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1.
The properties of the hydrogen burning shell in the envelope of an accreting neutron star have been studied for a range of mass accretion rates, neutron star radii, and metal abundances of the accreted matter. It is found that the hydrogen burning shells lie at densities ranging from 105–6×106 gm cm–3. For mass accretion rates in excess ofM c2 hydrogen and helium burn together. ForM c1MM c2, the hydrogen burning shell is stabilized by the limited CNO cycle. Implications of these results to the X-ray burst phenomena are briefly discussed.  相似文献   

2.
The life-time of the star on AGB is approximately 6 × 104 yr. We divide it into front half and back half of AGB (including to optical Mira variable and OH/IR star) according to their evolution character. The observations show that the star has non-pulsation, but constant mass loss rate ( 5 × 10–7 M yr–1) on front half of AGB. Its circumstellar envelope is formed. When the star has pulsation on back half of AGB, its mass loss rate is relative with time, and increases gradually. In this time the star is on the stage of optical Mira variable. When the mass loss rate reaches the value of 3 × 10–6 M yr–1, the star evoluted from the stage of optical variable into the stage of radio bright OH/IR star. On the end of AGB the mass loss rate reaches 10–4 M yr–1. (Band and Habing 1983, Hermen and Habing 1985).  相似文献   

3.
The final nuclear composition of the matter expanding from the density of a neutron star is investigated. It is assumed that starquakes cause the cracks which penetrate the neutron star crust and that the neutron star fluid can flow out through the cracks into space. The change with time of the nuclear composition of this matter is calculated by use of the compressible nuclear mass formula, and the hydrodynamics of the system is followed by the effect of nuclear transformation with time of the second fission of heavy neutron-rich nuclei, which is followed by a rapid rise to above 109 K. If the value of the -strength function exceeds about 10–5.5 MeV–1 s–1, the system proceeds to a state of nuclear equilibrium in the later expansion stage and the nuclear composition is reshuffled, finally to be transformed into neutron-excess, stable nuclei within the atomic mass region 80A120. It also becomes clear that if the strength function has a value smaller than the above critical value, then the neutron-rich nuclides withA[200, 400] are copiously produced. These results will also be applied in the cases of a neutron-star-black-hole collision and the explosion of a neutron star associated with the catastrophic phase transition within the neutron star core. The astrophysical implications are briefly discussed.  相似文献   

4.
It is suggested that the minimum mass of a star at the time of its formation is approximately 0.01M . Making use of this fact and the stellar mass functionF(M) M –, it is found that the hidden mass (or the missing mass) in the solar neighborhood may be explained by the presence of a large number of invisible stars of very low mass (0.01M M<0.07M ).  相似文献   

5.
The period variations of TV Cassiopeiae between 1901 and 1977 are discussed in the light of the period change model proposed by Biermann and Hall. During each period decrease 4.0×10–6 M of mass is transferred from the secondary star to the primary. The average observable mass transfer rate is found to be 4.3×10–7 M yr–1. This average rate corresponds to the thermal time-scale of the mass-losing star.  相似文献   

6.
The periodogram analysis of theV observations of the Scuti star HR 1225 has been carried out. Two frequencies of 6.415 cd (P 0=0 . d 1558) and 8.418 cd (P 1=0 . d 1188) have been determined. The period ratio ofP 1/P 0=0.762 indicates radial pulsation. The absolute magnitude, effective temperature and mass of the star are derived to be 1 . m 05, 7600 K and 1.9M , respectively.  相似文献   

7.
A plausible scenario for the formation of a stable supermassive star in the relativistic regime has been discussed. The onset of the negativity of the `distribution function' in the stable sequences of the star clusters [the stability of star clusters is assured by using the variational method (Chandrasekhar, 1964a,b) for equivalent gas spheres] described by Tolman's type VII solution with vanishing surface density has been regarded as an indication of the conversion of the cluster structure into a supermassive star. For the critical values of the `adiabatic index', (4/3) < crit (5/3) (forwhich a supermassive star represent neutrally stable system), the mass, and the size of this object comes out to be 6.87 × 107 M M 1.7 × 109M, and 2.74× 1014 cm a 1.43× 1015cm respectively, for the central temperature,T0 = 6× 107°K, which is sufficient for the release ofnuclear energy. The total energy released during their evolution rangesfrom 2.46× 1060 - 3.18× 1062 erg, which is sufficient to power these objects at least for a period of 106 - 107years. These figures agrees quite well with those cited for Quasi Stellar Objects (QSOs) in the literature.  相似文献   

8.
Structures of Newtonian super-massive stars are calculated with the opacity for Comptor effectK 0/(1 + T), whereK 0=0.21(1 +X and =2.2×10–9K–1. The track of the Main-Sequence is turned right in the upper part of the HR diagram. Mass loss will occur in a Main-Sequence stage for a star with mass larger than a critical mass. The cause of mass loss and the expansion of the radius is continuum radiation pressure. The critical mass for mass loss is 1.02×106 M for a Population I star, and 1.23×105 M for Population III star. Mass loss rates expected in these stars are 3.3×10–3 and 4.0×10–3 M yr–1, respectively.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.  相似文献   

9.
We have modeled stellar coronal loops in static conditions for a wide range of loop length, plasma pressure at the base of the loop and stellar surface gravity, so as to describe physical conditions that can occur in coronae of stars ranging from low mass dwarfs to giants as well as on a significant fraction of the Main-Sequence stars.Three alternative formulations of heat conduction have been used in the energy balance equation, depending on the ratio 0/L Tbetween electron mean free path and temperature scale height: Spitzer's formulation for 0/L Tless than 2 × 10–3, the Luciani, Mora, and Virmont non-local formulation for 0/L Tbetween 2 × 10–3 and 6.67 × 10–3 and the limited free-streaming formulation for 0/L Tlarger than 6.67 × 10–3.We report the characteristics of all loop models studied, and present examples to illustrate how the temperature and density stratification can be drastically altered by the different conductivity regimes. Significant differences are evident in the differential emission measure distribution vs temperature, an important observable quantity. We also show how physical conditions of coronal plasma, and in particular thermal conduction, change with stellar surface gravity.We have found that, for fixed loop length and stellar gravity, a minimum of loop-top plasma temperature occurs, corresponding to the highest value of base plasma pressure for which the limited free-streaming conduction occurs. This value of temperature satisfies the appropriate scalingT 10–9 L g, in cgs units.  相似文献   

10.
The continuum energy distributions of a totally eclipsing binary system GG Cas at phases 0 . P 903 and 0 . P 003 in the wavelenght range 3200–7600 Å have been presented and discussed. The flux of continuum of the secondary star seems to dominate longward of 4500 Å. The spectral types of the systemic components are found to be as B4+K0.  相似文献   

11.
In this paper the gravitational collapse of cosmic gas clouds and formation of star clusters has been considered. Hoyle's view of successive fragmentation has been taken as the basic mechanim in the present work. The initial masses of protostars have been estimated as the function of their distances from the centre of the cluster and the intensity of the magnetic field of the medium. It has been shown that the fragmentation process is greatly inhibited by the presence of a strong magnetic field. A model has been constructed showing how a protostar grows in mass by accretion from the surrounding medium, on the basis of the assumption that as the star moves at random in the cluster it picks up a fraction of the material through which it passes. It has been estimated that a protostar of initial mass of about 0.1M grows to one of 1.0M in a time period which ranges from a few multiples of 105 to a few multiples of 107 yr, depending on the parameters involved in the accretion process. The number of stars per unit mass range has also been estimated; it is found to be proportional tom –3.3,m being the mass of the star.  相似文献   

12.
It has been shown that the mass of neutron stars obtained from equations of state based on nuclear theory depend upon the number of baryons assembled in it but not on the type of interactions considered. On examining the behaviour of different equations of state based on nuclear theories, a simple polytropic equation of state,P = (K/N)(p –p s)N is proposed. The results obtained forN=1.75 cover the entire range of neutron star masses obtained from the equations of state based on nuclear theories and give a maximum mass of 2.8M . Depending upon various mechanisms for energy output the mass of Crab pulsar is estimated to range from 0.32M to 1.5M . The relation connecting the coordinate mass,M, and the rest mass,M 0, may be written asM/M 0.93 (M 0/M)0.9.  相似文献   

13.
The structure and stability of rapidly uniformly rotating supermassive stars is investigated using the full post-Newtonian equations of hydrodynamics. The standard model of a supermassive star, a polytrope of index three, is adopted. All rotation terms up to and including those of order 4, where is the angular velocity, are retained. The effects of rotation and post-Newtonian gravitation on the classical configuration are explicitly evaluated and shown to be very small. The dynamical stability of the model is treated by using the binding energy approach. The most massive objects are found to be dynamically unstable when =1/c 2.p c / c 2.2 × 10–3, wherep c and c are the central pressure and density, respectively. Hence, the higher-order terms considered in this analysis do not appreciably alter the previously known stability limits.The maximum mass that can be stabilized by uniform rotation in the hydrogen-burning phase is found to be 2.9×106 M , whereM is the solar mass. The corresponding nuclear-generated luminosity of 6×1044 erg/sec–1 is too small for the model to be applicable to the quasi-stellar objects. The maximum kinetic energy of a uniformly rotating supermassive star is found to be 3×10–5 Mc 2, whereM is the mass of the star. Masses in excess of 1010 M are required if an adequate store of kinetic energy is to be made available to a pulsar like QSO. However such large masses have rotation periods in excess of 100 yr and thus could not account for any short term periodic variability. It is concluded then that the uniformly rotating supermassive star does not provide a suitable base for a model of a QSO.  相似文献   

14.
Galaxies may have formed by fragmentation in a collapsing cloud of very large mass. The most massive galaxies were formed from fragments which were nearly but not quite opaque: the least massive galaxies were formed from fragments about as large as the Jeans mass. If the maximum mass of galaxies is 1013 M , then the minimum mass should be 106 M .  相似文献   

15.
With the assumption, the physical 3-spacet = constant in a superdense star is spheroidal and the matter-density on the boundary surface of the configurationa = 2 × 1014 g cm–3( the average matter density in a neutron star) Vaidya and Tikekar (1982) proposed an exact relativistic model for a neutron star. They suggested that their model can describe the hydrostatic equilibrium conditions in such a superdense star with densities in the range of 1014-1016 g cm–3. Based on this model Parui and Sarma (1991) estimated the maximum limit of the density variation parameter for a stable neutron star (both for charged and uncharged) which is equal to 0.68, i.e. max = 0.68.In this paper we have shown variation of central density per unit equilibrium radius (0/a), variation of mass, upper limit of density variation parameter both for charged and uncharged neutron stars at densities 1015 and 1016 g cm–3, respectively. We have obtained max = 0.68, i.e. the same as before. The important is that the duration of stability among the neutron star's constituents around max will be shorter and shorter at higher densities as we proceed near the centre of the neutron star. In case of a charged neutron star, once stability among the constituents has been established, then unstability appears gradually maintaining linear relation between change in central density per unit equilibrium radius and change in mass whereas in case of uncharged neutron star, linear relation does not maintain.  相似文献   

16.
We have mapped the nuclear region of the starburst galaxy NGC 253 in the3 P 1 3 P 0 line of neutral carbon using the JCMT. Carbon is widespread across the nuclear region with a similiar distribution to CO as expected. Previous studies of Galactic star-forming regions showed that carbon emission is enhanced in photon-dominated regions (where UV photons impinge upon molecular clouds). Previous observations of other PDR tracers such as ionized carbon and FIR continuum constrain the physical conditions in the PDR gas of NGC 253. The carbon we have observed is far brighter than predicted by theoretical models of PDRs with solar elemental values. This indicates that carbon emission is not a reliable diagnostic of the physical conditions in the nuclear region of a galaxy undergoing a burst of star formation.  相似文献   

17.
Two sets of observations of the WR-eclipsing binary CV Ser were carried out in 1982 and 1983 in the standard photometric systemUBV. Since 1982, the depth of the atmospheric eclipse has changed and does not show in theV data obtained in 1983. There is significant intrinsic variability (0 . m 05) in the light curves of CV Ser over both sets of observations and it is suggested that this is due to the changing mass-loss rate from the envelope of the WC8 star on time-scales from days to months. The observed mass-loss rate can change the mass by some 10–5 M y–1. An orbital inclinationi=72±2° is obtained.  相似文献   

18.
The young cluster NGC 654 is studied using UBV photographic photometry with a view to determining the distribution of interstellar matter in a region where star formation recently occurred.NGC 654 is found to be enclosed in a shell of interstellar matter of mass 1500M . The mass of all stars in the cluster is 4000M .  相似文献   

19.
An inhomogeneous model neutron star with a variable density profile of the type 0(r)=c[1–(2/3)r2/R2]exp(–r2/R2) is considered, where c is the central density, R is the star's radius, and is the inhomogeneity parameter in the radial mass distribution. This parameterization adequately reproduces the results of numerical evolutionary calculations of the density profile and enables one to obtain in analytical form the parameters of hydrostatic equilibrium and the eigenmodes of nonradial oscillations of a nonrotating neutron star, modeled by a spherical mass of incompressible, inviscid liquid. It is shown that a characteristic manifestation of the star's inhomogeneity is the presence of a stable dipole f-mode, the lowest one in the spectrum of natural oscillations. The presence of this mode serves as a general and primary criterion that evidently distinguishes all inhomogeneous hydrodynamic models from the homogeneous Kelvin model, in which the quadrupole mode is the lowest stable mode. Estimates obtained for the periods of nonradial pulsations coincide with the periods of micropulses observed in the average pulse profiles of c-pulsars. This suggests that the detected variations in emission intensity in the range of micropulse duration (on the order of 10–4 sec) are associated with nonradial stellar oscillations.Translated from Astrofizika, Vol. 39, No. 3, pp. 475–488, July–September, 1996.  相似文献   

20.
The fragmentation of a molecular cloud is modelled as a random process by the Monte Carlo method. The probability of the fragmentation is a function of the cloud initial mass and decreases rapidly for mass lower than critical mass, which is a defined parameter. The modelled IMF is compared with the mean mass function in open clusters assumed here as the observed IMF. The best fit was found for initial mass 3×103 M s and for the critical mass range 0.4 to 0.6M s . It also implies the star formation efficiency to be about 0.3.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.  相似文献   

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