar to 0.3 x 10(-3) yr(-1) according to our standard model, and that this channel can explain SNe Ia with short delay times (similar to 4.5 x 10(7)-1.4 x 10(8) yr). Meanwhile, these WD + He star systems may be related to the young supersoft X-ray sources prior to SN Ia explosions.

Initial-final mass relationship for stars of different metallicities

Context. The initial-final mass relationship (IFMR) for stars is important in many astrophysical fields of study, such as the evolution of galaxies, the properties of type Ia supernovae (SNe Ia) and the components of dark matter in the Galaxy. Aims. The purpose of this paper is to obtain the dependence of the IFMR on metallicity. Methods. We assume that the envelope of an asymptotic giant branch (AGB) or a first giant branch (FGB) star is lost when the binding energy of the envelope is equal to zero (Delta W = 0) and the core mass of the AGB star or the FGB star at the point (Delta W = 0) is taken as the final mass. Using this assumption, we calculate the IFMRs for stars of different metallicities. Results. We find that the IFMR depends strongly on the metallicity, i.e. Z = 0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.1. From Z = 0.04, the final mass of the stars with a given initial mass increases with increasing or decreasing metallicity. The difference in the final mass due to the metallicity may be up to 0.4 M(circle dot). A linear fit of the initial-final mass relationship in NGC 2099 (M37) shows the effect of metallicity on the IFMR. The IFMR for stars of Z = 0.02 obtained here matches well with those inferred observationally in the Galaxy. For Z >= 0.02, helium WDs are obtained from the stars of M(i) <= 1.0 M(circle dot) and this result is supported by the discovery of numerous low-mass WDs in NGC 6791, which is a metal-rich old open cluster. Using the IFMR for stars of Z = 0.02 obtained here, we have reproduced the mass distribution of DA WDs in Sloan DR4 except for some ultra-massive white dwarfs. Conclusions. The trend that the mean mass ofWDs decreases with effective temperature may originate from the increase of the initial metallicities of stars. We briefly discuss the potential effects of the IFMR on SNe Ia and at the same time, predict that metal-rich low-mass stars may become under-massive white dwarfs.

THE CONTRIBUTIONS OF INTERACTIVE BINARY STARS TO DOUBLE MAIN-SEQUENCE TURNOFFS AND DUAL RED CLUMP OF INTERMEDIATE-AGE STAR CLUSTERS

Double or extended main-sequence turnoffs (DMSTOs) and dual red clump (RC) were observed in intermediate-age clusters, such as in NGC 1846 and 419. The DMSTOs are interpreted as that the cluster has two distinct stellar populations with differences in age of about 200-300 Myr but with the same metallicity. The dual RC is interpreted as a result of a prolonged star formation. Using a stellar population-synthesis method, we calculated the evolution of a binary-star stellar population. We found that binary interactions and merging can reproduce the dual RC in the color-magnitude diagrams of an intermediate-age cluster, whereas in actuality only a single population exists. Moreover, the binary interactions can lead to an extended main-sequence turnoff (MSTO) rather than DMSTOs. However, the rest of the main sequence, subgiant branch, and first giant branch are hardly spread by the binary interactions. Part of the observed dual RC and extended MSTO may be the results of binary interactions and mergers.

THE HYBRID CONe WD + He STAR SCENARIO FOR THE PROGENITORS OF TYPE Ia SUPERNOVAE

The hybrid CONe white dwarfs (WDs) have been suggested to be possible progenitors of type Ia supernovae (SNe Ia). In this article, we systematically studied the hybrid CONe WD + He star scenario for the progenitors of SNe Ia, in which a hybrid CONe WD increases its mass to the Chandrasekhar mass limit by accreting He-rich material from a non-degenerate He star. According to a series of detailed binary population synthesis simulations, we obtained the SN Ia birthrates and delay times for this scenario. The SN Ia birthrates for this scenario are ~0.033-0.539*10^(-3)yr^(-1), which roughly accounts for 1-18% of all SNe Ia. The estimated delay times are ~28Myr-178Myr, which are the youngest SNe Ia predicted by any progenitor model so far. We suggest that SNe Ia from this scenario may provide an alternative explanation of type Iax SNe. We also presented some properties of the donors at the point when the WDs reach the Chandrasekhar mass. These properties may be a good starting point for investigating the surviving companions of SNe Ia, and for constraining the progenitor scenario studied in this work.

THE BIRTH RATE OF SNe Ia FROM HYBRID CONe WHITE DWARFS

Considering the uncertainties of the C-burning rate (CBR) and the treatment of convective boundaries, Chen et al. (2014) found that there is a regime where it is possible to form hybrid CONe white dwarfs (WDs), i.e. ONe WDs with carbon-rich cores. As these hybrid WDs can be as massive as 1.30

The birth rate of subluminous and overluminous type Ia supernovae

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Companion stars of Type Ia supernovae with different metallicities

, not much mass needs to be accreted for these objects to reach the Chandrasekhar limit and to explode as Type Ia supernovae (SNe Ia). We have investigated their contribution to the overall SN Ia birth rate and found that such SNe Ia tend to be relatively young with typical time delays between 0.1 and 1 Gyr, where some may be as young as 30 Myr. SNe Ia from hybrid CONe WDs may contribute several percent to all SNe Ia, depending on the common-envelope ejection efficiency and the CBR. We suggest that these SNe Ia may produce part of the 2002cx-like SN Ia class.

Distribution of 56 Ni Yields of Type Ia Supernovae and its Implication for Progenitors

Context. Based on the single degenerate (SD) scenario, a super-Chandrasekhar mass model derived from the rapid rotation of a progenitor star may account for the overluminous type Ia supernovae (SNe Ia) like SN 2003fg. Previous authors calculated a series of binary evolution and showed the parameter spaces for the super-Chandrasekhar mass model. Another team developed an equal-mass double degenerate (DD) model to explain subluminous SNe Ia like SN 1991bg. But they did not show the evolution of the birth rate of these peculiar SNe Ia or compared them with absolute birth rates from observations. Aims. We aim to show the evolution of the birth rates of these peculiar SNe Ia based on the results of these other works, and compare the birth rates with observations to check whether these model may account for all the peculiar SNe Ia. Methods. We carried out a series of binary population synthesis calculations and considered two methods of common envelope (CE) evolution, i.e. α-formalism and γ-algorithm. Results. We found that the evolution of the birth rate of these peculiar SNe Ia heavily dependen on how the CE evolution is treated. The overluminous SNe Ia may only occur for α-formalism with low CE ejection efficiency, and the delay time of the SNe Ia is between 0.4 and 0.8 Gyr. The upper limit of the contribution rate of the supernovae to all SN Ia is less than 0.3%. The delay time of subluminous SNe Ia from equal-mass DD systems is between 0.1 and 0.3 Gyr for α-formalism with α = 3.0, but longer than 9 Gyr for α = 1.0. The range of the delay time for γ-algorithm is very wide, i.e. longer than 0.22 Gyr, even as long as 15 Gyr. The subluminous SNe Ia from equal-mass DD systems may only account for no more than 1% of all SNe Ia observed. Conclusions. The super-Chandrasekhar mass model may account for a part of the 2003fg-like supernovae and the equal-mass DD model may explain some 1991bg-like events, too. In addition, based on the comparison between theories and observations, including the birth rate and delay time of the 1991bg-like events, we found that the γ-algorithm is more likely to be an appropriate prescription of the CE evolution of DD systems than the α-formalism if the equal-mass DD system is the progenitor of 1991bg-like SNe Ia.

The birth rate of supernovae from double-degenerate and core-degenerate systems

The single-degenerate model is the most widely accepted progenitor model of Type Ia supernovae (SNe Ia), where a carbon-oxygen white dwarf (CO WD) accretes hydrogen-rich material from its companion to increase its mass. The companion may be a main-sequence (MS) star or a subgiant star (WD + MS). When the CO WD approaches the Chandrasekhar mass limit, it explodes as a SN Ia and part of the supernova ejecta collides into the companion envelope. After the impact of the ejecta, the companion survives and may show some special properties. A good way to verify the single-degenerate model is to study the interaction between the supernova ejecta and the companion, and/or search for the companion in the remnant of a SN Ia. Following previous studies, we have carried out a series of binary population synthesis studies exploring the properties of the companions of SNe Ia for different metallicities Z. We present the distributions of the masses M SN 2 , radii R SN 2 of the companions, periods P SN and ratios of separations to radii A/R SN 2 of WD + MS systems for various Z at the moment of supernova explosion. These parameters can be applied to constrain the numerical simulation of the interaction between the ejecta of a supernova and its companion. We also show the distributions of some integral properties of the companions, i.e. the mass, the space velocity and the surface gravity, for various Z after the interaction. The distributions may help us to search for the companion in a supernova remnant. All the parameters above change significantly with Z. Incorporating the simulation results of the interaction between supernova ejecta and companions from other works into our binary population synthesis study, we found that more than 75 per cent of all supernovae have a strong enough polarization signal to be detected by spectropolarimetric observations. We also found that 13-14 per cent of SNe Ia belong to the class of supernovae like 1991T, which is consistent with observations within the errors. This may indicate that 1991 T-like SNe do not have any special physical properties except for the viewing angle of the observer.