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Although type Ia supernovae (SNe Ia) are very useful in many astrophysical fields, their exact progenitor nature is still unclear. A basic method to distinguish the different progenitor models is to search the signal from the single degenerate (SD) model, e.g. the signal for the existence of a non-degenerate companion before or after supernova explosion. Observationally, some SNe Ia show such signal, while the others do not. Here, we propose a universal model to explain these observations based on the spin-up/spin-down model, in which a white dwarf (WD) will experience a spin-down phase before supernova explosion, and the spin-down timescale is determined by its initial mass, i.e. the more massive the initial WD, the shorter the spin-down timescale and then the more likely the SN Ia to show the SD signature. Therefore, our model predicts that the SNe Ia from hybrid carbon-oxygen-neon WDs are more likely to show the SD signature observationally, as some peculiar SNe Ia showed., Comment: 2 figures, ApJ Letter on line

The double-degenerate (DD) model, involving the merging of massive double carbon-oxygen white dwarfs (CO WDs) driven by gravitational wave radiation, is one of the classical pathways for the formation of type Ia supernovae (SNe Ia). Recently, it has been proposed that the WD+He subgiant channel has a significant contribution to the production of massive double WDs, in which the primary WD accumulates mass by accreting He-rich matter from a He subgiant. We evolved about 1800 CO WD+He star systems and obtained a large and dense grid for producing SNe Ia through the DD model. We then performed a series of binary population synthesis simulations for the DD model, in which the WD+He subgiant channel is calculated by interpolations in this grid. According to our standard model, the Galactic birthrate of SNe Ia is about 2.4*10^{-3} yr^{-1} for the WD+He subgiant channel of the DD model; the total birthrate is about 3.7*10^{-3} yr^{-1} for all channels, reproducing that of observations. Previous theoretical models still have deficit with the observed SNe Ia with delay times <1 Gyr and >8 Gyr. After considering the WD+He subgiant channel, we found that the delay time distributions is comparable with the observed results. Additionally, some recent studies proposed that the violent WD mergers are more likely to produce SNe Ia based on the DD model. We estimated that the violent mergers through the DD model may only contribute to about 16% of all SNe Ia., Comment: 16 pages, 9 figures, submitted to MNRAS on August 28th 2017

Context. The companions of the exploding carbon-oxygen white dwarfs (CO WDs) for producing type Ia supernovae (SNe Ia) are still not conclusively confirmed. A red-giant (RG) star has been suggested to be the mass donor of the exploding WD, named as the symbiotic channel. However, previous studies on the this channel gave a relatively low rate of SNe Ia. Aims. We aim to systematically investigate the parameter space, Galactic rates and delay time distributions of SNe Ia from the symbiotic channel by employing a revised mass-transfer prescription. Methods. We adopted an integrated mass-transfer prescription to calculate the mass-transfer process from a RG star onto the WD. In this prescription, the mass-transfer rate varies with the local material states. Results. We evolved a large number of WD+RG systems, and found that the parameter space of WD+RG systems for producing SNe Ia is significantly enlarged. This channel could produce SNe Ia with intermediate and old ages, contributing to at most 5% of all SNe Ia in the Galaxy. Our model increases the SN Ia rate from this channel by a factor of 5. We suggest that the symbiotic systems RS Oph and T CrB are strong candidates for the progenitors of SNe Ia., Comment: 8 pages, 6 figures

Two blue-straggler sequences discovered in globular cluster M30 provide a strong constraint on the formation mechanisms of blue stragglers. We study the formation of the blue-straggler binaries through binary evolution, and find that binary evolution can contribute to the blue stragglers in both of the sequences. Whether a blue straggler is located in the blue sequence or red sequence depends on the contribution of the mass donor to the total luminosity of the binary, which is generally observed as a single star in globular clusters. The blue stragglers in the blue sequence have a cool white-dwarf companion, while the majority ($\sim 60\%$) of the objects in the red sequence are the binaries that are still experiencing mass transfer, but there are also some objects that the donors have just finished the mass transfer (the stripped-core stars, $\sim 10\%$) or the blue stragglers (the accretors) have evolved away from the blue sequence ($\sim 30\%$). Meanwhile, W UMa contact binaries found in both sequences may be explained by various mass ratios, that is, W UMa contact binaries in the red sequence have two components with comparable masses (e.g. mass ratio $q\sim$ 0.3-1.0), while those in the blue sequence have low mass ratio (e.g. $q<$ 0.3). However, the fraction of blue sequence in M30 cannot be reproduced by binary population synthesis if we assumed the initial parameters of a binary sample to be the same as that of the field. This possible indicates that dynamical effects on binary systems is very important in globular clusters., Comment: 27 pages, 9 figures, accepted for publication in ApJ

He accretion onto carbon-oxygen white dwarfs (CO WDs) plays a fundamental role when studying the formation of type Ia supernovae (SNe Ia). Employing the MESA stellar evolution code, we calculated the long-term evolution of He-accreting CO WDs. Previous studies usually supposed that a WD can grow in mass to the Chandrasekhar limit in the stable He burning region and finally produce a SN Ia. However, in this study we find that off-centre carbon ignition occurs in the stable He burning region if the accretion rate is above a critical value (~2.05*10^{-6}{M$_\odot$}{yr}^-1), resulting in accretion-induced collapse rather than a SN Ia. If the accretion rate is below the critical value, explosive carbon ignition will eventually happen in the centre producing a SN Ia. Taking into account the possibility of off-centre carbon ignition, we have re-determined the initial parameter space that produces SNe Ia in the He star donor channel, one of the promising channels to produce SNe Ia in young populations. Since this parameter space is smaller than was found in the previous study of Wang et al. (2009), the SN Ia rates are also correspondingly smaller. We also determined the chemical abundance profile of the He-accreting WDs at the moment of explosive carbon ignition, which can be used as initial input for SN Ia explosion models., Comment: 14 pages, 9 figures, accepted for publication in MNRAS

Early polarization observations on Type Ia supernovae (SNe Ia) may reveal the geometry of supernova ejecta, and then put constraints on their explosion mechanism and their progenitor model. We performed a literature search of SNe Ia with polarization measurements and determined the polarization and relative equivalent width (REW) of Si II 635.5-nm absorption feature at -5 days after the maximum light. We found that either the distribution of observed polarization degree is bimodal, i.e. the broad line SNe Ia have systematically higher polarization than all other SNe Ia, or all kind of SNe Ia share the same polarization sequence, i.e. the polarization of Si II 635.5-nm absorption feature increases with the REW. We also discussed the potential meaning of the discovery on the explosion mechanism and progenitor model of SNe Ia., Comment: 4 figures, accepted for publication in ApJ

KIC~8262223 is an eclipsing binary with a short orbital period ($P=1.61$ d). The {\it Kepler} light curves are of Algol-type and display deep and partial eclipses, ellipsoidal variations, and pulsations of Delta Scuti type. We analyzed the {\it Kepler} photometric data, complemented by phase-resolved spectra from the R-C Spectrograph on the 4-meter Mayall telescope at Kitt Peak National Observatory and determined the fundamental parameters of this system. The low mass and oversized secondary ($M_2=0.20M_{\odot}$, $R_2=1.31R_{\odot}$) is the remnant of the donor star that transferred most of its mass to the gainer, and now the primary star. The current primary star is thus not a normal $\delta$ Scuti star but the result of mass accretion from a lower mass progenitor. We discuss the possible evolutionary history and demonstrate with the MESA evolution code that the system can be understood as the result of non-conservative binary evolution similar to that for the formation of EL CVn type binaries. The pulsations of the primary star can be explained as radial and non-radial pressure modes. The equilibrium models from single star evolutionary tracks can match the observed mass and radius ($M_1=1.94M_{\odot}$, $R_1=1.67R_{\odot}$) but the predicted unstable modes associated with these models differ somewhat from those observed. This work presents a preliminary asteroseismic analysis of the abnormal Delta Scuti pulsators, and we discuss the need for better theoretical understanding of such pulsating mass gaining stars., Comment: ApJ accepted

Employing tidally enhanced stellar wind, we studied in binaries the effects of metallicity, mass ratio of primary to secondary, tidal enhancement efficiency and helium abundance on the formation of blue hook (BHk) stars in globular clusters (GCs). A total of 28 sets of binary models combined with different input parameters are studied. For each set of binary model, we presented a range of initial orbital periods that is needed to produce BHk stars in binaries. All the binary models could produce BHk stars within different range of initial orbital periods. We also compared our results with the observation in the Teff-logg diagram of GC NGC 2808 and {\omega} Cen. Most of the BHk stars in these two GCs locate well in the region predicted by our theoretical models, especially when C/N-enhanced model atmospheres are considered. We found that mass ratio of primary to secondary and tidal enhancement efficiency have little effects on the formation of BHk stars in binaries, while metallicity and helium abundance would play important roles, especially for helium abundance. Specifically, with helium abundance increasing in binary models, the space range of initial orbital periods needed to produce BHk stars becomes obviously wider, regardless of other input parameters adopted. Our results were discussed with recent observations and other theoretical models., Comment: 12 pages, 11 figures, 2 tables, accepted for publication in MNRAS

Recent observations suggest that some type Ia supernovae (SNe Ia) originate from the merging of two carbon-oxygen white dwarfs (CO WDs). Meanwhile, recent hydrodynamical simulations have indicated that the accretion-induced collapse may be avoided under certain conditions when double WDs merge violently. However, the properties of SNe Ia from this violent merger scenario are highly dependent on a particular mass-accretion stage, the so-called WD + He subgiant channel, during which the primary WD is able to increase its mass by accreting He-rich material from a He subgiant before the systems evolves into a double WD system. In this article, we aim to study this particular evolutionary stage systematically and give the properties of violent WD mergers. By employing the Eggleton stellar evolution code, we followed a large number of binary calculations and obtained the regions in parameter space for producing violent mergers based on the WD + He subgiant channel. According to these simulations, we found that the primary WDs can increase their mass by ~0.10-0.45 Msun during the mass-accretion stage. We then conducted a series of binary population synthesis calculations and found that the Galactic SN Ia birthrate from this channel is about 0.01-0.4*10-3 yr-1. This suggests that the violent WD mergers from this channel may only contribute to ~0.3%-10% of all SNe Ia in our Galaxy. The delay times of violent WD mergers from this channel are >1.7 Gyr, contributing to the SNe Ia in old populations. We also found that the WD + He subgiant channel is the dominent way for producing violent WD mergers that may be able to eventually explode as SNe Ia., Comment: 18 pages, 13 figures, accepted for publication in MNRAS

MSE is an 11.25m aperture observatory with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. More than 3200 fibres will feed spectrographs operating at low (R ~ 2000 - 3500) and moderate (R ~ 6000) spectral resolution, and approximately 1000 fibers will feed spectrographs operating at high (R ~ 40000) resolution. MSE is designed to enable transformational science in areas as diverse as tomographic mapping of the interstellar and intergalactic media; the in-situ chemical tagging of thick disk and halo stars; connecting galaxies to their large scale structure; measuring the mass functions of cold dark matter sub-halos in galaxy and cluster-scale hosts; reverberation mapping of supermassive black holes in quasars; next generation cosmological surveys using redshift space distortions and peculiar velocities. MSE is an essential follow-up facility to current and next generations of multi-wavelength imaging surveys, including LSST, Gaia, Euclid, WFIRST, PLATO, and the SKA, and is designed to complement and go beyond the science goals of other planned and current spectroscopic capabilities like VISTA/4MOST, WHT/WEAVE, AAT/HERMES and Subaru/PFS. It is an ideal feeder facility for E-ELT, TMT and GMT, and provides the missing link between wide field imaging and small field precision astronomy. MSE is optimized for high throughput, high signal-to-noise observations of the faintest sources in the Universe with high quality calibration and stability being ensured through the dedicated operational mode of the observatory. (abridged), Comment: 210 pages, 91 figures. Exposure draft. Appendices to the Detailed Science Case can be found at http://mse.cfht.hawaii.edu/docs

Theoretical modelling of the evolution of classical and recurrent novae plays an important role in studies of binary evolution, nucleosynthesis and accretion physics. However, from a theoretical perspective the observed statistical properties of novae remain poorly understood. In this paper, we have produced model populations of novae using a hybrid binary population synthesis approach for differing star formation histories (SFHs): a starburst case (elliptical-like galaxies), a constant star formation rate case (spiral-like galaxies) and a composite case (in line with the inferred SFH for M31). We found that the nova rate at 10\;Gyr in an elliptical-like galaxy is $\sim 10-20$ times smaller than a spiral-like galaxy with the same mass. The majority of novae in elliptical-like galaxies at the present epoch are characterized by low mass white dwarfs (WDs), long decay times, relatively faint absolute magnitudes and long recurrence periods. In contrast, the majority of novae in spiral-like galaxies at 10\;Gyr have massive WDs, short decay times, are relatively bright and have short recurrence periods. The mass loss time distribution for novae in our M31-like galaxy is in agreement with observational data for Andromeda. However, it is possible that we underestimate the number of bright novae in our model. This may arise in part due to the present uncertainties in the appropriate bolometric correction for novae., Comment: 14 pages, 12 figures, MNRAS accepted

We carried out a series of binary stellar evolution calculations, in which the effect of tidally enhanced wind on the evolution of WD + RG systems is incorporated. The WDs increase their mass to the Chandrasekhar mass limit, and then explode as SNe Ia. Based on the binary evolution results, we estimated the X-ray/radio (the excess UV) luminosity from the interactions between supernova ejecta and the CSM (the secondary) via some published standard models. We found that the X-ray flux may be high enough to be detected for a nearby SN Ia from a symbiotic system, while the radio flux is more likely to de detected when the companion is an asymptotic giant branch (AGB) star, and for a first giant branch (FGB) companion, the radio flux is generally lower than the detection limit. For two well observed SNe Ia, 2011fe and 2014J, almost all symbiotic systems are excluded by X-ray observations, but WD + FGB systems may not be ruled out by radio observations. The excess UV luminosity resulting from the collision of supernova ejecta with the RG secondary may be high enough to be detected if the secondary fills its Roche lobe at the moment of supernova explosion. The X-ray/radio emissions are more prevalent in SNe Ia from WD + AGB systems, although SNe Ia from such systems are rare. The UV luminosity from the collision of supernova ejecta to RG secondary is high enough, but only 1 in every few hundred SNe Ia manifests the signal from the collision., Comment: 12 pages, 5 figures, accepted for publication in A&A, after language editor

We present a catalog of 166 spectroscopically identified hot subdwarf stars from LAMOST DR1, 44 of which show the characteristics of cool companions in their optical spectra. Atmospheric parameters of 122 non-composite spectra subdwarf stars were measured by fitting the profiles of hydrogen (H) and helium (He) lines with synthetic spectra from non-LTE model atmospheres. Most of the sdB stars scatter near the Extreme Horizontal Branch in the $T_{\rm eff}-\log{g}$ diagram and two well defined groups can be outlined. A clustering of He-enriched sdO stars appears near $T_{\rm eff}=45\,000$ K and $\log(g) = 5.8$. The sdB population separates into several nearly parallel sequences in the $T_{\rm eff}-{\rm He}$ abundance diagram with clumps corresponding to those in the $T_{\rm eff}-\log{g}$ diagram. Over $38\,000$ K (sdO) stars show abundance extremes, they are either He-rich or He-deficient and we observe only a few stars in the $ -1 < \log(y) < 0$ abundance range. With increasing temperature these extremes become less prominent and the He abundance approaches to $\log(y)\sim-0.5$. A unique property of our sample is that it covers a large range in apparent magnitudes and galactic latitudes, therefore it contains a mix of stars from different populations and galactic environments. Our results are consistent with the findings of Hirsch (2009) and we conclude that He-rich and He-deficient sdB stars ($\log(y) < 1$) probably origin from different populations. We also find that most sdO and sdB stars lie in a narrow strip in the luminosity and helium abundance plane, which suggests that these atmospheric parameters are correlated., Comment: Accepted To The Astrophysical Journal

Supernovae of type Ia (SNe Ia) are believed to be thermonuclear explosions of carbon-oxygen white dwarfs (CO WDs). However, the mass accretion process onto CO WDs is still not completely understood. In this paper, we study the accretion of He-rich matter onto CO WDs and explore a scenario in which a strong wind forms on the surface of the WD if the total luminosity exceeds the Eddington limit. Using a stellar evolution code called modules for experiments in stellar astrophysics (MESA), we simulated the He accretion process onto CO WDs for WDs with masses of 0.6-1.35Msun and various accretion rates of 10^{-8}-10^{-5}Msun/yr. If the contribution of the total luminosity is included when determining the Eddington accretion rate, then a super-Eddington wind could be triggered at relatively lower accretion rates than those of previous studies based on steady-state models. The super-Eddington wind can prevent the WDs with high accretion rates from evolving into red-giant-like He stars. We found that the contributions from thermal energy of the WD are non-negligible, judging by our simulations, even though the nuclear burning energy is the dominating source of luminosity. We also provide the limits of the steady He-burning regime in which the WDs do not lose any accreted matter and increase their mass steadily, and calculated the mass retention efficiency during He layer flashes for various WD masses and accretion rates. These obtained results can be used in future binary population synthesis computations., Comment: 6 pages, 5 figures, accepted for publication in Astronomy and Astrophysics, after language editing

In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We survey here adiabatic mass loss from Population I stars of mass 0.10 Msun to 100 Msun from the zero age main sequence to the base of the giant branch, or to central hydrogen exhaustion for lower main sequence stars. For intermediate- and high-mass stars, dynamical mass transfer is preceded by an extended phase of thermal time scale mass transfer as the star is stripped of most of its envelope mass. The critical mass ratio qad above which this delayed dynamical instability occurs increases with advancing evolutionary age of the donor star, by ever-increasing factors for more massive donors. Most intermediate- or high-mass binaries with nondegenerate accretors probably evolve into contact before manifesting this instability. As they approach the base of the giant branch, however, and begin developing a convective envelope, qad plummets dramatically among intermediate-mass stars, to values of order unity, and a prompt dynamical instability occurs. Among low-mass stars, the prompt instability prevails throughout main sequence evolution, with q_ad declining with decreasing mass. Our calculated qad agree well with the behavior of timedependent models by Chen & Han (2003) of intermediate-mass stars initiating mass transfer in the Hertzsprung gap. Application of our results to cataclysmic variables, as systems which must be stable against rapid mass transfer, nicely circumscribes the range in qad as a function of orbital period in which they are found. These results are intended to advance the verisimilitude of population synthesis models of close binary evolution., Comment: 56 pages, 11 figures, 4 tables, Accepted to be published in ApJ

Type Ia supernovae luminosities can be corrected to render them useful as standard candles able to probe the expansion history of the universe. This technique was successful applied to discover the present acceleration of the universe. As the number of SNe Ia observed at high redshift increases and analysis techniques are perfected, people aim to use this technique to probe the equation of state of the dark energy. Nevertheless, the nature of SNe Ia progenitors remains controversial and concerns persist about possible evolution effects that may be larger and harder to characterize than the more obvious statistical uncertainties., Comment: 21 pages, 9 figures, invited review published in IJMPD

Accreting white dwarfs (WDs) with non-degenerate companions are expected to emit in soft X-rays and the UV, if accreted H-rich material burns stably. They are an important component of the unresolved emission of elliptical galaxies, and their combined ionizing luminosity may significantly influence the optical line emission from warm ISM. In an earlier paper we modeled populations of accreting WDs, first generating WD with main-sequence, Hertzsprung gap and red giant companions with the population synthesis code \textsc{BSE}, and then following their evolution with a grid of evolutionary tracks computed with \textsc{MESA}. Now we use these results to estimate the soft X-ray (0.3-0.7keV), H- and He II-ionizing luminosities of nuclear burning WDs and the number of super-soft X-ray sources for galaxies with different star formation histories. For the starburst case, these quantities peak at $\sim 1$ Gyr and decline by $\sim 1-3$ orders of magnitude by the age of 10 Gyr. For stellar ages of $\sim$~10 Gyr, predictions of our model are consistent with soft X-ray luminosities observed by Chandra in nearby elliptical galaxies and He II 4686$\AA/\rm{H}{\beta}$ line ratio measured in stacked SDSS spectra of retired galaxies, the latter characterising the strength and hardness of the UV radiation field. However, the soft X-ray luminosity and He~II~4686$\AA/\rm{H}{\beta}$ ratio are significantly overpredicted for stellar ages of $\lesssim 4-8$ Gyr. We discuss various possibilities to resolve this discrepancy and tentatively conclude that it may be resolved by a modification of the typically used criteria of dynamically unstable mass loss for giant stars., Comment: 13 pages, 12 figures, MNRAS accepted

The formation mechanism for blue hook (BHk) stars in globular clusters (GCs) is still unclear. Following one of the possible scenario, named late hot flash scenario, we proposed that tidally enhanced stellar wind in binary evolution may provide the huge mass loss on the red giant branch (RGB) and produce BHk stars. Employing the detailed stellar evolution code, Modules for Experiments in Stellar Astrophysics (MESA), we investigated the contributions of tidally enhanced stellar wind as a possible formation channel for BHk stars in GCs. We evolved the primary stars with different initial orbital periods using the binary module in MESA (version 6208) from zero age main-sequence (ZAMS) to post horizontal branch (HB) stage, and obtained their evolution parameters which are compared with the observation. The results are consistent with observation in the color-magnitude diagram (CMD) and the logg-Teff plane for NGC 2808, which is an example GC hosting BHk stars. However, the helium abundance in the surface for our models is higher than the one obtained in BHk stars. This discrepancy between our models and observation is possibly due to the fact that gravitational settling and radiative levitation which are common processes in hot HB stars are not considered in the models as well as the fact that the flash mixing efficiency may be overestimated in the calculations. Our results suggested that tidally enhanced stellar wind in binary evolution is able to naturally provide the huge mass loss on the RGB needed for late hot flash scenario and it is a possible and reasonable formation channel for BHk stars in GCs., Comment: 11 pages, 5 figures, 2 tables, accepted for publication in MNRAS

The super-Eddington wind scenario has been proposed as an alternative way for producing type Ia supernovae (SNe Ia). The super-Eddington wind can naturally prevent the carbon--oxygen white dwarfs (CO WDs) with high mass-accretion rates from becoming red-giant-like stars. Furthermore, it works in low-metallicity environments, which may explain SNe Ia observed at high redshifts. In this article, we systematically investigated the most prominent single-degenerate WD+MS channel based on the super-Eddington wind scenario. We combined the Eggleton stellar evolution code with a rapid binary population synthesis (BPS) approach to predict SN Ia birthrates for the WD+MS channel by adopting the super-Eddington wind scenario and detailed mass-accumulation efficiencies of H-shell flashes on the WDs. Our BPS calculations found that the estimated SN Ia birthrates for the WD+MS channel are ~0.009-0.315*10^{-3}{yr}^{-1} if we adopt the Eddington accretion rate as the critical accretion rate, which are much lower than that of the observations (<10% of the observed SN Ia birthrates). This indicates that the WD+MS channel only contributes a small proportion of all SNe Ia. The birthrates in this simulation are lower than previous studies, the main reason of which is that new mass-accumulation efficiencies of H-shell flashes are adopted. We also found that the critical mass-accretion rate has a significant influence on the birthrates of SNe Ia. Meanwhile, the results of our BPS calculations are sensitive to the values of the common-envelope ejection efficiency., Comment: 14 pages, 9 figures, 1 table, accepted for publication in Astronomy and Astrophysics

Among the initial results from Kepler were two striking light curves, for KOI 74 and KOI 81, in which the relative depths of the primary and secondary eclipses showed that the more compact, less luminous object was hotter than its stellar host. That result became particularly intriguing because a substellar mass had been derived for the secondary in KOI 74, which would make the high temperature challenging to explain; in KOI 81, the mass range for the companion was also reported to be consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI 74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km s[superscript -1]. As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination and primary mass derived from the eclipses and primary spectral type, we infer a secondary mass of 0.22 ± 0.03 M ☉. We use our estimates to consider the likely evolutionary paths and mass-transfer episodes of these binary systems.