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 31 毫秒
1.
We present spectroscopic evolutionary synthesis calculations for starburst galaxies of various metallicities in order to model the broad emission lines HeII 4686 and CIII 4650 produced by Wolf-Rayet (WR) stars in the spectra of WR galaxies. The strengths of both lines strongly decrease with decreasing metallicity. The presence of WR emission features is a clear indicator of very recent star formation less than 4 to 7 × 106 yr ago. Bursts of duration 1 to 5 × 106 yr which lead to an increase in the total stellar mass in the galaxy by 0.1 to 10 % are compatible with the equivalent width of Hell »4686 observed in WR galaxies.  相似文献   

2.
The bright stellar content for fifteen binary star clusters and their adjoining fields in the Large Magellanic Cloud (LMC) were studied here. Film copies of plates taken with the 1.2 U.K. Schmidt telescope were used for deriving the spectral types of the stars in the studied regions. All classified stars are brighter thanV=17.5 mag and situated in large areas around each pair and in a neighbouring field. Seven of the pairs, the brightest and most populous are young clusters (located mainly at the north part of the parent galaxy). The derived distributions of spectral types of their stars give strong evidence that each pair consists of similar stellar content with ages 0.6–8×107 yr.Eight more binary star clusters were studied as well, selected among the rest of the binaries in the LMC. It is found that their stars were faint for our limit of detection so the poor statistics did not allow a comparison among the two cluster members of each pair. However the bright limit of their stars implies ages >6×108 yr. Considering that these objects were randomly selected it is unlikely that all are projected pairs. So it seems that binarity in star clusters is a phenomenon (favourable in the LMC) which did not happen only once in the life of this galaxy.  相似文献   

3.
Space velocities of 146 chromospherically active binary stars have been calculated. Containing F to M spectral types on the Main Sequence together with G and K giants, this very heterogeneous sample has been divided into subsamples in order to segregate stars so that they have similar kinematics and ages. Dispersions of space velocity components and other kinematical quantities (velocity averages and |Z| distributions) of these groups imply their ages as: Both old and young Main-Sequence systems (<10×109 yr and 1×109 yr, respectively) exist in the sample. Systems containing subgiants (single- and double-lined) or double-lined giants as their companions have ages about 2–3×109 yr. Single-lined giants appear to be older than intermediate disk population stars (>5×109 yr). The possible existence of white dwarfs as invisible companions of some of the single lined giants is suggested in order to explain why these systems are older than double-lined giants.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.  相似文献   

4.
The stellar population of the blue compact dwarf galaxy SBS 1415+437 is investigated using the archive database of the Hubble space telescope. The color index-magnitude diagram for stars reaches a magnitude of 29 m in the V and I bands. It comprises young main-sequence stars, blue and red supergiants, and the old population of red giant branch and asymptotic giant branch. The tip of the red giant branch αTRGB) was used to calculate the distance modulus, which turned out to be m ? M = 30.65 ± 0.08 m . The corresponding distance to the galaxy is D = 13.5 ± 1.0 Mpc. The youngest stars are distributed irregularly near the bright H II region in the southwest part of SBS 1415+437. The old population occupies a larger area, it is distributed more evenly and forms the galactic halo. The spatial distribution of young stars shows that the star formation in the galaxy spread in the direction from northeast to southwest over the last 5 × 107 yr with an average rate of 60 km/s. The TRGB of SBS 1415+437 was found to be appreciably shifted to the blue range: (V ? I) TRGB ≈ 1.30. The galaxy age turns out to be not smaller than the age of Galactic globular clusters (about 1010 yr), provided that the galaxy originally had a very low metallicity (our photometric estimate is [Fe/H] = ?2.4). If the metallicity of SBS 1415+437 changed almost not at all in the course of evolution and was equal to [Fe/H] = ?1.3 (as estimated from the emission lines of ionized gas), the galaxy age is no more than 2 × 109 yr.  相似文献   

5.
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   

6.
The evolution of a first-generation 3M star from the threshold of stability through the stage of helium exhaustion in the core has been studied. The total time elapsed is 4.174×108 yr and most of this time is spent in the blue-giant region of theH-R diagram. Hydrogen-burning near the Main Sequence occurs at a high central temperature via the proton-proton chain until the triplealpha reactions generate a small amount of C12 toward the end of the hydrogen-burning phase. The corresponding evolution time is longer than that of a normal population I star with the same mass. The ignition of the triple-alpha processes begins in a mildly degenerate, small convective core while the star still has a high surface temperature. Helium-burning in the core, coupled with hydrogenburning in the shell, occupies a period of about 1.8×107 yr, which is only one-third that of a normal star. The mass of the star interior to the hydrogen shell source has increased to a value of 0.50M near the end of core helium exhaustion. This region maintains an inhomogenous composition composed of helium, carbon and oxygen.  相似文献   

7.
A model of -bursts is considered that treats the flares of neutron stars as a result of convectiveoscillation instability associated with the stars having strong internal magnetic fields ( 1013 to 1014 G). In the context of this model only sufficiently old (104 to 107 yr), drastically cooled-down neutron stars may be sources of -bursts. The paper shows that major characteristics of a -burster in the Supernova N 49 remnant (energy release during burst up to 1044 erg, age 104 yr, burst-to-burst interval (I to 3)×106s; rotation period P=8 s) may be explained under the assumption that the mass of the neutron star is about 0.14M · while its mean magnetic field strength is 1.5×1014 G abd 1013 G within the star and on its surface, respectively. The observational tests of the model discussed conclude the paper.  相似文献   

8.
Observations of nearby star-forming regions suggest that the initial mass function is deficient in low-mass stars when the ambient star formation rate is high. We investigate the consequences of a truncated initial mass function on the photometric evolution of starburst galaxies using new models of stellar population synthesis. A major prediction with respect to the case of a standard initial mass function is that the spectra of the galaxies become unusually red when the turnoff mass reaches the cutoff of the initial mass function. This red phase can last for more than 109 yr if the lower cutoff is less than 2M . The amplitude of the reddening depends only weakly on the lower cutoff but increases with the fraction of the galaxy mass involved in the starburst. Such unusually red (V - K > 3.5) post-starburst galaxies can be distinguished from galaxies reddened by dust or with abnormally high metallicities by their unusually strong 4000 Å break and the presence of stellar absorption features typical of late-type giants.  相似文献   

9.
We present the results of our stellar photometry and spectroscopy for the new Local Group galaxy VV124 (UGC4879) obtainedwith the 6-m BTAtelescope. The presence of a fewbright supergiants in the galaxy indicates that the current star formation process is weak. The apparent distribution of stars with different ages in VV 124 does not differ from the analogous distributions of stars in irregular galaxies, but the ratio of the numbers of young and old stars indicates that VV 124 belongs to the rare Irr/Sph type of galaxies. The old stars (red giants) form the most extended structure, a thick disk with an exponential decrease in the star number density to the edge. Definitely, the young population unresolvable in images makes a great contribution to the background emission from the central galactic regions. The presence of young stars is also confirmed by the [OIII] emission line visible in the spectra that belongs to extensive diffuse galactic regions. The mean radial velocity of several components (two bright supergiants, the unresolvable stellar population, and the diffuse gas) is υ h = −70 ± 15 km s−1 and the velocity with which VV 124 falls into the Local Group is υ LG = −12 ± 15 km s−1. We confirm the distance to the galaxy (D = 1.1 ± 0.1 Mpc) and the metallicity of red giants ([Fe/H] = −1.37) found by Kopylov et al. (2008). VV 124 is located on the periphery of the Local Group approximately at the same distance from M31 and our Galaxy and is isolated from other galaxies. The galaxy LeoA nearest to it is 0.5 Mpc away.  相似文献   

10.
We present some results of the photometric analysis of the stellar population of the irregular dwarf galaxy KK 230 on the basis of the archive database of the Hubble space telescope. The color index-magnitude diagram for KK 230 gets to magnitude 27 m in the V and I bands, and it comprises stellar populations of various ages. The age of the youngest main-sequence stars is 3.2 × 107 yr. These stars are distributed along the north-south direction in the picture plane, and this fact can be linked to the observed kinematics of the neutral gas in the galaxy. Older blue and red supergiants are no less than 1.6 × 108 years old, and such an age implies that the star formation was episodic over the last several hundreds of millions of years. As judged from the position of the tip of the red giant branch, the distance modulus for KK 230 is m ? M = 26.5 m . The corresponding distance is D = 2 Mpc. Based on the average absolute magnitude M I,RC and color index (V ? I)I,RC of the red clump, we conclude that the majority of KK 230 stars have an age of no more than (2–3) × 109 yr, their metallicity being Z ≈ 0.0004.  相似文献   

11.
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   

12.
On the basis of our age estimations of Population I pulsating stars in our Galaxy (Tsvetkov, 1986a), the mean ages of 6 open star clusters containing 21 Delta Scuti-variables and of 8 star clusters and associations containing 13 classical cepheids, have been evaluated. These mean cluster age estimations weighted according to the probabilities for different evolutionary phases of the pulsating stars, are obtained in the evolutionary track systems of Iben (1967) and Paczyski (1970); the cluster ages are larger in the former system. Our results are compared with those obtained from various methods by other authors. Clusters with classical cepheids and with Delta Scuti-stars have ages, respectively, in the ranges 107–108 years and 106–109 years. It is shown that the use of simple period-age(-colour) relations for Population I pulsating stars gives sufficiently accurate cluster age estimations. By use of our period-age relations for classical cepheids (Tsvetkov, 1986a), the mean ages of 56 other star clusters and associations in our Galaxy, the Magellanic Clouds, and M 31 galaxy have been estimated in both systems of tracks. The results are generally in agreement with those obtained from various methods by other authors. The use of Population I pulsating stars in star clusters and associations is one of the simplest and most easily applied methods for determining cluster ages; but there are some limitations in its application.  相似文献   

13.
We use long-slit spectra taken with the William Herschel Telescope on La Palma and high-resolution Hubble Space Telescope imaging to study the gas kinematic in the halo of the ultraluminous infrared/radio galaxy PKS1345+12 (z=0.122). Our long-slit spectra show line splitting at the locations of massive star clusters ( $10^{6}<M_{\mathrm{SSC}}^{\mathrm{YSP}}<10^{7}$ M⊙), indicating that they are moving at up to 450 km?s?1 with respect to the local ambient gas. Given their kinematics, it is plausible that these super star clusters have been formed either in fast-moving gas streams or tidal tails that are falling back into the nuclear regions as part of the merger process, or as a consequence of jet-induced star formation linked to the extended, diffuse radio emission detected in the halo of the galaxy.  相似文献   

14.
We have searched for CO outflows in eight embedded IRAS sources located in the Taurus molecular cloud using the 45m telescope of Nobeyama Radio Observatory. We have detected CO wing emission in four of these sources. CO outflow associated with TMC1A (04365+2535) is strong and spatially compact (radius 0.04 pc). The dynamical timescale of 2.5 × 103 yr suggests this outflow is the youngest one in Taurus.We have combined our data with previously published survey data and have analyzed the physical properties of the outflow sources. We found that 12 out of 16 embedded sources ( 75 %) have CO outflows associated with them; this indicates that almost all stars experience a phase of molecular outflow in their embedded stage. The IRAS color of the outflow sources suggests that the outflows appear in considerably early phase of the evolution of YSOs, that is, as early as YSOs became observable with IRAS and that visible outflow sources are in a transient phase of evolution between embedded sources and visible T Tauri stars without outflow. Visible outflow sources are systematically more luminous than visible no-outflow sources, while embedded outflow sources have comparable luminosities with visible no-outflow sources. Such luminosity function suggests that the YSOs with outflow undergo mass accretion and increase their stellar mass as they progress from embedded sources to visible outflow sources. Typical mass accretion rate derived from the bolometric luminosity is 2 ×10–6 M yr –1. The timescale for mass accretion to acquire typical stellar mass, 0.5 – 0.8M , is 2.5 – 4 × 105 yr.  相似文献   

15.
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   

16.
Physical arguments suggest the spiral arms may be manifestations of the galaxy not being in dynamical equilibrium — in the sense that the kinetic energy of tis stars and gas is less relative to its binding energy than that dictated by the virial theorem. Without constant cooling of the galactic disk (i.e., a progressive increase in the binding energy of the galaxy) such a departure from dynamical equilibrium would be corrected and the spiral arms destroyed in about 109 yr due to an increase in the velocity dispersion of the stars in the disk resulting from their interacting with the spiral arms. The rate of cooling required to maintain the spiral arms, about 6×104 L , may be provided by mass loss from stars in the disk population. The cooling arises from the average scale-heights and velocities of these stars being larger than that of the gas in the disk, so that there is a net loss of kinetic energy and an increase in the binding energy of the galaxy due to the ejected gas settling down to a lower terminal velocity and scale-height in the galactic disk.  相似文献   

17.
In this paper the magnetic superstar model is used to discuss QSO luminosity and density evolution. Our main hypotheses are that (i) mass loss from old stars in massive galaxies cools and then falls into the centre to form a nuclear disc (Bailey, 1980); and (ii) magnetic superstars in galactic nuclei condense out of gaseous material at the centre of a supermassive-magnetised disc (Kundt, 1979). On this generalised model we find that the non-thermal (synchrotron) optical luminosity scales asL opt L 3 t –7/3, whereL is the total blue luminosity of old stars in the galaxy and t is cosmic time. In addition we show that QSO co-moving density follows the lawD(t)exp-(t/t Evol)16/15 with an evolution timescalet Evol = 1.95 × 109 yr. The model as a whole is in good agreement with observations.  相似文献   

18.
The evolution of young (≲ 10 Myr) star clusters with a density exceeding about 105 star pc−3 are strongly affected by physical stellar collisions during their early lifetime. In such environments the same star may participate in several tens to hundreds of collisions ultimately leading to the collapse of the star to a black hole of intermediate mass. At later time, the black hole may acquire a companion star by tidal capture or by dynamical – three-body – capture. When the captured star evolves it starts to fill its Roche-lobe and transfers mass to its accompanying black hole. This then leads to a bright phase of X-ray emission, which lasts for the remaining main-sequence lifetime of the donor. If the star captured by the intermediate mass black hole is relatively low mass ≲ 2 M⊙) the binary will also be visible as a bright source in gravitational waves. Based on empirical models we argue that, for as long as the donor remains on the main sequence, the source will be ultraluminous Lx >rsim 1040 ergs-1 for about a week every few month. When the donor star is more massive >15 M⊙, or evolved off the main sequence the bright time is longer, but the total accretion phase lasts much shorter.  相似文献   

19.
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   

20.
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   

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