首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
In the present paper we combine an N-body code that simulates the dynamics of young dense stellar systems with a massive star evolution handler that accounts in a realistic way for the effects of stellar wind mass loss. We discuss two topics.
  1. The formation and the evolution of very massive stars (with masses >120 M⊙) is followed in detail. These very massive stars are formed in the cluster core as a consequence of the successive (physical) collisions of the 10–20 most massive stars in the cluster (this process is known as ‘runaway merging’). The further evolution is governed by stellar wind mass loss during core hydrogen and core helium burning (the WR phase of very massive stars). Our simulations reveal that, as a consequence of runaway merging in clusters with solar and supersolar values, massive black holes can be formed, but with a maximum mass ≈70 M⊙. In low-metallicity clusters, however, it cannot be excluded that the runaway-merging process is responsible for pair-instability supernovae or for the formation of intermediate-mass black holes with a mass of several 100 M⊙.
  2. Massive runaways can be formed via the supernova explosion of one of the components in a binary system (the Blaauw scenario), or via dynamical interaction of a single star and a binary or between two binaries in a star cluster. We explore the possibility that the most massive runaways (e.g. ζ Pup, λ Cep, BD+43°3654) are the product of the collision and merger of two or three massive stars.
  相似文献   

2.
Evolutionary tracks from the zero age main sequence to the asymptotic giant branch were computed for stars with initial masses 2 M ⊙ ≤ M ZAMS ≤ 5 M ⊙ and metallicity Z = 0.02. Some models of evolutionary sequences were used as initial conditions for equations of radiation hydrodynamics and turbulent convection describing radial stellar pulsations. The early asymptotic giant branch stars are shown to pulsate in the fundamental mode with periods 30 day ? Π ? 400day. The rate of period change gradually increases as the star evolves but is too small to be detected (Π?/Π < 10?5 yr?1). Pulsation properties of thermally pulsing AGB stars are investigated on time intervals comprising 17 thermal pulses for evolutionary sequences with initial masses M ZAMS = 2 M ⊙ and 3 M ⊙ and 6 thermal pulses for M ZAMS = 4 M ⊙ and 5 M ⊙. Stars with initial masses M ZAMS ≤ 3 M ⊙ pulsate either in the fundamental mode or in the first overtone, whereas more massive red giants (M ZAMS ≥ 4 M ⊙) pulsate in the fundamental mode with periods Π ? 103 day. Most rapid pulsation period change with rate ?0.02 yr?1 ? Π?/Π ? ?0.01 yr?1 occurs during decrease of the surface luminosity after the maximum of the luminosity in the helium shell source. The rate of subsequent increase of the period is Π?/Π ? 5 × 10?3 yr?1.  相似文献   

3.
The results of calculations of graphite grain formation in the atmospheres of R CrB stars are given. The parameters for the models wereM=1M ⊙,M bol=?6 mag. The effective temperature ranged from 5300K to 8300K. The chemical composition corresponded to the hydrogen-deficient carbon rich mixture:X=0,Y=0.9,Z c=0.1. The results obtained show the existence of a critical mass loss rate which is ranged fromM *≈10?6 M ⊙yr?1 forT eff=5300 K toM *≈10?5 M ⊙ yr?1 forT eff=8300 K. As soon as the rate of mass loss exceedsM * by 3–5 times the degree of condensation of carbon changes from 0 to 0.7. The finite radii of grains are about from 0.01 μm to 0.6 μm depending on the density near the condensation point, the velocity of matter outflow, and the stellar effective temperature. The duration of grain growth should amount to some dozens of days. It is supposed that the most probable explanation of dust-shell formation around R CrB stars is graphite condensation behind a shock wave arising from nonlinear stellar pulsation.  相似文献   

4.
The results of investigations of a number of eclipsing Wolf-Rayet binaries are presented. The ‘core’ radiuses, the ‘core’ temperatures and masses of WR stars in the eclipsing WR+OB binary systems V 444 Cyg, CX Cep, CQ Cep, and CV Ser are obtained (see Table I). The results obtained from the light curves analysis of the V 444 Cyg in the range λλ2460 Å-3.5μ give strong evidence for the Beals (1944) model of WR phenomenon. The chromospheric-coronal effects in the WN5 extended atmosphere are not observed up to a distance ofr?20R ⊙. In the Hertzsprung—Russell diagram all the WR stars lie on the left side from the main sequence between the main sequence and the sequence of uniform helium stars (see Figure 9). Their locations are close to those of the helium remnants formed as a result of mass exchange in massive close binary systems. The period variations in the systems V 444 Cyg and CQ Cep have been discovered and a reliable value of the mass loss rateM=10?5 M ⊙yr?1 is obtained, for the two WR stars. The results of the photometric and spectroscopic investigations of the WR stars with low mass companions (post X-ray binary stage?) are presented too (see Table II). The masses of the companions are (1–2)M ⊙, their optical luminosity is ～1036, erg s?1 which implies that these companions cannot be the normal stars. It is possible that these companions are neutron stars accreting from the stellar wind of the WR stars. Low values of the X-ray luminosities of such WR stars with low mass companions imply that the accretion of matter in such systems is distinct from the accretion process in classical X-ray binary systems. It is noted also that the parameters of low massive companions coupled with WR stars are close to those of helium stars.  相似文献   

5.
The evolution of Population I stars with initial masses 70M ⊙ ≤ M ZAMS ≤ 130M ⊙ is considered. The computations were performed under various assumptions about the mass loss rate and were terminated at the phase of gravitational contraction after core helium exhaustion. The mass loss rate at the helium burning phase, ?3α , is shown to be the main parameter that determines the coefficients of the mass—luminosity relation for Wolf—Rayet stars. Several more accurate mass—luminosity relations for mass loss rates ? = f 3α ?3α , where 0.5 ≤ f 3α ≤ 3, are suggested, along with the mass—luminosity relation that combines all of the evolutionary sequences considered. The results of the stellar evolution computations were used as initial conditions in solving the hydrodynamic equations describing the spherically symmetric motions of a self-gravitating gas. The outer layers of massive Population I stars are unstable against radial oscillations throughout the helium burning phase. The oscillation amplitude is largest at enhanced carbon and oxygen abundances in the outer stellar layers, i.e., at a lower initial stellar mass M ZAMS or a lower mass loss rate during the entire preceding evolution. In the course of evolution, the radial oscillation amplitude decreases and the small nonlinearity of the oscillations at M < 10M ⊙ allow the integral of mechanical work W done by an elementary spherical layer of gas in a closed thermodynamic cycle to be calculated with the necessary accuracy. The maximum of the radial dependence of W is shown to be located in layers with a gas temperature T ～ 2 × 105 K, where the oscillations are excited by the iron Z-bump κ-mechanism. Comparison of the radial dependences of the integral of mechanical work W and the amplitude of the radiative flux variations suggests that the nonlinear radial oscillations of more massive Wolf—Rayet stars are also excited by the κ-mechanism.  相似文献   

6.
The main results of a study of a catalogue of physical parameters of 1041 spectroscopic binaries are presented. The distribution of spectroscopic binaries over all main parametersM 1, a, e, M1/M2, P, and certain dependencies between some of them have been found.
  1. It appears that among bright (m v?3 m –5 m ) stars withM?1M ⊙, about 40% are apparently spectroscopic binaries with comparable masses of components.
  2. The majority of spectroscopic binaries with the ratio of the large semiaxis of the orbit to the radius of the primarya/R 1?20, have eccentricities close to zero. This is probably a consequence of the tidal circularization of orbits of close binaries by viscous friction.
  3. The discovery of duplicity of double-line spectroscopic binaries is possible only if the semiamplitude of radial velocityK 1 is almost 10 times higher than the semiamplitude of the radial velocity of a single-line spectroscopic binary of the same mass.
  4. Double-line spectroscopic binaries witha/R ⊙?6(M 1/M ⊙)1/3,M 1≈M 2?1.5M ⊙ are almost almost absent, and the number of stars witha/R ⊙?6(M 1/M ⊙)1/3,M 1≈1.5M ⊙ is relatively low.
  5. The distribution of unevolved SB stars over the large semiaxis may be described by the expression d(N d/Nt)≈0.2 d loga for 6(M 1/M ⊙)1/3?a/R ⊙?100.
  6. The intial mass-function for primaries of spectroscopic binaries is the same Salpeter function dN d≈M 1 ?2.35 dM 1 for 1?M 1/M ⊙?30.
  7. It is possible to explain the observed ratio of the number of single-line spectroscopic binaries to the number of double-line binaries if one assumes that the average initial mass ratio is close to 1 and that the mass of the postmass-exchange remnant of the primary exceeds the theoretical one and/or that half of the angular momentum of the system is lost during mass-exchange.
  8. The above-mentioned distributions ofM 1 anda and assumptions on the mass of remnant and/or momentum loss also allow us to explain the observed shapes of dN/dM, dN/dq, and dN/da distributions after some selection effects are taken into account.
  相似文献   

7.
《New Astronomy》2007,12(2):95-103
Low metallicity very massive stars with an initial mass between 140M⊙ and 260M⊙ can be subdivided into two groups: those between 140M⊙ and 200M⊙ which produce a relatively small amount of Fe, and those with a mass between 200M⊙ and 260M⊙ where the Fe-yield ejected during the supernova explosion is enormous. We first demonstrate that the inclusion of the second group into a chemical evolutionary model for the Solar Neighbourhood predicts an early temporal evolution of Fe, which is at variance with observations whereas it cannot be excluded that the first group could have been present. We then show that a low metallicity binary with very massive components (with a mass corresponding to the first group) can be an efficient site of primary 14N production through the explosion of a binary component that has been polluted by the pair instability supernova ejecta of its companion. When we implement these massive binary 14N yields in a chemical evolution model, we conclude that very massive close binaries may be important sites of 14N enrichment during the early evolution of the Galaxy.  相似文献   

8.
We suggest that planets, brown dwarfs, and even low mass stars can be formed by fragmentation of protoplanetary disks around very massive stars (M ? 100 M⊙). We discuss how fragmentation conditions make the formation of very massive planetary systems around very massive stars favorable. Such planetary systems are likely to be composed of brown dwarfs and low mass stars of ～0.1–0.3 M⊙, at orbital separations of ～ few × 100–104 AU. In particular, scaling from solar-like stars suggests that hundreds of Mercury-like planets might orbit very massive stars at ～103 AU where conditions might favor liquid water. Such fragmentation objects can be excellent targets for the James Webb Space Telescope and other large telescopes working in the IR bands. We predict that deep observations of very massive stars would reveal these fragmentation objects, orbiting in the same orbital plane in cases where there are more than one object.  相似文献   

9.
We investigate the possibility that gravity modes can be stochastically excited by turbulent convection in massive main-sequence (MS) stars. We build stellar models of MS stars with masses M=10?M ⊙,15?M ⊙, and 20?M ⊙. For each model, we then compute the power supplied to the modes by turbulent eddies in the convective core (CC) and the outer convective zones (OCZ). We found that, for asymptotic gravity modes, the major part of the driving occurs within the outer iron convective zone, while the excitation of low n order modes mainly occurs within the CC. We compute the mode lifetimes and deduce the expected mode amplitudes. We finally discuss the possibility of detecting such stochastically-excited gravity modes with the CoRoT space-based mission.  相似文献   

10.
N. Audard  J. Provost 《Solar physics》1994,152(1):316-316
The structure of stars more massive than about 1.2M ⊙ is characterized by a convective core. We have studied the evolution with age and mass of acoustic frequencies if high radial ordern and low degree ? for models of stars of 1, 1.5 and 2M ⊙. Using a polynomial approximation for the frequency, the p-mode spectrum can be characterized by derived global asteroseimic coefficients, i.e. the mean separationv 0～v n,? ?v n,?1,? and the small frequency separationΔv 0,2～v n,?=0 ?v n,?1,?=2 . The diagram(v 0,Δ(v 0,2/v 0 plotted along the evolutionary tracks would help to separate the effects of age and mass. We study of sensitivity of these coefficients and other observable quantities, like the radius and luminosity, the stellar parameters in the vicinity of 1M ⊙ and 2M ⊙; this sensitivity substantially depends on the stellar mass and must be taken into account for asteroseismic calibration of stellar clusters. Considering finally some rapid variations of the internal structure, we show that the second frequency differenceδ2 v=v n,? ?2v n,?1,? +v n,?2,? exhibits and oscillatory behaviour well related to the rapid variation of the adiabatic exponent γ in the HeII ionization zone. A more complete discussion is given in Audard N, Provost J, ‘Seismological properties of intermediate-mass stars’,A&A, 1993, in press.  相似文献   

11.
《New Astronomy Reviews》2000,44(4-6):329-334
Heavy element abundances derived from high-quality ground-based and Hubble Space Telescope (HST) spectroscopic observations of low-metallicity blue compact galaxies (BCGs) with oxygen abundances 12+log O/H between 7.1 and 8.3 are discussed. None of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12+log O/H≤7.6 (Z≤Z⊙/20). This constancy implies that all these heavy elements have a primary origin and are produced by the same massive (M≥10 M⊙) stars responsible for O production. The dispersion of the C/O and N/O ratios in these galaxies is found to be remarkably small, being only ±0.03 dex and ±0.02 dex respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest-metallicity BCGs, and hence against production of these elements by intermediate-mass (3 M⊙≤M≤9 M⊙) stars at very low metallicities, as commonly thought.In higher metallicity BCGs (7.6<12+log O/H<8.2), the Ne/O, Si/O, S/O, Ar/O and Fe/O abundance ratios retain the same constant value they had at lower metallicities. By contrast, there is an increase of the C/O and N/O ratios along with their dispersions at a given O. We interpret this increase as due to the additional contribution of C and primary N production in intermediate-mass stars, on top of that by high-mass stars. BCGs show the same O/Fe overabundance with respect to the Sun (∼0.4 dex) as galactic halo stars, suggesting the same chemical enrichment history.  相似文献   

12.
We re‐discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M⊙ are concentrated towards the centre of the main‐sequence band. In contrast, stars with masses M > 3 M⊙ seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M⊙ and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main‐sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities β is detected in both higher and lower mass stars. It is quite possible that the angle β becomes close to 0. in slower rotating stars of mass M > 3 M⊙ too, analog to the behaviour of angles β in slowly rotating stars of M < 3 M⊙. The obliquity angle distribution as inferred from the distribution of r ‐values appears random at the time magnetic stars become observable on the H‐R diagram. After quite a short time spent on the main sequence, the obliquity angle β tends to reach values close to either 90. or 0. for M < 3 M⊙. The evolution of the obliquity angle β seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of β with the elapsed time on the main sequence for stars with M > 3 M⊙, no similar trend is found for stars with M < 3 M⊙. However, the predominance of high values of β at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
Excitation of radial pulsations in red supergiants of Magellanic Clouds is investigated using the stellar evolution calculations and the self-consistent solution of the equations of radiation hydrodynamics and turbulent convection. The stars with initial masses 6M ⊙ ≤ M ZAMS ≤ 28M ⊙ and the initial chemical composition X = 0.7, 0.004 ≤ Z ≤ 0.008 are shown to be unstable against fundamental mode oscillations with periods from 17 to 1200 days as they become helium burning red supergiants. The period-luminosity relation slightly depends on the mass loss rate varying with a factor of three, whereas its dependence on the metal abundance is given by δM bol = 0.89δ log Z. In comparison with galactic red supergiants the low metal abundances in red supergiants of Magellanic Clouds are responsible for their higher effective temperatures and substantially narrower ranges of evolutionary radius change during helium burning. Therefore on the period-mass diagram the red supergiants of Magellanic Clouds are located within the strip with width of δ logM ≈ 0.09, so that the uncertainty of mass evaluation of the red supergiant with the known pulsation period is nearly 25%.  相似文献   

14.
In this paper, we report a rare reflection effect eclipsing sdB+dM binary, 2M?1533+3759. It is the seventh eclipsing sdB+dM binary that has been discovered to date. This system has an orbital period of 0.16177042 day and a velocity semi-amplitude of 71.1 km?s?1. Using a grid of zero-metallicity NLTE model atmospheres, we derived T eff=29250 K, log?g=5.58 and [He/H]=?2.37 from spectra taken near the reflection effection minimum. Lightcurve modeling resulted in a system mass ratio of 0.301 and an orbital inclination angle of 86.6°. The derived primary mass for 2M?1533+3759, 0.376±0.055 M ⊙, is significantly lower than the canonical mass (0.48 M ⊙) found for most previously investigated sdB stars. This implies an initial progenitor mass >1.8 M ⊙, at least a main sequence A star and perhaps even one massive enough to undergo non-degenerate helium ignition.  相似文献   

15.
Mass is a fundamental parameter, but the masses are not well known for most hot subdwarfs. We propose a method of determining the masses of hot subdwarfs. Using this method, we studied the masses of hot subdwarfs from the ESO supernova Ia progenitor survey and the Hamburg quasar survey. The study shows that most of the subdwarf B stars have masses between 0.42 and 0.54 M ⊙, whilst most sdO stars are in the range 0.40～0.55 M ⊙. Comparing our study to the theoretical mass distributions of Han et al. (Mon. Not. R. Astron. Soc. 341:669, 2003), we found that sdO stars with mass less than ～0.5 M ⊙ may evolve from sdB stars, whilst most high-mass (>0.5 M ⊙) sdO stars result from mergers directly.  相似文献   

16.
Models for the chemical evolution of the galaxy are constructed in which the time evolution is imposed by the contraction rate of the galaxy and present observations of stellar metal abundances as a function of height above the galactic plane. Stars with massm?3.5m ⊙ do not contribute to the metal enrichment of the interstellar gas, and we argue that the interstellar metal abundance at this epoch should be fairly insensitive to the size of the mass fraction of the galaxy that condenses into such stars. The birth rate for stars more massive than 3.5m ⊙ is assumed proportional toV gal ?n , whereV gal is the contracting volume of the main body of the galaxy. If a dynamic time-scale is adopted for the contraction of the galaxy, then the assumed power-law birth rate yields suitable chemical evolution models only if observed Population II metals are synthesized in stars more massive than about 8.5m ⊙. This mass range is consistent with the predictions of current stellar-evolution theory. Provided the birth function does include stars more massive than 8.5m ⊙, the relation between the value of the parametern in the birth rate and the observed chemical evolution rate is not particularly sensitive to the specific form adopted for the initial mass spectrum, or to the proportionality constant in the birth rate. We find (i)n?1.4, in general, and (ii)n is close to 1.4 if the contraction of the galaxy to a heighth=400 pc above the plane occurs at close to the free-fall rate. These results are independent of the form of the initial mass spectrum, providedS 3 is small. HereS is the total mass fraction of the galaxy that cycles through stars during its contraction. Numerical models, with an explicit initial mass spectrum, indicate that the same restrictions on the values ofn apply approximately whenS 3 is not small. To introduce low mass stars, we allow the birth rate for stars more massive than 3.5m ⊙ to level off at a time intervalt L just before the contraction of the galaxy stops, while the total birth rate remains a simple power law. We find that reasonable models are obtained witht L ?1.5×107 yr if the galaxy contracts at a dynamic rate. However, aside from these restrictions on the values ofn andt L , there is no uniquely favored model. For any suitable model, the supernova rate must be small enough so that shock waves from neighboring supernovae do not collide during the adiabatic expansion stage. Otherwise, the interstellar gas would not have time to cool, and its high temperature would tend to impede both star formation and the rapid contraction of the galaxy. The supernova rates in the numerical models given here are small enough to avoid this problem, but large enough to achieve a uniform metal abundance on a time scale short compared to the chemical-evolution time scale. At the epoch considered here, the interstellar metal abundance is approximately less than 0.4Z ⊙, and the models are assumed to apply before galactic-scale inhomogeneities, such as the galactic nucleus, become important. Therefore, the chemical mixing time scales imply that most Population II stars of the same age should have approximately the same initial metal abundance, unless the clustering of supernova explosions associated with massive Population II stars is significant. It is shown that collisions between shock waves from neighboring supernovae can produce local regions of significantly enhanced density. The peak bolometric luminosity of the galaxy during its contraction is similar to that predicted by Partridge and Peebles (1967a), but it occurs during the final stages of contraction to the disc. Numerical models give values between 13 and 34 yr?1 for the average number of supernova explosions per year during this bright phase. The X-ray luminosity of the galaxy from these supernovae may be comparable to that of Seyfert galaxies.  相似文献   

17.
We present the results of Monte Carlo mass-loss computations for hot low-mass stars, specifically for subdwarf B (sdB) stars. It is shown that the mass-loss rates on the Horizontal Branch (HB) computed from radiative line-driven wind models are not high enough to create sdB stars. We argue, however, that mass loss plays a role in the chemical abundance patterns observed both in field sdB stars, as well as in cluster HB stars. The derived mass loss recipe for these (extremely) hot HB stars may also be applied to other groups of hot low-mass stars, such as post-HB (AGB-manqué, UV-bright) stars, over a range in effective temperatures between ?10 000 and 50 000 K. Finally, we present preliminary spectral synthesis on the more luminous sdB stars for which emission cores in Hα have been detected (Heber, U., et al.: 2003, in:Stellar Atmosphere Modeling, ASP Conference Proceedings, p. 251). We find that these line profiles can indeed be interpreted as the presence of a stellar wind with mass loss on the order of 10?11?M ⊙ yr ?1.  相似文献   

18.
We performed hydrodynamic computations of nonlinear stellar pulsations of population I stars at the evolutionary stages of the ascending red giant branch and the following luminosity drop due to the core helium flash. Red giants populating this region of the Hertzsprung–Russel diagram were found to be the fundamental mode pulsators. The pulsation period is the largest at the tip of the red giant branch and for stars with initial masses from 1.1 M ⊙ to 1.9 M ⊙ ranges from ∏ ≈ 254 day to ∏ ≈ 33 day , respectively. The rate of period change during the core helium flash is comparable with rates of secular period change in Mira type variables during the thermal pulse in the helium shell source. The period change rate is largest (∏?/∏ ≈ ?10?2 yr?1) in stars with initial mass M ZAMS = 1.1 M ⊙ and decreases to ∏?/∏ ～ ?10?3 yr?1 for stars of the evolutionary sequence M ZAMS = 1.9 M ⊙. Theoretical light curves of red giants pulsating with periods ∏ > 200 day show the presence of the secondary maximum similar to that observed in many Miras.  相似文献   

19.
We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-Newton. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (<2M ⊙), 36 intermediate mass (2–10M ⊙) and 16 massive (>10M ⊙) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature <2 keV, and mean L X/L bol of 10???6.9. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ~1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ~4–14 Myr. These observed values of L X/L bol are found to have a mean value of 10??3.6±0.4, which is below the X-ray saturation level. The L X/L bol values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their L X/L bol values, however have been found to be significantly different from each other with a confidence level greater than 99.999% and the strength of X-ray activity in the intermediate mass stars is found to be lower compared to the low mass stars. However, the possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out.  相似文献   

20.
Stellar evolution calculations were carried out from the main sequence to the final stage of the asymptotic giant branch for stars with initial masses 1 M⊙ ≤ MZAMS ≤ 2 M⊙ and metallicity Z = 0.01. Selected models of evolutionary sequences were used as initial conditions for solution of the equations of radiation hydrodynamics and time–dependent convection describing radial stellar pulsations. The study was aimed to construct the hydrodynamic models of Mira–type stars that show the secular decrease in the pulsation period Π commenced in 1970th at Π = 315 day. We show that such a condition for the period change is satisfied with evolutionary sequences 1 M⊙ ≤ MZAMS ≤ 1.2 M⊙ and the best agreement with observations is obtained for MZAMS = 1.2 M⊙. The pulsation period reduction is due to both the stellar radius decrease during the thermal pulse of the helium burning shell and mode switch from the fundamental mode to the first overtone. Theoretical estimates of the fundament parameters of the star at the onset of pulsation period reduction are as follows: the mass is M = 0.93 M⊙, the luminosity is L = 4080 L⊙, and the radius is R = 220 R⊙. The mode switch occurs 35 years after the onset of period reduction.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号