arXiv subjects

Zhanwen Han

Publications and source records attributed to Zhanwen Han.

At least 217 records · Page 12Linked to original sources

arXiv2009

Primordial Binary Evolution and Blue Stragglers

In this paper, we systematically studied blue stragglers produced from primordial binary evolution via a binary population synthesis approach, and examined their contribution to the integrated spectral energy distributions of the host clusters. The mass transfer efficiency, $β$, is an important parameter for the final products (then blue stragglers) after mass transfer, and it is set to be 0.5 except for case A binary evolution. The study shows that primordial binary evolution may produce blue stragglers at any given times and that different evolutionary channels are corresponding for blue stragglers in different visual magnitude regions (in V band) on the colour-magnitude diagram (CMD) of clusters. The specific frequency of blue stragglers obtained from primordial binary evolution decreases with time first, and then increases again when the age is larger than 10Gyr, while that from angular momentum loss induced by magnetic braking in low-mass binaries increases with time and exceeds that of primordial binary evolution in a population older than 3 Gyr. Meanwhile, blue stragglers resulting from primordial binary evolution are dominant contributors to the ISEDs in ultraviolet and blue bands in a population between 0.3 and 2.0 Gyr. The value of $β$ significantly affects on the final results. For old open clusters, the assumption of $β=1$ when the primary is in HG at the onset of mass transfer matches the observations better than that of $β=0.5$ from the locations of BSs on the CMDs. Our study also shows that, for most Galactic open clusters, the specific frequency of blue stragglers obtained from our simulations is lower than that of observations, which is puzzling.

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arXiv2008

On type Ia supernovae and the formation of single low-mass white dwarfs

There is still considerable debate over the progenitors of type Ia supernovae (SNe Ia). Likewise, it is not agreed how single white dwarfs with masses less than ~0.5 Msun can be formed in the field, even though they are known to exist. We consider whether single low-mass white dwarfs (LMWDs) could have been formed in binary systems where their companions have exploded as a SN Ia. In this model, the observed single LMWDs are the remnants of giant-branch donor stars whose envelopes have been stripped off by the supernova explosion. We investigate the likely remnants of SNe Ia, including the effects of the explosion on the envelope of the donor star. We also use evolutionary arguments to examine alternative formation channels for single LMWDs. In addition, we calculate the expected kinematics of the potential remnants of SNe Ia. SN Ia in systems with giant-branch donor stars can naturally explain the production of single LMWDs. It seems difficult for any other formation mechanism to account for the observations, especially for those single LMWDs with masses less than ~0.4 Msun. Independent of those results, we find that the kinematics of one potentially useful population containing single LMWDs is consistent with our model. Studying remnant white-dwarf kinematics seems to be a promising way to investigate SN Ia progenitors. The existence of single LMWDs appears to constitute evidence for the production of SNe Ia in binary systems with a red-giant donor star. Other single white dwarfs with higher space velocities support a second, probably dominant, population of SN Ia progenitors which contained main-sequence or subgiant donor stars at the time of explosion. The runaway stars LP400-22 and US 708 suggest the possibility of a third formation channnel for some SNe Ia in systems where the donor stars are hot subdwarfs.

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arXiv2008

Hot Subdwarfs in Binaries as the Source of the Far-UV Excess in Elliptical Galaxies

The excess of far-ultraviolet (far-UV) radiation in elliptical galaxies has remained one of their most enduring puzzles. In contrast, the origin of old blue stars in the Milky Way, hot subdwarfs, is now reasonably well understood: they are hot stars that have lost their hydrogen envelopes by various binary interactions. Here, we review the main evolutionary channels that produce hot subdwarfs in the Galaxy and present the results of binary population synthesis simulations that reproduce the main properties of the Galactic hot-subdwarf population. Applying the same model to elliptical galaxies, we show how this model can explain the main observational properties of the far-UV excess, including the far-UV spectrum, without the need to invoke ad hoc physical processes. The model implies that the UV excess is not a sign of age, as has been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population.

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arXiv2008

The evolutionary status of W Ursae Majoris-type systems

Well-determined physical parameters of 130 W UMa systems have been collected from the literature. Based on these data, the evolutionary status and dynamical evolution of W UMa systems are investigated. It is found that there is no evolutionary difference between W- and A-type systems in $M-J$ diagram which is consistent with the results derived from the analysis of observed spectral type, $M-R$ and $M-L$ diagrams of W UMa systems. $M-R$ and $M-L$ diagrams of W- and A-type systems indicate that a large amount of energy should be transferred from the more massive to the less massive component so that they are not in thermal equilibrium and undergo thermal relaxation oscillation (TRO). Meanwhile, the distribution of angular momentum, together with the distribution of mass ratio, suggests that the mass ratio of the observed W UMa systems is decreased with the decrease of their total mass. This could be the result of the dynamical evolution of W UMa systems which suffer angular momentum loss (AML) and mass loss due to magnetic stellar wind (MSW). Consequently, the tidal instability forces these systems towards the lower q values and finally to fast rotating single stars.

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arXiv2008

The single degenerate channel for the progenitor of type Ia supernovae

We have carried out a detailed study of the single-degenerate channel for the progenitors of type Ia supernovae (SNe Ia). In the model, a carbon-oxygen white dwarf (CO WD) accretes hydrogen-rich material from an unevolved or a slightly evolved non-degenerate companion to increase its mass to Chandrasekhar mass limit. Incorporating the prescription of \cite{HAC99a} for the accretion efficiency into Eggleton's stellar evolution code and assuming that the prescription is valid for all metallicities, we performed binary stellar evolution calculations for more than 25,000 close WD binary systems with various metallicities. The initial parameter spaces for SNe Ia are presented in an orbital period-secondary mass ($\log P_{\rm i}, M_{\rm 2}^{\rm i}$) plane for each $Z$. Adopting the results above, we studied the birth rate of SNe Ia for various $Z$ via binary population synthesis. From the study, we see that for a high $Z$, SNe Ia occur systemically earlier and the peak value of the birth rate is larger if a single starburst is assumed. The Galactic birth rate from the channel is lower than (but comparable to) that inferred from observations. We also showed the distributions of the parameters of the binary systems at the moment of supernova explosion and the distributions of the properties of companions after supernova explosion. The former provides physics input to simulate the interaction between supernova ejecta and its companion, and the latter is helpful to search for the companions in supernova remnants.

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arXiv2008

How binary interactions affect spectral stellar population synthesis

Single-star stellar population (ssSSP) models are usually used for stellar population studies. However, more than 50% of stars are in binaries and evolve differently from single stars. This suggests that the effects of binary interactions should be considered when modeling the stellar populations of galaxies and star clusters. Via a rapid spectral stellar population synthesis (RPS) model, we give detailed studies of the effects of binary interactions on the Lick indices and colours of stellar populations, and on the determination of the stellar ages and metallicities of populations. The results show that binary interactions make stellar populations less luminous, bluer, with larger age-sensitive Lick index (Hbeta) and less metallicity-sensitive indices (e.g., Mgb, Fe5270 and Fe5335) compared to ssSSPs. It also shows that when ssSSP models are used to determine the ages and metallicities of stellar populations, lower ages or metallicities will be obtained, taking two line indices (Hbeta and [MgFe]) and two colours (e.g., u-R and R-K), respectively. Some relations for linking the stellar-population parameters obtained by ssSSPs to those obtained by binary-star stellar populations (bsSSPs) are presented in the work. This can help us to get some absolute values for stellar-population parameters and is useful for absolute studies. However, it is found that the relative luminosity-weighted stellar ages and metallicities obtained via ssSSPs and bsSSPs are similar. This suggests that ssSSPs can be used for most stellar population studies, except in some special cases.

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arXiv2008

Chemical Abundances in Symbiotic Stars

We have carried out a study of the chemical abundances of $^1$H, $^4$He, $^{12}$C, $^{13}$C, $^{14}$N, $^{15}$N, $^{16}$O, $^{17}$O, $^{20}$Ne and $^{22}$Ne in symbiotic stars (SSs) by means of a population synthesis code. We find that the ratios of the number of O-rich SSs to that of C-rich SSs in our simulations are between 3.4 and 24.1, depending on the third dredge-up efficiency $λ$ and the terminal velocity of the stellar wind $v(\infty)$. The fraction of SSs with $extrinsic$ C-rich cool giants in C-rich cool giants ranges from 2.1% to 22.7%, depending on $λ$, the common envelope algorithm and the mass-loss rate. Compared with the observations, the distributions of the relative abundances of $^{12}$C/$^{13}$C vs. [C/H] of the cool giants in SSs suggest that the thermohaline mixing in low-mass stars may exist. The distributions of the relative abundances of C/N vs. O/N, Ne/O vs. N/O and He/H vs. N/O in the symbiotic nebulae indicate that it is quite common that the nebular chemical abundances in SSs are modified by the ejected materials from the hot components. Helium overabundance in some symbiotic nebulae may be relevant to a helium layer on the surfaces of white dwarf accretors.

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arXiv2008

Mass transfer from a giant star to a main sequence companion and its contribution to long-orbital-period blue stragglers

Binary population synthesis shows that mass transfer from a giant star to a main-sequence (MS) companion may account for some observed long-orbital period blue stragglers. However, little attention {\bf is paid to this blue straggler formation scenario} as dynamical instability often happens when the mass donor is a giant star. In this paper, we have studied the critical mass ratio, $q_{\rm c}$, for dynamically stable mass transfer from a giant star to a MS companion using detailed evolution calculations. The results show that a more evolved star is generally less stable for Roche lobe overflow. Meanwhile, $q_{\rm c}$ almost linearly increases with the amount of the mass and angular momentum {\bf lost} during mass transfer, but has little dependance on stellar wind. To conveniently use the result, we give a fit of $q_{\rm c}$ as a function of the stellar radius at the onset of Roche lobe overflow and of the mass transfer efficiency during the Roche lobe overflow. To examine the formation of blue stragglers from mass transfer between giants and MS stars, we have performed Monte Carlo simulations with various $q_{\rm c}$. {\bf The simulations show that some binaries with the mass donor on the first giant branch may contribute to blue stragglers with $q_{\rm c}$ obtained in this paper but will not from previous $q_{\rm c}$. Meanwhile, from our $q_{\rm c}$, blue stragglers from the mass transfer between an AGB star and a MS companion may be more numerous and have a wider range of orbital periods than those from the other $q_{\rm c}$.

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arXiv2008

Stellar age versus mass of early-type galaxies in the Virgo Cluster

The flux excess of elliptical galaxies in the far-ultraviolet can be reproduced by population synthesis models when accounting for the population of old hot helium-burning subdwarf stars. This has been achieved by Han and coworkers through a quantitative model of binary stellar evolution. Here, we compare the resulting evolutionary population synthesis model to the GALEX far-near ultraviolet colors (FUV-NUV) of Virgo cluster early-type galaxies that were published by Boselli and coworkers. FUV-NUV is reddest at about the dividing luminosity of dwarf and giant galaxies, and becomes increasingly blue for both brighter and fainter luminosities. This behavior can be easily explained by the binary model with a continuous sequence of longer duration and later truncation of star formation at lower galaxy masses. Thus, in contrast to previous conclusions, the GALEX data do not require a dichotomy between the stellar population properties of dwarfs and giants. Their apparently opposite behavior in FUV-NUV occurs naturally when the formation of hot subdwarfs through binary evolution is taken into account.

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arXiv2008

Companion Stars of Type Ia Supernovae

The WD+MS channel of the single-degenerate scenario is currently favourable for progenitors of type Ia supernovae (SNe Ia). Incorporating the results of detailed binary evolution calculations for this channel into the latest version of a binary population synthesis code, I obtained the distributions of many properties of the companion stars at the moment of SN explosion. The properties can be verified by future observations.

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arXiv2008

An Isochrone Database and a Rapid Model for Stellar Population Synthesis

We first presented an isochrone database that can be widely used for stellar population synthesis studies and colour-magnitude diagram (CMD) fitting. The database consists of the isochrones of both single star and binary star simple stellar populations (ss-SSPs and bs-SSPs). The ranges for the age and metallicity of populations are 0--15 Gyr and 0.0001--0.03, respectively. All data are available for populations with Salpeter IMF and Chabrier IMF. Then based on the isochrone database, we built a rapid stellar population synthesis (RPS) model and calculated the high-resolution (0.3 AA) integrated spectral energy distributions (SEDs), Lick indices and colour indices for bs-SSPs and ss-SSPs. In particular, we calculated the UBVRIJHKLM colours, ugriz colours and some composite colours that consist of magnitudes on different systems. As an example for applying the isochrone database for CMD fitting, we fitted the CMDs of star clusters M67 and NGC1868 and obtained their distance moduli, colour excesses, stellar metallicities and ages. The results showed that the isochrones of bs-SSPs are closer to those of real star clusters. The isochrone database, colours, Lick indices of populations can be downloaded via http://www.ynao.ac.cn/~bps/zhongmu/download.htm

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arXiv2008

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 $\triangle m_{15}$, 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., $M_{\rm Ni}=0.1 - 1.3$ $M_{\odot}$), which cannot be explained in terms of the standard Chandrasekhar-mass model (with a $^{56}$Ni mass production of 0.4 -- 0.8 $M_{\odot}$). 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 $M_{\rm Ni}$ $>$ 0.8 $M_{\odot}$) 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 $Δm_{15} < 1.2$ while this ratio is only 18% in E/S0 galaxies. On the other hand, the nickel-poor SNe Ia (with $M_{\rm Ni}$ $<$ 0.2 $M_{\odot}$) 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.

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arXiv2007

Fitting Formulae for the Effects of Binary Interactions on Lick Indices and Colours of Stellar Populations

More than about 50% stars of galaxies are in binaries, but most stellar population studies take single star-stellar population (ss-SSP) models, which do not take binary interactions into account. In fact, the integrated peculiarities of ss-SSPs are various from those of stellar populations with binary interactions (bs-SSPs). Therefore, it is necessary to investigate the effects of binary interactions on the Lick indices and colours of populations detailedly. We show some formulae for calculating the difference between the Lick indices and colours of bs-SSPs, and those of ss-SSPs. Twenty-five Lick indices and 12 colours are studied in the work. The results can be conveniently used for estimating the effects of binary interactions on stellar population studies and for adding the effects of binary interactions into existing ss-SSP models. The results and a few procedures can be obtained on request to the authors or via http://www.ynao.ac.cn/~bps/zhongmu/download.htm

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arXiv2007

Colour pairs for constraining the age and metallicity of stellar populations

Using a widely used stellar population synthesis model, we study the ability of using pairs of AB system colours to break the well-known stellar age--metallicity degeneracy and give constraints on the two stellar-population parameters (age and metallicity). The relative age and metallicity sensitivities of AB system colours that relate to u, B, g, V, r, R, i, I, z, J, H, and K bands are presented, and the abilities of various colour pairs for breaking the age--metallicity degeneracy are quantified by an uncertainty parameter (UP) method. Our results suggest that a few pairs of colours can be used to constrain the two above stellar-population parameters. This will be very useful for exploring the stellar populations of distant galaxies. In detail, colour pairs [(r-K), (u-R)] and [(r-K), (u-r)] are shown to be the best pairs for estimating stellar ages and metallicities. They can constrain two stellar-population parameters on average with age uncertainties less than 3.89 Gyr and metallicity uncertainties less than 0.34 dex for typical uncertainties in colours. Some other colour pairs, such as [(R-K), (u-R)], [(I-K), (u-R)], [(R-K), (u-r)] and [(i-J), (u-R)], can possibly be used for constraining the two parameters, too. As a whole, our results suggest that colours relating to both UBVRIJHK and ugriz magnitudes are much better than either UBVRIJHK colours or ugriz colours for breaking the well-known degeneracy. The results also show that the stellar ages and metallicities of galaxies observed by the Sloan Digital Sky Survey (SDSS) and the Two-Micron All-Sky Survey (2MASS) can be estimated via photometry data. It is also shown that the colours can be used in conjunction with line indices to measure stellar-population parameters.

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arXiv2007

Binary coalescence from case A evolution -- mergers and blue stragglers

We constructed some main-sequence mergers from case A binary evolution and studied their characteristics via Eggleton's stellar evolution code. Both total mass and orbital angular momentum are conservative in our binary evolutions. Some mergers might be on the left of the ZAMS as defined by normal surface composition on a CMD because of enhanced surface helium content. The study also shows that central hydrogen content of the mergers is independent of mass. As a consequence, we fit the formula of magnitude and B-V of the mergers when they return back to thermal equilibrium with maximum error 0.29 and 0.037, respectively. Employing the consequences above, we performed Monte Carlo simulations to examine our models in NGC 2682 and NGC 2660. In NGC 2682, binary mergers from our models cover the region with high luminosity, but its importance is much less than that of AML. Our results are well-matched to the observations of NGC2660 if there is about 0.5Mo of mass loss in the merger process.

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arXiv2007

How young stellar populations affect the ages and metallicities of galaxies

Some studies show that there are recent star formations in galaxies including early type galaxies. This suggests that we possibly did not obtain the accurate ages and metallicities of the dominating stellar populations of galaxies from colors. We study how young populations (YSPs) in composite stellar populations affect the colors of star systems and analyze how the stellar ages and metallicities derived from colors possibly deviate from those of the dominating populations of composite star systems. Some possible distributions of the differences between stellar ages and metallicities determined by colors and those of the dominating stellar populations of composite stellar systems are presented in this work. Furthermore, some possible distributions of the differences between colors of composite stellar populations and those of their dominating stellar populations are also shown in this paper. In addition, we compare the stellar ages and metallicities determined respectively by a color pair and two line strength indices. The results obtained by a color pair and those obtained by a few independent colors are also compared in the paper. We also discuss about how the YSPs may affect the Fundamental Plane and Kormendy relation of early type galaxies. The main results can be downloaded from the CDS.

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arXiv2007

Binary Stars as the Source of the Far-UV Excess in Elliptical Galaxies

The discovery of an excess of light in the far-ultraviolet (UV) spectrum in elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. This has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population.

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arXiv2007

Potential of colors for determining age and metallicity of stellar populations

Colors are usually not used for constraining stellar populations because they are thought to have the well-known agemetallicity degeneracy, but some recent works show that colors can also be used. A simple stellar population synthesis model is widely used, but there is no analysis for its colors. We try to find colors that can potentially be used to determine the age and metallicity of stellar populations by the standard model. Principal component analysis and relative sensitive parameter techniques are used in this work. As a result, U-K, U-H, U-J, B-K, B-H, U-I, B-J, and V-K are found to be more important for studying populations than others. Pairs of colors such as B-K and B-V are found to be able to disentangle the stellar age-metallicity degeneracy via the high-resolution model, while pairs such as U-K and R-I may be used instead when the low-resolution model is used. Furthermore, the u-g and r-i colors of the low-resolution model seem to have the same potential, but there are no such colors for the high-resolution one. In conclusion, some colors have been shown to have the potential to determine the age and metallicity of stellar populations, but relative metallicity and age sensitivities of colors in different stellar population synthesis models are usually different. In addition, minor star formations will make star systems look younger and more metal rich than their dominating populations.

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