arXiv subjects

Zhanwen Han

Publications and source records attributed to Zhanwen Han.

229 records · Page 13Linked to original sources

arXiv2006

Population synthesis for symbiotic stars with white dwarf accretors

We have carried out a detailed study of symbiotic stars with white dwarf accretors by means of a population synthesis code. We estimate the total number of symbiotic stars with white dwarf accretors in the Galaxy as 1,200 - 15,000. This range is compatible with observational estimates. Two crucial physical parameters that define the birthrate and number of symbiotic stars are the efficiency of accretion by white dwarfs (which greatly depends on the separation of components after common envelope stage and stellar wind velocity) and the mass of the hydrogen layer which the white dwarf can accumulate prior to the hydrogen ignition. The theoretical estimate of the Galactic occurrence rate of symbiotic novae ranges from about 1.3 to about 13.5 per year out of which weak symbiotic novae comprise about 0.5 to 6.0 per year, depending on the model assumptions. We simulate the distributions of symbiotic stars over orbital periods, masses of components, mass-loss rates of cool components, mass-accretion rates of hot components and luminosity of components. Agreement with observations is reasonable.

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arXiv2006

Binary Stellar Population Synthesis Study of Elliptical Galaxies

We determine relative stellar ages and metallicities mainly for about 80 elliptical galaxies in low and high density environments via the latest binary stellar population (BSP) synthesis model and test a latest hierarchical formation model of elliptical galaxies which adopted the new $Λ$CDM cosmology for the first time. The stellar ages and metallicities of galaxies are estimated from two high-quality published spectra line indices, i.e. H$β$ and [MgFe]. The results show that elliptical galaxies have stellar populations older than 3.9 Gyr and more metal rich than 0.02. Most of our results are in agreement with predictions of the model: First, elliptical galaxies in denser environment are redder and have older populations than field galaxies. Second, elliptical galaxies with more massive stellar components are redder while have older and more metal rich populations than less massive ones. Third, the most massive galaxies are shown to have the oldest and most metal rich stars. However, some of our results are found to be different with predictions of the galaxy formation model, i.e. the metallicity distributions of low- and high-density elliptical galaxies and the relations relating to cluster-centric distance.

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arXiv2006

A single-degenerate model for the progenitor of the Type Ia supernova 2002ic

Supernova 2002ic was an atypical Type Ia supernova (SN Ia) with evidence for substantial amounts of hydrogen associated with the system. Contrary to previous claims, we show that its unusual properties can be understood within the framework of one of the most favoured progenitor models, the so-called supersoft channel. This requires that the donor star was initially relatively massive ($\sim 3\Ms$) and that the system experienced a delayed dynamical instability, leading to a large amount of mass loss from the system in the last few $10^4 $yr before the explosion. This can produce the inferred hydrogen-rich circumstellar environment, most likely with a disc-like geometry. However, to apply these models requires a larger accretion efficiency onto the white dwarf than is assumed in present parameterizations. If this is confirmed, it would most likely increase estimates for the frequency of the single-degenerate channel. Based on population synthesis simulations we estimate that not more than 1 in 100 SNe Ia should belong to this subgroup of SNe Ia.

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arXiv2005

Evolutionary Population Synthesis for Binary Stellar Population at High Spectral Resolution: Integrated Spectral Energy Distributions and Absorption-feature Indices

Using EPS we present high resolution (0.3 Å, HRes) ISEDs from 3000 to 7000 Åand Lick/IDS absorption-line indices, for an extensive set of instantaneous burst binary stellar populations (BSPs) with binary interactions. The ages of the BSPs are in the range 1 - 15 Gyr and the metallicities are in the range 0.004 - 0.03. This HRes synthesis results can satisfy the needs of modern spectroscopic galaxy surveys, and are available on request. By comparing the synthetic continuum of BSPs at high and low resolution (LRes) we show that there is a good agreement for Z=0.02 and a tolerable disagreement for non-solar metallicity. The strength of the Balmer lines at HRes is greater than that at LRes for all metallicities. The comparison of Lick/IDS absorption-line indices at LRes and HRes, both of which are obtained by the fitting functions (FFs), shows that the discrepancies in all indices except for TiO_1 and TiO_2 are insignificant for BSPs with Z=0.004 and Z=0.02. The HRes Ca4227, Fe5015 and Mg_b indices are redder than the corresponding LRes one for BSPs with Z=0.01 and Z=0.03, this effect lowers the derived age and metallicity of the BSP. The high resolution Mg_1, Fe5709 and Fe5782 indices are bluer than those at LRes, it raises the age and metallicity. The discrepancy in these six indices is greater for BSPs with Z=0.03 in comparison to Z=0.01. At HRes we compare the Lick/IDS spectral absorption indices obtained by using the FFs with those measured directly from the synthetic spectra, and see that Ca4455, Fe4668, Mg_b and Na D indices obtained by the use of the FFs are redder for all metallicities, Fe5709 is redder at Z=0.03 and becomes to be bluer at Z=0.01 and 0.004, and other indices are bluer for all metallicities than the corresponding values measured directly from the synthetic spectra.

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arXiv2004

Inclusion of Binaries in Evolutionary Population Synthesis

Using evolutionary population synthesis we present integrated colours, integrated spectral energy distributions and absorption-line indices defined by the Lick Observatory image dissector scanner (referred to as Lick/IDS) system, for an extensive set of instantaneous burst binary stellar populations with and without binary interactions. The ages of the populations are in the range $1-15 $Gyr and the metallicities are in the range $0.0001-0.03$. By comparing the results for populations with and without binary interactions we show that the inclusion of binary interactions makes the integrated $\rm U-B$, $\rm B-V$, $\rm V-R$ and $\rm R-I$ colours and all Lick/IDS spectral absorption indices (except for $\rm H_β$) substantially smaller. In other words binary evolution makes a population appear bluer. This effect raises the derived age and metallicity of the population. We calculate several sets of additional solar-metallicity binary stellar populations to explore the influence of input parameters to the binary evolution algorithm (the common-envelope ejection efficiency and the stellar wind mass-loss rate) on the resulting integrated colours. We also look at the dependence on the choice of distribution functions used to generate the initial binary population. The results show that variations in the choice of input model parameters and distributions can significantly affect the results. However, comparing the discrepancies that exist between the colours of various models, we find that the differences are less than those produced between the models with and those without binary interactions. Therefore it is very necessary to consider binary interactions in order to draw accurate conclusions from evolutionary population synthesis work.

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arXiv2004

Structure and evolution of low mass W UMa type systems

The structure and evolution of low mass W UMa type contact binaries are discussed by employing Eggleton's stellar evolution code. Assuming that these systems completely satisfy Roche geometry, we calculate the relative radii of both components of contact binaries in different contact depth between inner and outer Roche lobes. We obtain a radius grid of contact binaries, and can ensure the surfaces of two components lying on an equipotential surface by interpolation using this radius grid when we follow the evolution of the contact binaries. We assume that the energy transfer takes place in the different regions of the common envelope to investigate the effects of the region of energy transfer on the structure and evolution of contact binaries. We find that the region of energy transfer has significant influence on the structure and evolution of contact binaries, and conclude that the energy transfer may occur in the outermost layers of the common convective envelope for W-type systems, and this transfer takes place in the deeper layers of the common envelope for A-type systems. Meanwhile, if we assume that the energy transfer takes place in the outermost layers for our model with low total mass, and find that our model steadily evolves towards a system with a smaller mass ratio and a deeper envelope, suggesting that some A-type W UMa systems with low total mass could be considered as the later evolutionary stages of W-subtype systems, and that the surface temperature of the secondary excesses that of the primary during the time when the primary expands rapidly, or the secondary contracts rapidly, suggesting that W-subtype systems may be caused by expansion of the primary, or by the contraction of the secondary.

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arXiv2004

Evolutionary Population Synthesis for Binary Stellar Populations

We present integrated colours, integrated spectral energy distributions, and absorption-line indices, for instantaneous burst solar-metallicity binary stellar populations with ages in the range 1 - 15Gyr. By comparing the results for populations with and without binary interactions we show that the inclusion of binary interactions makes the appearance of the population substantially bluer -- this is the case for each of the quantities we have considered. This effect raises the derived age and metallicity of the population. Therefore it is necessary to consider binary interactions in order to draw accurate conclusions from evolutionary population synthesis work.

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arXiv2004

Integrated Spectral Energy Distributions and Absorption Feature Indices of Single Stellar Populations

Using evolutionary population synthesis, we present integrated spectral energy distributions and absorption-line indices defined by the Lick Observatory image dissector scanner (referred to as Lick/IDS) system, for an extensive set of instantaneous burst single stellar populations (SSPs). The ages of the SSPs are in the range 1-19 Gyr and the metallicities [Fe/H] are in the range -2.3 - 0.2. Our models use the rapid single stellar evolution algorithm of Hurley, Pols and Tout for the stellar evolutionary tracks, the empirical and semi-empirical calibrated BaSeL-2.0 model of Lejeune, Cuisinier and Buser for the library of stellar spectra and the empirical fitting functions of Worthey, Faber, Gonzalez and Burstein for the Lick/IDS spectral absorption-feature indices. Applying our synthetic Lick/IDS absorption-line indices to the merit function, we obtain the age and the metallicity of the central region of M32, which can be well explained by an instantaneous SSP with an age of 6.5 Gyr and a metallicity similar to solar. Applying the derived age and the metallicity from the merit function to a number of index-index diagrams, we find that the plots of Hbeta-Fe5015 and Hbeta-Fe5782 are the best index-index diagrams from which we can directly obtain reasonable age and metallicity.

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arXiv2003

The single degenerate channel for the progenitors of type Ia supernovae

We have carried out a detailed study of one of the most popular evolutionary channels for the production of Type Ia supernova (SN Ia) progenitors, the semi-degenerate channel (CO+MS), where a carbon/oxygen (CO) white dwarf (WD) accretes matter from an unevolved or slightly evolved non-degenerate star until it reaches the Chandrasekhar mass limit. Employing Eggleton's stellar evolution code and adopting the prescription of Hachisu et al. (1999) for the accretion efficiency, we have carried out full binary evolution calculations for about 2300 close WD binary systems and mapped out the initial parameters in the orbital period -- secondary mass ($P$--$M_2$) plane (for a range of WD masses) which lead to a successful Type Ia supernova. We obtained accurate, analytical fitting formulae to describe this parameter range which can be used for binary population synthesis (BPS) studies. The contours in the $P$--$M_2$ plane differ from those obtained by Hachisu et al. (1999) for low-mass CO WDs, which are more common than massive CO WDs. We show that white dwarfs with a mass as low as $0.67M_\odot$ can accrete efficiently and reach the Chandrasekhar limit. We have implemented these results in a BPS study to obtain the birthrates for SNe Ia and the evolution of birthrates with time of SNe Ia for both a constant star formation rate and a single star burst. The birthrates are somewhat lower than (but comparable to) those inferred observationally.

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arXiv2003

On the formation and evolution of black-hole binaries

We present the results of a systematic study of the formation and evolution of binaries containing black holes and normal-star companions with a wide range of masses. We first reexamine the standard formation scenario for close black-hole binaries, where the spiral-in of the companion in the envelope of a massive star causes the ejection of the envelope. We estimate the formation rates for different companion masses and different assumptions about the common-envelope structure and other model parameters. We find that black-hole binaries with intermediate- and high-mass secondaries can form for a wide range of assumptions, while black-hole binaries with low-mass secondaries can only form with apparently unrealistic assumptions (in agreement with previous studies). We then present detailed binary evolution sequences for black-hole binaries with secondaries of 2 to 17 Msun and demonstrate that in these systems the black hole can accrete appreciably even if accretion is Eddington limited (up to 7 Msun for an initial black-hole mass of 10 Msun) and that the black holes can be spun up significantly in the process. We discuss the implications of these calculations for well-studied black-hole binaries (in particular GRS 1915+105), ultra-luminous X-ray sources and Cygnus X-1. Finally, we discuss how some of the assumptions in the standard model could be relaxed to allow the formation of low-mass, short-period black-hole binaries which appear to be very abundant in Nature. (Abstract abridged)

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arXiv2003

The origin of sdB stars (II)

We have carried out a detailed binary populations synthesis (BPS) study of the formation of subdwarf B (sdB) stars and related objects (sdO, sdOB stars) using the latest version of the BPS code developed by Han et al.(1994, 1995a, 1995b, 1998, 2001). We systematically investigate the importance of the five main evolutionary channels in which the sdB stars form after one or two common-envelope (CE) phases, one or two phases of stable Roche-lobe overflow (RLOF) or as the result of the merger of two helium white dwarfs (WD) (see Han et al. 2002, Paper I). Our best BPS model can satisfactorily explain the main observational characteristics of sdB stars, in particular their distributions in the orbital period - minimum companion mass diagram and in the effective temperature - surface gravity diagram, their distributions of orbital period, log (g theta^4), and mass function, their binary fraction and the fraction of sdB binaries with WD companions, their birthrates and their space density. We obtain a Galactic formation rate, a total number in the Galaxy, the intrinsic binary fraction for sdB stars. We also predict a distribution of masses for sdB stars that is wider than is commonly assumed and that some sdB stars have companions of spectral type as early as B. The percentage of A type stars with sdB companions can in principle be used to constrain some of the important parameters in the binary evolution model. We conclude that (a) the first RLOF phase needs to be more stable than is commonly assumed; (b) mass transfer in the first stable RLOF phase is non-conservative, and the mass lost from the system takes away a specific angular momentum similar to that of the system; (c) common-envelope ejection is very efficient.

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arXiv2002

The Origin of Subdwarf B Star (I): the Formation Channels

Subdwarf B (sdB) stars (and related sdO/sdOB stars) are believed to be helium core-burning objects with very thin hydrogen-rich envelopes. In recent years it has become increasingly clear from observational surveys that a large fraction of these objects are members of binary systems. To better understand their formation, we here present the results of a detailed investigation of the three main binary evolution channels that can lead to the formation of sdB stars: the common envelope (CE) ejection channel, the stable Roche lobe overflow (RLOF) channel and the double helium white dwarfs (WDs) merger channel. We obtained the conditions for the formation of sdB stars from each of these channels using detailed stellar and binary evolution calculations where we modelled the detailed evolution of sdB stars and carried out simplified binary population synthesis simulations. The observed period distribution of sdB stars in compact binaries strongly constrains the CE ejection parameters. We also present the distribution of sdB stars in the $T_{\rm eff}$ - $\log g$ diagram, the Hertzsprung-Russell diagram and the distribution of mass functions.

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arXiv2000

Low and intermediate-mass close binary evolution and the initial - final mass relation

Using Eggleton's stellar evolution code, we carry out 150 runs of Pop I binary evolution calculations, with the initial primary mass between 1 and 8 solar masses the initial mass ratio between 1.1 and 4, and the onset of Roche lobe overflow (RLOF) at an early, middle, or late Hertzsprung-gap stage. We assume that RLOF is conservative in the calculations, and find that the remnant mass of the primary may change by more than 40 per cent over the range of initial mass ratio or orbital period, for a given primary mass. This is contrary to the often-held belief that the remnant mass depends only on the progenitor mass if mass transfer begins in the Hertzsprung gap. We fit a formula, with an error less than 3.6 per cent, for the remnant (white dwarf) mass as a function of the initial mass of the primary, the initial mass ratio, and the radius of the primary at the onset of RLOF. We also find that a carbon-oxygen white dwarf with mass as low as 0.33 solar masses may be formed if the primary's initial mass is around 2.5 solar masses.

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