Fenghui Zhang

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 the 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 U-B, B-V, V-R and R-I colours and all Lick/IDS spectral absorption indices (except for H) 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 the binary evolution algorithm input parameters (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.

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

Evolutionary population synthesis for binary stellar population at high spectral resolution: integrated spectral energy distributions and absorption-feature indices

1 {-} 15\,

The correlations between the spin frequencies and kHz QPOs of neutron stars in LMXBs

Gyr. 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.

The evolutionary status of W Ursae Majoris‐type systems

Using evolutionary population synthesis, we present high-resolution (0.3 angstrom) integrated spectral energy distributions from 3000 to 7000 angstrom and absorption-line indices defined by the Lick Observatory Image Dissector Scanner (Lick/IDS) system, for an extensive set of instantaneous-burst binary stellar populations with binary interactions. The ages of the populations are in the range 1-15 Gyr and the metallicities are in the range 0.004-0.03. These high-resolution synthesis results can satisfy the needs of modern spectroscopic galaxy surveys, and are available on request. By comparing the synthetic continuum of populations at high and low resolution, we show that there is good agreement for solar metallicity and tolerable disagreement for non- solar metallicity. The strength of the Balmer lines at high spectral resolution is greater than that at low resolution for all metallicities. The comparison of Lick/IDS absorption-line indices at low and high resolution, both of which are obtained by the fitting functions, shows that the discrepancies in all indices except for TiO1 and TiO2 are insignificant for populations with Z = 0.004 and 0.02. The high-resolution Ca4227, Fe5015 and Mg-b indices are redder than the corresponding low-resolution ones for populations with Z = 0.01 and 0.03; this effect lowers the derived age and metallicity of the population. The high-resolution Mg-1, Fe5709 and Fe5782 indices are bluer than those at low resolution; this effect raises the age and metallicity. The discrepancy in these six indices is greater for populations with Z = 0.03 in comparison to Z = 0.01. At high resolution we compare the Lick/IDS spectral absorption indices obtained by using the fitting functions with those measured directly from the synthetic spectra. We find that the Ca4455, Fe4668, Mg-b and Na D indices obtained by the use of the fitting functions are redder for all metallicities, Fe5709 is redder at Z = 0.03 and becomes bluer at Z = 0.01 and 0.004, and the other indices are bluer for all metallicities than the corresponding values measured directly from the synthetic spectra.

Structure and evolution of low-mass W UMa-type systems

Aims. We studied the correlations between spin frequencies and kilohertz quasi-periodic oscillations ( kHz QPOs) in neutron star low-mass X-ray binaries. Methods. The updated data on kHz QPOs and spin frequencies are statistically analyzed. Results. We find that when two simultaneous kHz QPOs are present in the power spectrum, the minimum frequency of upper kHz QPO is at least 1.3 times higher than the spin frequency, i. e. nu(s) = -( 0.19 +/- 0.05)nu(s) + ( 389.40 +/- 21.67) Hz. If we shift this correlation in the direction of the peak separation by a factor of 1.5, this correlation matches the data points of the two accretionpowered millisecond X- ray pulsars, SAX J1808.4- 3658 and XTE J1807-294.

Structure and evolution of low-mass W Ursae Majoris type systems - II. With angular momentum loss

Well-determined physical parameters of 130 W Ursae Majoris (W UMa) systems were 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 the M-J diagram, which is consistent with the results derived from the analysis of observed spectral type and of 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. Moreover, the distribution of angular momentum, together with the distribution of the mass ratio, suggests that the mass ratio of the observed W UMa systems decreases with decreasing total mass. This could be the result of the dynamical evolution of W UMa systems, which suffer angular momentum loss and mass loss as a result of the magnetic stellar wind. Consequently, the tidal instability forces these systems towards lower q values and finally to rapidly rotating single stars.

Potential of colors for determining age and metallicity of stellar populations

The structure and evolution of low-mass W UMa type contact binaries are discussed by employing Eggleton’s stellar evolution code (Eggleton 1971, 1972, 1973). Assuming that these systems completely satisfy Roche geometry, for contact binaries with every kind of mass ratios (0.02�1.0), we calculate the relative radii (R1,2/A, where R1,2 are the radii of both stars, and A the orbital separation) 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. Serious uncertainties concern mainly the transfer of energy in these systems, i.e., it is unclear that how and where the energy is transferred. 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.

The dynamical stability of W Ursae Majoris-type systems

In a previous paper, using Eggletons stellar evolution code we have discussed the structure and evolution of low-mass W Ursae Majoris (W UMa) type contact binaries without angular momentum loss (AML). The models exhibit cyclic behaviour about a state of marginal contact on a thermal time-scale. Part of the time of each cycle is spent in contact and part in a semidetached state. According to observations, W UMa systems suffer AML. We present the models of low-mass contact binaries with AML due to gravitational wave radiation or magnetic stellar wind (MSW). We find that gravitational radiation cannot prevent the cyclic evolution of W UMa systems, and the effect of gravitational radiation on the cyclic behaviour of contact binary evolution is almost negligible. We also find that the most likely AML mechanism for W UMa systems is magnetic braking, and that magnetic braking effects can increase the period of the cyclic evolution and shorten the fraction of the time spent in the poor thermal contact state exhibiting EB light curve. If W UMa stars do not undergo cyclic evolution, and their AML is caused simultaneously by MSW of both components, we find that the value of the parameter, lambda, should be taken as about 3.8 for W UMa systems, which is larger than the largest value of similar single stars derived from observations. This indicates that the AML efficiency in W UMa systems may be lowered in comparison with non-contact stars because of less mass contained in the convective envelopes of the components in W UMa systems or some feedback mechanism which may have an effect on W UMa systems. If W UMa systems lose their angular momentum at a constant rate, an angular momentum rate of d ln J/dt approximate to 1.6 x 10(-9) yr(-1) can prevent the cyclic behaviour of the model, and the model can keep in good contact with an essentially constant depth of contact.

Probability Distribution of Terrestrial Planets in Habitable Zones around Host Stars

Context. 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. Aims. We try to find colors that can potentially be used to determine the age and metallicity of stellar populations by the standard model. Methods. Principal component analysis and relative sensitive parameter techniques are used in this work. Results. 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. Conclusions. 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.