arXiv subjects

Zhongmu Li

Publications and source records attributed to Zhongmu Li.

At least 37 records · Page 2Linked to original sources

arXiv2013

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

More than about 50% stars are in binaries, but most stellar population studies take single star stellar population (ssSSP) models, which do not take binary interactions into account. In fact, the integrated peculiarities of ssSSPs are various from those of stellar populations with binary interactions (bsSSPs). 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 bsSSPs, and those of ssSSPs. Twenty-five Lick indices and 12 colours are studied in the work. The results can be conveniently used for calculating the effects of binary interactions on stellar population studies and for adding the effects of binary interactions into present ssSSP models. The electrical data and fortran procedures of the paper can be obtained on request to the authors.

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arXiv2013

Potential Importance of Binary Evolution in UV-Optical Spectral Fitting of Early-Type Galaxies

Binaries are very common in galaxies, and more than half of Galactic hot subdwarf stars, which are thought as a possible origin of UV-upturn of old stellar populations, are found in binaries. Previous works showed that binary evolution can make the spectra of binary star populations significantly different from those of single star populations. However, the effect of binary evolution has not been taken into account in most works of spectral fitting of galaxies. This paper studies the role of binary evolution in spectral fitting of early-type galaxies, via a stellar population synthesis model including both single and binary star populations. Spectra from ultraviolet to optical band are fitted to determine a few parameters of galaxies. The results show that the inclusion of binaries in stellar population models may lead to obvious change in the determination of some galaxy parameters and therefore it is potentially important for spectral studies. In particular, the ages of young components of composite stellar populations become much older when using binary star population models instead of single star population models. This implies that binary star population models will measure significantly different star formation histories (SFHs) for galaxies compared to single star population models. In addition, stellar population models with binary interactions measure larger dust extinctions than single star population models on average. It suggests that when using binary star population models instead of single star population models, negative extinctions are possibly unnecessary in spectral fitting of early-type galaxies.

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arXiv2013

Combined Effects of Binaries and Stellar Rotation on the Color-Magnitude Diagrams of Intermediate-Age Star Clusters

About seventy percent of intermediate-age star clusters in the Large Magellanic Clouds have been confirmed to have broad main sequence, multiple or extended turn-offs and dual red giant clumps. The observed result seems against the classical idea that such clusters are simple stellar populations. Although many models have been used for explaining the results via factors such as prolonged star formation history, metallicity spread, differential redenning, selection effect, observational uncertainty, stellar rotation, and binary interaction, the reason for the special color-magnitude diagrams is still uncertain. We revisit this question via the combination of stellar rotation and binary effects. As a result, it shows "golf club" color-magnitude diagrams with broad or multiple turn-offs, dual red clump, blue stragglers, red stragglers, and extended main sequences. Because both binaries and massive rotators are common, our result suggests that most color-magnitude diagrams including extended turn-off or multiple turn-offs can be explained using simple stellar populations including both binary and stellar rotation effects, or composite populations with two components.

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arXiv2013

Using colours to determine the stellar ages and metallicities of distant galaxies

The determination of stellar populations of galaxies are important for studying the formation and evolution of galaxies, because all galaxies contain many stars and they evolve with galaxies. Spectra data are usually used to determine the stellar populations of nearby galaxies as they have the ability to disentangle the degeneracy between stellar age and metallicity. However, it is difficult to give similar studies to distant (e.g., $z >$ 0.3) galaxies because of the lack of reliable spectra data. This is actually limited by current observational equipments and methods. In fact, the information of the stellar ages and metallicities of distant galaxies are crucial for solving the problem of galaxy formation and evolution. Colours can give us some information of the stellar populations of distant galaxies. In the paper, I introduce our works about using colours to estimate the ages and metallicities of the stellar populations of galaxies. Some potential colours for studying stellar-population parameters (age and metallicity) and their sensitivities to stellar-population parameters are reviewed. A new composite stellar population model that includes both single stars and binary stars is also introduced.

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arXiv2013

Rapid Binary Stellar Population Synthesis

Binary stars are common and it is necessary to model stellar populations using binary stars. We introduce a method to model binary-star stellar populations quickly. The method can also be used to model single-star stellar populations. The method is called rapid stellar population synthesis and it is based on a statistical isochrone database of both single- and bianry-star stellar populations. The database can be obtained easily from the CDS (or on request to the authors) and can be used conveniently for most stellar population studies and colour-magnitude fits. The main feature of using such a technique for modeling stellar populations is that it can save much computing time. Comparing to the method which evolves binary-stars using stellar evolution codes directly, the rapid stellar population synthesis method takes only one of 200,000 of the computing time. Although we can use the database to model stellar populations quickly, the involved uncertainties in spectra synthesis is small ($\sim$ 0.8%). I also introduce the method that uses the spectral energy distributions (SEDs) of single stellar populations to model the SEDs of composite populations.

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arXiv2013

Application of Binary Evolution in Astrophysical Studies

Binary stars evolve differently from single stars, thus binary evolution is very useful for astrophysical studies. This paper discusses the application of binary evolution in the studies of stars, star clusters, galaxies, and cosmology. In particular, I concentrate on the use of binary evolution in colour-magnitude diagram and spectral energy distribution studies of star clusters and galaxies.

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arXiv2013

A Binary Study of Colour-Magnitude Diagrams of 12 Globular Clusters

Binary stars are common in star clusters and galaxies, but the detailed effects of binary evolution are not taken into account in some colour-magnitude diagram (CMD) studies. This paper studies the CMDs of twelve globular clusters via binary-star stellar populations. The observational CMDs of star clusters are compared to those of binary-star populations, and then the stellar metallicities, ages, distances and reddening values of these star clusters are obtained. The paper also tests the different effects of binary and single stars on CMD studies. It is shown that binaries can fit the observational CMDs of the sample globular clusters better compared to single stars. This suggests that the effects of binary evolution should be considered when modeling the CMDs and stellar populations of star clusters and galaxies.

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arXiv2012

Photometric study of the short-period RS CVn eclipsing binary DV Psc

We present our new photometry of DV Psc obtained in 2010 and 2011, and new spectroscopic observation on Feb. 14, 2012. During our observations, three flare-like events might be detected firstly in one period on DV Psc. The flare rate of DV Psc is about 0.017 flares per hour. Using Wilson-Devinney program, we derived the preliminary starspot parameters. Moreover, the magnetic cycle is 9.26(+/-0.78) year analyzed by variabilities of Max.I - Max.II.

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arXiv2012

Magnetic activity of several low-mass eclipsing binary

We present our new photometric and spectroscopic observations of NSVS 02500276, NSVS 07453183, NSVS 11868841, NSVS 06550671 and NSVS 10653195. The first flare-like event was detected on NSVS07453183. Using the Wilson-Devinney program, the preliminary orbital solutions and starspot parameters are derived. The chromospheric activity indicators show NSVS10653195 and NSVS06550671 are active. Then, we discuss the starspot evolution on the short and long term scale. In the end, we give our future plan.

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arXiv2012

Integrated spectral energy distributions of binary star composite stellar populations

This paper presents theoretical integrated spectral energy distributions (SEDs) of binary star composite stellar populations (bsCSPs) in early-type galaxies, and how the bsCSP model can be used for spectral studies of galaxies. All bsCSPs are built basing on three adjustable inputs (metallicity, ages of old and young components). The effects of binary interactions and stellar population mixture are taken into account. The results show some UV-upturn SEDs naturally for bsCSPs. The SEDs of bsCSPs are affected obviously by all of three stellar population parameters, and the effects of three parameters are degenerate. This suggests that the effects of metallicity, and the ages of the old (major in stellar mass) and young (minor) components of stellar populations should be taken into account in SED studies of early-type galaxies. The sensitivities of SEDs at different wavelengths to the inputs of a stellar population model are also investigated. It is shown that UV SEDs are sensitive to all of three stellar population parameters, rather than to only stellar age. Special wavelength ranges according to some SED features that are relatively sensitive to stellar metallicity, young-component age, and old-component age of bsCSPs are found by this work. For example, the shapes of SEDs with wavelength ranges of 5110-5250AA, 5250--5310AA, 5310--5350AA, 5830--5970AA, 20950--23550AA are relatively sensitive to the stellar metallicity of bsCSPs. The shapes of SEDs within 965-985AA, 1005--1055AA, 1205--1245AA are sensitive to old-component age, while SED features within the wavelength ranges of 2185--2245AA, 2455--2505AA, 2505--2555AA, 2775--2825AA, 2825--2875AA to young-component age.

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arXiv2012

Characteristics of solar-like oscillations of secondary red clump stars

We calculated the populations of core-helium-burning (CHeB) stars and found that the secondary red clump (SRC) stars can form an SRC peak in the distributions of the frequency of maximum seismic amplitude ($ν_{max}$) and mean large-frequency separation ($Δν$) of CHeB stars when metallicity $Z \geq$ 0.02. The $ν_{max}$ and $Δν$ of CHeB stars are dependent not only on He core mass but on H-shell burning. The SRC peak is composed of the CHeB stars with mass roughly between the critical mass M_{Hef} and M_{Hef}+0.2 while He core mass is between about 0.33 and 0.36 M_{sun}. The location of the SRC peak can be affected by the mixing-length parameter $α$, metallicity $Z$, and overshooting parameter $δ_{ov}$. A decrease in $α$ or increase in $Z$ or $δ_{ov}$ leads to a movement of the SRC peak towards a lower frequency. However, the change in $Z$ and $α$ only slightly affects the value of M_{Hef} but the variation in $δ_{ov}$ can significantly affects the value of M_{Hef}. Thus the SRC peak might aid in determining the value of M_{Hef} and calibrating $δ_{ov}$. In addition, the effects of convective acceleration of SRC stars and the $ν_{max}$ of `semi-degenerate' stars decreasing with mass result in the appearance of a shoulder between about 40 and 50 $μ$hz in the \dnu{} distribution. However, the convective acceleration of stars with M < M_{Hef} leads to the deficit in the $ν_{max}$ distribution between about 9 and 20 $μ$hz{}. Moreover, the value of the parameter $b$ of the relation between $ν_{max}$ and $Δν$ for the populations with M > M_{Hef} is obviously larger than that for the populations with $M <$ \dmhef{}.

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arXiv2011

The effect of metallicity on the delay-time distribution of type Ia supernova

Measuring the delay-time distribution (DTD) of type Ia supernova(SNe Ia) is an important way to constrain the progenitor nature of SNe Ia. Recently, Strolger et al. (2010) obtained a very delayed DTD, which is much different from other measurements. They suggested that metallicity could be the origin of their delayed DTD. In this paper, we show the effect of metallicity on the DTD of SNe Ia from single-degenerate models (including WD + MS and WD+RG channels). Via a binary population synthesis approach, we find that the DTD from a low metallicity population is significantly delayed compared with that from a high metallicity one. In addition, we also find that a substantial fraction of SNe Ia have a delay time shorter than 1 Gyr, and the fraction of SNe Ia with short delay times increases with metallicity, i.e. about 35% for Z=0.001, while more than 70% for Z=0.02. These results would help to qualitatively explain the result of Strolger et al. (2010). Furthermore, we noticed that the contribution of WD + RG channel from the low metallicity population is higher than that from the high metallicity one. However, we can not quantitatively obtain a DTD consistent with the results of Strolger et al. (2010) by changing metallicity. As a consequence, metallicity may partly contribute to the DTD of SNe Ia and should therefore be checked carefully when one derives the DTD of SNe Ia from observations.

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arXiv2011

Characteristics of solar-like oscillations of clusters simulated by stellar population synthesis

Using a stellar population-synthesis method, we studied the distributions of ν_max and Δν of simulated clusters with various ages and metallicities. Except for the confirmed peak (RC peak) of Δν of red-clump (RC) stars, i.e. core-helium burning stars, there are a gap and a main sequence (MS) peak in the distributions of ν_max and Δν of young clusters. The gap corresponds mainly to the Hertzsprung gap phase of evolution. The RC peak is caused by the fact that the radius of many RC stars near the zero-age horizontal branch concentrates in a certain range. The MS peak also results from the fact that many MS stars which are located in a certain mass range have an approximate radius in the early phase of MS. The MS peak barely exists in the simulated clusters with age < 1.0 Gyr. The location of the MS peak moves to a lower frequency with increasing age or metallicity, which may be applied to constrain the age and metallicity of young clusters. For the simulated clusters with Z = 0.02, the frequency of the location of the dominant RC peak increases with age when age < 1.2 Gyr, and then decreases with age when age > 1.2 Gyr; but it scarcely varies when age > 2.4 Gyr. This is relative to the degeneracy of the hydrogen-exhausted core at the time of helium ignition. In addition, the RC peak is not sensitive to the metallicity, especially for the clusters with age > 2.4 Gyr. Asteroseismical observation for clusters with age < 2.4 Gyr may aid in testing the theory of the degeneracy of the hydrogen-exhausted core. Moreover, for the clusters with 1.1 M{\odot} < M_hook < 1.3 M{\odot}, there are a MS gap and a peak on the left of the MS gap in the distributions ofν_max and Δν, which may be applied to constrain the central hydrogen abundance of stars in the MS gap and the peak.

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arXiv2010

The birth rate of subluminous and overluminous type Ia supernovae

Based on the results of Chen & Li (2009) and Pakmor et al. (2010), we carried out a series of binary population synthesis calculations and considered two treatment of common envelope (CE) evolution, i.e. $α$-formalism and $γ$-algorithm. We found that the evolution of birth rate of these peculiar SNe Ia is heavily dependent on how to treat the CE evolution. The over-luminous SNe Ia may only occur for $α$-formalism with low CE ejection efficiency and the delay time of the SNe Ia is between 0.4 and 0.8 Gyr. The upper limit of the contribution rate of the supernovae to all SN Ia is less than 0.3%. The delay time of sub-luminous SNe Ia from equal-mass DD systems is between 0.1 and 0.3 Gyr for $α$-formalism with $α=3.0$, while longer than 9 Gyr for $α=1.0$. The range of the delay time for $γ$-algorithm is very wide, i.e. longer than 0.22 Gyr, even as long as 15 Gyr. The sub-luminous SNe Ia from equal-mass DD systems may only account for no more than 1% of all SNe Ia observed. The super-Chandrasekhar mass model of Chen & Li (2009) may account for a part of 2003fg-like supernovae and the equal-mass DD model developed by Pakmor et al. (2010) may explain some 1991bg-like events, too. In addition, based on the comparison between theories and observations, including the birth rate and delay time of the 1991bg-like events, we found that the $γ$-algorithm is more likely to be an appropriate prescription of the CE evolution of DD systems than the $α$-formalism if the equal-mass DD systems is the progenitor of 1991bg-like SNe Ia.

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arXiv2010

The effects of stellar population synthesis on the distributions of the asteroseismic observables ν_max and Δν of red-clump stars

The distributions of the frequencies of the maximum oscillation Power (ν_max) and the large frequency separation ( Δν) of red giant stars observed by CoRoT have a dominant peak. Miglio et al. (2009) identified that the stars are red-clump stars. Using stellar population synthesis method, we studied the effects of Reimers mass loss, binary interactions, star formation rate and the mixing-length parameter on the distributions of the ν_max and Δν of red-clump stars. The Reimers mass loss can result in an increase in the ν_max and Δν of old population which lost a considerable amount of mass. However, it leads to a small decrease in those of middle-age population which lost a little bit of mass. Furthermore, a high mass-loss rate impedes the low-mass and low-metal stars evolving into core-helium burning (CHeB) stage. Both Reimers mass loss and star formation rate mainly affect the number of CHeB stars with ν_max and Δν, but hardly affect the peak locations of ν_max and Δν. Binary interactions also can lead to an increase or decrease in the ν_max and Δν of some stars. However, the fraction of CHeB stars undergoing binary interactions is very small in our simulations. Therefore, the peak locations are also not affected by binary interactions. The non-uniform distributions of ν_max and Δν are mainly caused by the most of red-clump stars having an approximate radius rather than mass. The radius of red-clump stars decreases with increasing the mixing-length parameter. The peak locations of ν_max and Δν can, thus, be affected by the mixing-length parameter.

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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

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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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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