17434779
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 $1 {-} 15\,$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.
1551177
The dynamical stability of W Ursae Majoris-type systems
Theoretical study indicates that a contact binary system would merge into a rapidly rotating single star due to tidal instability when the spin angular momentum of the system is more than a third of its orbital angular momentum. Assuming that W Ursae Majoris (W UMa) contact binary systems rigorously comply with the Roche geometry and the dynamical stability limit is at a contact degree of about 70 per cent, we obtain that W UMa systems might suffer Darwin's instability when their mass ratios are in a region of about 0.076-0.078 and merge into the fast-rotating stars. This suggests that the W UMa systems with mass ratio q <= 0.076 cannot be observed. Meanwhile, we find that the observed W UMa systems with a mass ratio of about 0.077, corresponding to a contact degree of about 86 per cent would suffer tidal instability and merge into the single fast-rotating stars. This suggests that the dynamical stability limit for the observed W UMa systems is higher than the theoretical value, implying that the observed systems have probably suffered the loss of angular momentum due to gravitational wave radiation (GR) or magnetic stellar wind (MSW).
15791171
Energy transfer and its effects on the secondaries in W Ursae Majoris type contact binaries
Based on the physical parameters of 133 W Ursae Majoris (W UMa) type contact binaries, energy transfer and its effects on the secondaries in W UMa contact binaries are investigated. Relations are given between the mass ratio (q) for W UMa contact binaries and the relative energy transfer rates, i.e. U(1), the ratio of the transferred luminosity to the surface luminosity of the primary, and U(2), the ratio of the transferred luminosity to the nuclear luminosity of the secondary. The theoretical curves(U(1) versus q and U(2) versus q) are derived based on various assumptions: that the two components in each W UMa system are nearly identical in effective temperature and just fill their inner Roche lobes, and the primaries are zero-age main-sequence stars. Although these curves can reflect the distribution of U(1) and U(2) versus q, some observational systems deviate significantly from these curves. This results mainly from the difference in effective temperatures of the components of W UMa systems. The radius and the density of the secondary are related to the relative energy transfer rate U(2): the higher U(2), the greater the expansion and the lower the density of the secondary in a W UMa system. In addition, it is found that the temperature difference of W UMa binary components is correlated with the relative energy transfer rate U(1) and decreases with increasing U(1). This might suggest that there is a thermal coupling between the two components in W UMa contact binaries, and that the classification of W UMa contact binaries into A or W types depends on the energy transfer from the primary to the secondary. The temperature difference of W UMa binary components is poorly correlated with the mass of the primary. This suggests that the properties of the common envelope of a W UMa contact binary might not have a significant effect on the energy transfer between the two components.
17105580
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.
119229638
The short-period limit of contact binaries
The stability of mass transfer is important in the formation of contact binaries from detached binaries when the primaries of the initially detached binaries fill their Roche lobes. Using Eggletons stellar evolution code, we investigate the formation and the short-period limit of contact binaries by considering the effect of the instability of mass transfer. It is found that with decreasing initial primary mass from 0.89 to 0.63 M?, the range of the initial mass ratio decreases for detached binaries that experience stable mass transfer and evolve into contact. If the initial primary mass is less than 0.63 M?, detached binaries would experience dynamically unstable mass transfer when the primaries of detached binaries fill their Roche lobes. These systems would evolve into a common envelope situation and probably then to a complete merger of two components on a quite short time-scale. This results in a low mass limit at about 0.63 M? for the primary mass of contact binaries, which might be the main reason why the period distribution of contact binaries has a short limit of about 0.22 d. By comparing the theoretical period distribution of contact binaries with the observational data, it is found that the observed contact binaries are above the low mass limit for the primary mass of contact binaries and no observed contact binaries are below this limit. This suggests that the short-period limit of contact binaries can be explained by the instability of the mass transfer that occurs when the primaries of the initially detached binaries fill their Roche lobes.
13861127
Evolutionary population synthesis for binary stellar population at high spectral resolution: integrated spectral energy distributions and absorption-feature indices
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.
118614193
Reexamination of reaction rates for a key stellar reaction of $^{14}$O($\alpha$,p)$^{17}$F
The reaction rates of the key stellar reaction of O(α,p)F have been reexamined. The previous conclusion, the 6.15-MeV state (Jπ=1−) dominating this reaction rate, has been overthrown by a careful reanalysis of the previous experimental data [J. Gómez del Campo et al., Phys. Rev. Lett. 86, 43 (2001)]. According to the present R-matrix analysis, the previous 1− assignment for the 6.15MeV state is definitely wrong. Most probably, the 6.286-MeV state is the 1− state and the 6.15-MeV state is a 3− one, and hence the resonance at Ex=6.286 MeV (J π=1−) dominates the reaction rates in the temperature region of astrophysical interests. The newly calculated reaction rates for the O(α,p)F reaction are quite different from the previous ones, for instance, it’s only about 1/6 of the previous value around 0.4 GK, while it’s about 2.4 times larger than the previous value around 2 GK. The astrophysical implications have been discussed based on the present conclusions.
17648096
Non-Axisymmetric g-Mode and p-Mode Instability in a Thin Accretion Disk
It has been suggested that quasi-periodic oscillations of accreting X-ray sources may relate to the modes named in the title. We idealize a thin accretion disk by means of the shearing sheet model and derive a wave equation for nonaxisymmetric perturbations. We show that the equation can be transformed into the Whittaker equation, which enables us to write down analytical solutions. For modes with vertical wave number n > 0, there are four distinct permitted regions for waves, separated by two forbidden regions and a corotation resonance. We show that each forbidden region behaves like a wave amplifier so that any wave incident on it is reflected with a larger amplitude. The corotation resonance, however, behaves like a strong wave absorber. Following Kato (2001b, 2002), we look for internally trapped growing g-modes in which most of the wave action is concentrated in the region between the two forbidden zones. We find that there are no such modes in the shearing sheet. We discuss whether such modes may be present in more general shear flows that deviate from the idealized shearing sheet. We conclude that instability, though not ruled out, is likely to be rare. We also analyse externally trapped p-modes in which the wave action is concentrated in an external cavity between one of the forbidden zones and the inner edge of the disk. Unstable modes are certainly possible in this case. Moreover, the wave amplification can be substantially larger for n = 1 than for the previously studied case of n = 0. Therefore, n = 1 p-modes might have significant growth rates in realistic disks, provided that the disk inner edge reflects waves reasonably well. Subject headings: accretion, accretion disks — instabilities — waves — X-rays: binaries
119185282
Yunnan-III models for evolutionary population synthesis
We build the Yunnan-III evolutionary population synthesis (EPS) models by using the stellar evolution code, BaSeL stellar spectra library and the initial mass functions (IMFs) of Kroupa and Salpeter, and present colours and integrated spectral energy distributions (ISEDs) of solar-metallicity stellar populations (SPs) in the range of 1Myr-15Gyr. The main characteristic of the Yunnan-III EPS models is the usage of a set of self-consistent solar-metallicity stellar evolutionary tracks (the masses of stars are from 0.1 to 100M⊙). This set of tracks is obtained by using the state-of-the-art code. code can evolve stellar models through thermally pulsing asymptotic giant branch (TP-AGB) phase for low- and intermediate-mass stars. By comparisons, we confirm that the inclusion of TP-AGB stars make the V K, V J and V R colours of SPs redder and the infrared flux larger at ages log(t/yr) > 7.6 (the differences reach the maximum at log(t/yr)�8.6, �0.5-0.2mag for colours, � 2 times for K band flux). We also find that the colour-evolution trends of Model with-TPAGB at intermediate and large ages are similar to those from the code, which employs the Padova-AGB stellar library, BaSeL spectral library and the Kroupa IMF. At last, we compare the colours with the other EPS models comprising TP-AGB stars (such as CB07, M05, V10 and POPSTAR), and find that the B V colour agrees with each other but the V K colour exists larger discrepancy among these EPS models (�1mag when 8< log(t/yr)< 9). The stellar evolutionary tracks, isochrones, colours and ISEDs can be obtained on request from the first author or from our website (http://www1.ynao.ac.cn/�zhangfh/). Using the isochrones, you can build your EPS models. Now the format of stellar evolutionary tracks is the same as that in the code, you can put them into the code and get the SP’s results. Moreover, the colours involving other passbands or on other systems (for example, HST F439W F555W colour on AB system) can also be obtained on request.
17303015
Must time machines be unstable against vacuum fluctuations
In the appearance of an absorption material, the quantum vacuum fluctuations of all kinds of fields may be smoothed out and the spacetime with time machines may be stable against vacuum fluctuations. The chronology protection conjecture may break down and the anti-chronology protection conjecture may hold: there is no law of physics preventing the appearance of closed timelike curves.
118472204
Binary Stellar Population Synthesis Model
Using Yunnan evolutionary population synthesis (EPS) models, we present integrated colours, integrated spectral energy distributions (ISEDs) 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 (BSPs) with interactions. By comparing the results for populations with and without interactions we show that the inclusion of binary interactions makes the appearance of the population substantially bluer. This effect raises the derived age and metallicity of the population. To be used in the studies of modern spectroscopic galaxy surveys at intermediate/high spectral resolution, we also present intermediate- (3A) and high-resolution (~0.3A) ISEDs and Lick/IDS absorption-line indices for BSPs. To directly compare with observations the Lick/IDS absorption indices are also presented by measuring them directly from the ISEDs.