A Model for Contact Binary Systems

A model for contact binary systems is presented, which incorporates the following special features: a) The energy exchange between the components is based on the understand-ing that the energy exchange is due to the release of potential, kinetic and thermal energies of the exchanged mass. b) A special form of mass and angular momentum loss occurring in contact binaries is losses via the outer Lagrangian point. c) The effects of spin, orbital rota-tion and tidal action on the stellar structure as well as the effect of meridian circulation on the mixing of the chemical elements are considered. d) The model is valid not only for low-mass contact binaries but also for high-mass contact binaries. For illustration, we used the model to trace the evolution of a massive binary system consisting of one 12M and one 5M star. The result shows that the start and end of the contact stage fall within the semi-detached phase during which the primary continually transfers mass to the secondary. The time span of the contact stage is short and the mass transfer rate is very large. Therefore, the contact stage can be regarded as a special part of the semi-detached phase with a large mass transfer rate. Both mass loss through the outer Lagrangian point and oscillation between contact and semi-contact states can occur during the contact phase, and the effective temperatures of the primary and the secondary are almost equal.     

The Relation between the Critical Accretion Rate of Progenitors of SNe Ia and Metallicity

A carbon-oxygen white dwarf may explode in a Type Ia supernova by accreting matter from its companion via either Roche lobe overflow or from winds, but there exists a critical accretion rate of the progenitor system for the explosion. We study the relation between the critical accretion rate and the metallicity via an AGB star approach. The result indicates that the critical accretion rate depends not only on the hydrogen mass fraction and the white dwarf mass, but also on the metallicity. The effect of the metallicity is smaller than that of the white dwarf mass. We show that it is reasonable to use the model with stellar mass 1.6 M⊙for real white dwarfs.     

Neutrino luminosity of stars with different masses

Neutrinos play an important role in stellar evolution. They are produced by nuclear reactions or thermal processes. Using the stellar evolution code Modules for Experiments in Stellar Astrophysics(MESA), we study stellar neutrino luminosity with different masses. The neutrino luminosities of stars with different initial masses at different evolutionary stages are simulated. We find that the neutrino flux of a star with 1 M⊙mass at an evolutionary age of 4.61 × 10~9 yr is consistent with that of the Sun. In general, neutrinos are produced by nuclear reactions, and the neutrino luminosity of stars is about one or two magnitudes lower than the photo luminosity. However, neutrino luminosity can exceed photo luminosity during the helium flash which can occur for stars with a mass lower than 8 M_⊙. Although the helium flash does not produce neutrinos, plasma decay, one of the thermal processes, can efficiently make neutrinos during this stage. Due to the high mass-loss rate, a star with a mass of 9 M_⊙does not undergo the helium flash. Its neutrinos mainly originate from nuclear reactions until the end of the AGB stage. At the end of the AGB stage, its neutrino luminosity results from plasma decay which is triggered by the gravitational energy release because of the stellar core contracting.     

He-shell flashes on the surface of oxygen-neon white dwarfs

Accretion induced collapse(AIC)may be responsible for the formation of some interesting neutron star binaries(e.g.,millisecond pulsars,intermediate-mass binary pulsars,etc).It has been suggested that oxygen-neon white dwarfs(ONe WDs)can increase their mass to the Chandrasekhar limit by multiple He-shell flashes,leading to AIC events.However,the properties of He-shell flashes on the surface of ONe WDs are still not well understood.In this article,we aim to study He-shell flashes on the surface of ONe WDs in a systematic approach.We investigated the long-term evolution of ONe WDs accreting He-rich material with various constant mass-accretion rates by time-dependent calculations with the stellar evolution code Modules for Experiments in Stellar Astrophysics(MESA),in which the initial ONe WD masses range from 1.1 to 1.35 M_⊙.We found that the mass-retention efficiency increases with the ONe WD mass and the mass-accretion rate,whereas both the nova cycle duration and the ignition mass decrease with the ONe WD mass and the mass-accretion rate.We also present the nuclear products in different accretion scenarios.The results presented in this article can be used in the future binary population synthesis studies of AIC events.     

Grids of stellar models including second harmonic and colors:solar composition(Z = 0.0172,X = 0.7024)

Grids of stellar evolution are required in many fields of astronomy/astrophysics,such as planet hosting stars,binaries,clusters,chemically peculiar stars,etc.In this study,a grid of stellar evolution models with updated ingredients and recently determined solar abundances is presented.The solar values for the initial abundances of hydrogen,heavy elements and mixing-length parameter are 0.0172,0.7024 and 1.98,respectively.The mass step is small enough(0.01 M_⊙)that interpolation for a given star mass is not required.The range of stellar mass is 0.74 to10.00 M_⊙.We present results in different forms of tables for easy and general application.The second stellar harmonic,required for analysis of apsidal motion of eclipsing binaries,is also listed.We also construct rotating models to determine the effect of rotation on stellar structure and derive fitting formulae for luminosity,radius and the second harmonic as a function of rotational parameter.We also compute and list colors and bolometric corrections of models required for transformation between theoretical and observational results.The results are tested for the Sun,the Hyades cluster,the slowly rotating chemically peculiar Am stars and eclipsing binaries with apsidal motion.The theoretical and observational results along isochrones are in good agreement.The grids are also applicable to rotating stars provided that equatorial velocity is given.     

The surviving companions in type Ia supernova remnants

The single-degenerate(SD)model is one of the most popular progenitor models of type Ia supernovae(SNe Ia),in which the companion star can survive after an SN Ia explosion and show peculiar properties.Therefore,searching for the surviving companion in type Ia supernova remnants(SNRs)is a potential method to verify the SD model.In the SN 1604 remnant(Kepler’s SNR),although Chandra X-ray observation suggests that the progenitor is most likely a WD+AGB system,the surviving companion has not been found.One possible reason is rapid rotation of the white dwarf(WD),causing explosion of the WD to be delayed for a spin-down timescale,and then the companion evolved into a WD before the supernova explosion,so the companion is too dim to be detected.We aim to verify this possible explanation by carrying out binary evolution calculations.In this paper,we use Eggleton’s stellar evolution code to calculate the evolution of binaries consisting of a WD+red giant(RG).We assume that the rapidly rotating WD can continuously increase its mass when its mass exceeds the Chandrasekhar mass limit(MCh=1.378 M_⊙)until the mass-transfer rate decreases to be lower than a critical value.Eventually,we obtain the final masses of a WD in the range 1.378 M_⊙ to 2.707 M_⊙.We also show that if the spin-down time is less than 10~6 yr,the companion star will be very bright and easily observed;but if the spin-down time is as long as~10~7 yr,the luminosities of the surviving companion would be lower than the detection limit.Our simulation provides guidance in hunting for the surviving companion stars in SNRs,and the fact that no surviving companion has been found in Kepler’s SNR may not be definite evidence disfavoring the SD origin of Kepler’s SN.     

KIC 5197256: an eclipsing binary containing a δ Scuti variable star

This paper reports analysis of an eclipsing binary system KIC 5197256 with a δ Sct variable component.Utilizing light-curve modeling,several stellar parameters are derived,e.g.,temperature,mass,and mass ratio.The O-C diagram is a straight line with a negative slope which means that its period is almost constant for about 2 yr.Frequency analyses are performed for the residual light curve after subtracting the binary variations.The frequency spectrum reveals that one component star is a δ Scuti variable.Large frequency separation is cross-identified with the histogram graph and the Fourier transform method.Based on the large separation and density relationship,the mean density of the δ Sct component is estimated to be 0.05 g·cm~(-3).Five frequencies with the same frequency spacing in the range of 25 d~(-1)-34 d~(-1) are detected.Statistically,the pulsation amplitudes of δ Sct stars increase with decreasing of rotations,so we propose that KIC 5197256 might have a relatively large rotational velocity,and the frequency f_(10) might be the rotation frequency.     

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

The amount of 56Ni produced in Type Ia supernova (SN Ia) explosion is probably the most important physical parameter underlying the observed correlation of SN Ia lumi-nosities with their light curves. Based on an empirical relation between the 56Ni mass and the light curve parameter △m15, we obtained rough estimates of the 56Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived 56Ni masses for different SNe Ia could vary by a factor of ten (e.g., MNi = 0.1 - 1.3 M⊙),which cannot be explained in terms of the standard Chandraseldaar-mass model (with a 56Ni mass production of 0.4 - 0.8 M⊙). Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich (with MNi 0.8 M⊙) SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more 56Ni to power the light curve. This is also consistent with the find-ing that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have △m15 < 1.2 while this ratio is only 18% in E/S0 galaxies. The nickel-poor SNe Ia (with MNi < 0.2 M⊙) may invoke the sub-Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.     

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

The amount of 56↑Ni produced in Type Ia supernova （SN Ia） explosion is probably the most important physical parameter underlying the observed correlation of SN Ia luminosities with their light curves. Based on an empirical relation between the 56↑Ni mass and the light curve parameter △m15, we obtained rough estimates of the 56↑Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived 56↑Ni masses for different SNe Ia could vary by a factor of ten （e.g., MNi = 0.1 - 1.3 M⊙）, which cannot be explained in terms of the standard Chandrasekhar-mass model （with a 56↑Ni mass production of 0.4 - 0.8 M⊙）. Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich （with MNi 〉 0.8 M⊙） SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more 56↑Ni to power the light curve. This is also consistent with the finding that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have △ml5 〈 1.2 while this ratio is only 18% in E/S0 galaxies. The nickel-poor SNe Ia （with MNi 〈 0.2 M⊙） may invoke the sub- Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.     

FUV and NIR size of the HI selected low surface brightness galaxies

How low surface brightness galaxies(LSBGs)form stars and assemble stellar mass is one of the most important questions related to understanding the LSBG population.We select a sample of 381 HI bright LSBGs with both far ultraviolet(FUV)and near infrared(NIR)observations to investigate the star formation rate(SFR)and stellar mass scales,and the growth mode.We measure the FUV and NIR radii of our sample,which represent the star-forming and stellar mass distribution scales respectively.We also compare the FUV and H band radius-stellar mass relation with archival data,to identify the SFR and stellar mass structure difference between the LSBG population and other galaxies.Since galaxy HI mass has a tight correlation with the HI radius,we can also compare the HI and FUV radii to understand the distribution of HI gas and star formation activities.Our results show that most of the HI selected LSBGs have extended star formation structure.The stellar mass distribution of LSBGs may have a similar structure to disk galaxies at the same stellar mass bins,but the star-forming activity of LSBGs happens at a larger radius than the high surface density galaxies,which may help to identify the LSBG sample from the wide-field deep u band image survey.The HI is also distributed at larger radii,implying a steeper(or not)Kennicutt-Schmidt relation for LSBGs.     

Contributions of type II and Ib/c supernovae to Galactic chemical evolution

Type II and Ib/c supernovae(SNe II and Ib/c) have made major stellar nucleosynthetic contributions to the inventories of stable nuclides during chemical evolution of the Galaxy. A case study is performed here with the help of recently developed numerical simulations of Galactic chemical evolution in the solar neighborhood to understand the contributions of SNe II and Ib/c by comparing the stellar nucleosynthetic yields obtained by two leading groups in this field. These stellar nucleosynthetic yields differ in terms of their treatment of stellar evolution and nucleosynthesis. The formulation describing Galactic chemical evolution is developed with the recently revised solar metallicity of ～0.014. Furthermore, the recent nucleosynthetic yields of stellar models based on the revised solar metallicity are also used. The analysis suggests that it could be difficult to explain, in a self-consistent manner, the various features associated with the elemental evolutionary trends over Galactic timescales by any single adopted stellar nucleosynthetic model that incorporates SNe II and Ib/c.     

初始质量为七个太阳质量的AGB星的氢燃烧壳层的特性和物质抛射机理

The property of the H-burning shell and the mecanism of the mass ejection of the AGB star are investigated by means of detailed computations carried out for the evolution of a star with its initial mass of 7. The energe generation rate εH of the H-burning shell changes periodically and has a phase-shifting with respect to that of the He-burning shell which undergoes thermonuclear runaway. For the most time of a period the value of εH is about one to three order of magnitude larger than that of the He-burning shell εHe . So the dominative energy supply of the star in the AGB phase comes from the H-burning shell. The luminosity of the star changes periodically and cynchronizes with the variation of εH . The upper limit of the luminosity of an AGB star can be obtained by the condition that the inward gravitational acceleration is balanced by the outward acceleration owing to the radiations, rotation and turbulent forces. Comparing the luminosity of the star with its upper limit, we can find that the lu     

Analysis of the X-ray emission from OB stars Ⅲ: low-resolution spectra of OB stars

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars.In our two previous papers,we study the high-resolution X-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field,mass loss rate and terminal wind velocity.Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent.In the present paper,we analyzed low-resolution X-ray spectra,using model-independent X-ray hardness values for checking the above mentioned dependencies.We establish that X-ray luminosities L_X weakly depend on the stellar magnetic field.At the same time,Lx ∝ M~(0.5) and L_X ∝ E_(kin)~(0.5),where M is the mass loss rate and E_(kin) is the kinetic energy of the wind.The X-ray luminosities decrease with growing magnetic confinement parameter η.We also argue that there is an additional(probably non-thermal) component contributed to the stellar X-ray emission.     

Investigation of near-contact semi-detached binary W UMi through observations and evolutionary models

W UMi is a near contact, semi-detached, double-lined eclipsing binary star with an orbital period of 1.7 d. Simultaneous analysis of new BV R multi-color light curves and radial velocity data yields the main astrophysical parameters of the binary and its component stars. We determined mass and radius to be M_1 = 3.22 ± 0.08 M_⊙, R_1 = 3.63 ± 0.04 R_⊙ for the primary star and M_2 = 1.44 ± 0.05 M_⊙,R_2 = 3.09 ± 0.03 R_⊙ for the secondary star. Based on analysis of mid-eclipse times, variation in the orbital period is represented by a cyclic term and a downward parabola. Mass loss from the system is suggested for a secular decrease(-0.02 s yr~(-1)) in the period. Both the mechanisms of a hypothetical tertiary star orbiting around W UMi and the surface magnetic activity of the less massive cooler companion were used to interpret periodic changes. Observational parameters were found to be consistent with binary stellar evolution models produced in the non-conservative approach of MESA at a higher metallicity than the Sun and an age of about 400 Myr for the system. Evidence that the system is rich in metal was obtained from spectral and kinematic analysis as well as evolution models. W UMi, a high mass ratio system compared to classical semi-detached binaries, is an important example since it is estimated from binary evolutionary models that the system may reach its contact phase in a short time interval.     

Database of molecular masers and variable stars

We present the database of maser sources in H2 O, OH and Si O lines that can be used to identify and study variable stars at evolved stages. Detecting the maser emission in H2 O, OH and Si O molecules toward infrared-excess objects is one of the methods for identifing long-period variables(LPVs, including miras and semiregulars), because these stars exhibit maser activity in their circumstellar shells. Our sample contains 1803 known LPV objects. Forty-six percent of these stars(832 objects) manifest maser emission in the line of at least one molecule: H2 O, OH or Si O. We use the database of circumstellar masers in order to search for LPVs which are not included in the General Catalogue of Variable Stars(GCVS). Our database contains 4806 objects(3866 objects without associations in GCVS) with maser detection in at least one molecule. Therefore it is possible to use the database in order to locate and study the large sample of LPV stars. The database can be accessed at http://maserdb.net.     

Kinematics of solar neighborhood stars and its dependence on age and metallicity

We have constructed a catalog containing the best available astrometric,photometric, radial velocity and astrophysical data for mainly F-type and G-type stars(called the Astrometric Catalog associated with Astrophysical Data, ACAD). This contains 27 553 records and is used for the purpose of analyzing stellar kinematics in the solar neighborhood. Using the Lindblad-Oort model and compiled ACAD, we calculated the solar motion and Oort constants in different age–metallicity bins. The evolution of kinematical parameters with stellar age and metallicity was investigated directly. The results show that the component of the solar motion in the direction of Galactic rotation(denoted S2) linearly increases with age, which may be a consequence of the scattering processes, and its value for a dynamical cold disk was found to be 8.0 ± 1.2 km s-1. S2 also linearly increases with metallicity, which indicates that radial migration is correlated to the metallicity gradient. On the other hand, the rotational velocity of the Sun around the Galactic center has no clear correlation with ages or metallicities of stars used in the estimation.     

An empirical model to form and evolve galaxies in dark matter halos

Based on the star formation histories of galaxies in halos with different masses, we develop an empirical model to grow galaxies in dark matter halos. This model has very few ingredients, any of which can be associated with observational data and thus be efficiently assessed. By applying this model to a very high resolution cosmological N-body simulation, we predict a number of galaxy properties that are a very good match to relevant observational data. Namely, for both centrals and satellites, the galaxy stellar mass functions up to redshift z ≈ 4 and the conditional stellar mass functions in the local universe are in good agreement with observations. In addition, the two point correlation function is well predicted in the different stellar mass ranges explored by our model. Furthermore, after applying stellar population synthesis models to our stellar composition as a function of redshift, we find that the luminosity functions in the 0.1_u,0.1_g,0.1_r,0.1_i and 0.1_z bands agree quite well with the SDSS observational results down to an absolute magnitude at about –17.0. The SDSS conditional luminosity function itself is predicted well. Finally, the cold gas is derived from the star formation rate to predict the HI gas mass within each mock galaxy. We find a remarkably good match to observed HI-to-stellar mass ratios. These features ensure that such galaxy/gas catalogs can be used to generate reliable mock redshift surveys.     

Time-dependent Behaviour of the Low Amplitude δ Scuti Star HD 52788

Wavelet transform is applied to reanalyze the low amplitude δ Scuti star HD 52788, which exhibits complex light variations with uncertain frequency solutions. We gain an insight into the strong instability of pulsation present in the star. Based on an estimate of the star‘s physical parameters, its evolutionary status is determined. An attempt of asteroseismic modelling failed to predict the observed dense frequencies. Because of its varying pulsation spectrum, HD 52788 is a distinctive and very interesting object among δ Sct stars for testing current models of stellar evolution and pulsation.     

压强电离对太阳演化模型的影响

We investigated the uncertainties in the formalism for pressure ionization and their influences on solar opacity calculations. The study is based on the formalism of the atomic configuration probability and the occupation probability for interpreting pressure ionization of stellar gases. Bound-free absorption for heavy elements is found to be affected by the effect of pressure ionization which reduces the solar interior opoacity considerably. The opacity reduction in solar interiors due to the effect of pressure ionization can reach as much as tens percent. We also calculated solar evolution models using the opacity data computed from the different formalisms for pressure ionization. The results indicate sensitivities of the calculation of solar evolution models to the opacity data of different models. The resulting structures of the Sun at present age have much different chracteristics. The central temperature has an uncertainty from 15.24 106 K to 14.52 106 K. The depth of the convection zone also has     

On the Physical Processes in Contact Binary Systems

Three importantphysical processes occurringin contact binarysystems are studied. The first one is the effect of spin, orbital rotation and tide on the structure of the components, which includes also the effect of meridian circulation on the mixing of the chemical elements in the components. The second one is the mass and energy exchange between the components. To describe the energy exchange, a new approach is introduced based on the understanding that the exchange is due to the release of the potential, kinetic and thermal energy of the exchanged mass. The third is the loss of mass and angular momentum through the outer Lagrangian point. The rate of mass loss and the angular momentum carried away by the lost mass are discussed. To show the effects of these processes, we follow the evolution of a binary system consisting of a 12M and a 5M star with mass exchange between the components and mass loss via the outer Lagrangian point, both with and without considering the effects of rotation and tide. The result shows that the effect of rotation and tide advances the start of the semi-detached and the contact phases, and delays the end of the hydrogen-burning phase of the primary. Furthermore, it can change not only the occurrence of mass and angular momentum loss via the outer Lagrangian point, but also the contact or semi-contact status of the system. Thus, this effect can result in the special phenomenon of short-term variations occurring over a slow increase of the orbital period. The occurrence of mass and angular momentum loss via the outer Lagrangian point can affect the orbital period of the system significantly, but this process can be influenced, even suppressed out by the effect of rotation and tide. The mass and energy exchange occurs in the common envelope. The net result of the mass exchange process is a mass transfer from the primary to the secondary during the whole contact phase.