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1.
There is no generally accepted evolutionary scheme for high mass star formation yet. A simple approach to address this problem is to cover several of the known stages during the formation of massive stars in the same cloud and then investigate their properties trying to construct an evolutionary sequence. Here we present such a project conducted with complementary APEX and ATCA observations. These observations show a compact and bright single hot core in the G327.3-0.6 region on a 0.03 pc scale with a mass of 500 M⊙ and 0.5–1.5 105 L⊙. Additionally a clumpy filament is seen in N2H+. Together with cm continuum observations, the data reveal like pearls on a string several stages of massive star formation, with likely the youngest stages hiding in the cold N2H+ cores analysed with a multilevel study of the APEX and ATCA observations.  相似文献   

2.
This paper presents the first analysis of spectroscopic and photometric observations of the neglected southern eclipsing binary star, QY Tel. Spectroscopic observations were carried out at the South African Astronomical Observatory in 2013. New radial velocity curves from this study and V light curves from the All Sky Automated Survey were solved simultaneously using modern light and radial velocity curve synthesis methods. The final model describes QY Tel as a detached binary star where both component stars fill at least half of their Roche limiting lobes. The masses and radii were found to be 1.32 (± 0.06) M⊙, 1.74 (± 0.15) R⊙ and 1.44 (± 0.09) M⊙, 2.70 (± 0.16) R⊙ for the primary and secondary components of the system, respectively. The distance to QY Tel was calculated as 365 (± 40) pc, taking into account interstellar extinction. The evolution case of QY Tel is also examined. Both components of the system are evolved main-sequence stars with an age of approximately 3.2  Gy, when compared to Geneva theoretical evolution models.  相似文献   

3.
Population synthesis is used to model the number of neutron stars in globular clusters that are observed as low-mass X-ray sources and millisecond radio pulsars. The dynamical interactions between binary and single stars in a cluster are assumed to take place only with a continuously replenished “background” of single stars whose properties keep track of the variations in parameters of the cluster as a whole and the evolution of single stars. We use the hypothesis that the neutron stars forming in binary systems from components with initial masses of ～8–12 M ⊙ during the collapse of degenerate O-Ne-Mg cores through electron captures do not acquire a high space velocity. The remaining neutron stars (from single stars with masses >8 M ⊙ or from binary components with masses >12 M ⊙) are assumed to be born with high space velocities. According to this hypothesis, a sizeable fraction of the forming neutron stars remain in globular clusters (about 1000 stars in a cluster with a mass of 5 × 105 M ⊙). The number of millisecond radio pulsars forming in such a cluster in the case of accretion-driven spinup in binary systems is found to be ～10, in agreement with observations. Our modeling also reproduces the observed shape of the X-ray luminosity function for accreting neutron stars in binary systems with normal and degenerate components and the distribution of spin periods for millisecond pulsars.  相似文献   

4.
We have obtained new estimates of the Sun’s distance from the symmetry plane Z ⊙ and the vertical disk scale height h using currently available data on stellar OB associations, Wolf–Rayet stars, HII regions, and Cepheids. Based on individual determinations, we have calculated the mean Z ⊙ = ?16 ± 2 pc. Based on the model of a self-gravitating isothermal disk for the density distribution, we have found the following vertical disk scale heights: h = 40.2 ± 2.1 pc from OB associations, h = 47.8 ± 3.9 pc from Wolf–Rayet stars, h = 48.4 ± 2.5 pc from HII regions, and h = 66.2 ± 1.6 pc from Cepheids. We have estimated the surface, Σ = 6 kpc?2, and volume, D(Z ⊙) = 50.6 kpc?3, densities from a sample of OB associations. We have found that there could be ～5000 OB associations in the Galaxy.  相似文献   

5.
《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   

6.
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   

7.
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   

8.
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   

9.
《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   

10.
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   

11.
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   

12.
The fine structure of the nucleus of the Seyfert galaxy NGC 1275 was investigated in 2005–2010 at a wavelength of 2 cm with a resolution as high as 50 μas. The structure consists of two parallel identical systems, eastern and western, spaced 0.5 pc apart in the plane of the sky. Each of them contains an ejector and a bipolar outflow. There are extended regions, lobes, at the extension of the bipolar outflows in the ?10° and 170° directions at distances of 5 pc northward and 6.5 pc southward of the active zone. The observed difference between the jet and counterjet sizes by a factor of ~3 and between the distances to the lobes by a factor of 0.8 is determined by the difference between their velocities and by the change of sign of the outflow acceleration in the period of silence. The high-velocity bipolar outflows are surrounded by three pairs of low-velocity components. The diameters of the low-velocity coaxial outflows and the third component are Ø1 ≈ 0.3 pc, Ø2 ≈ 0.8 pc, and Ø3 ≈ 1.4 pc at the detection limit. The outer low-velocity components of the outflows encompass both high-velocity outflows. The velocities of the outflows and their brightness temperatures increase exponentially as the center of the high-velocity outflows is approached. The brightness temperatures of the high-velocity outflows at the ejector exit are T b > 1012 K. The spectral line velocities in the nuclear region differ by ~600 km s?1 due to the velocity difference between the two systems. In the case of Keplerian motion, the revolution period is ~5 × 103 yr, and the mass of the central massive bodies, black holes, is M ≈ 107M⊙. The fine structure suggests a vortical nature of the formation. In the case under consideration, two parallel vortices spaced ~0.5 pc apart and shifted by ~0.5 pc relative to each other were formed. The surrounding material inflows onto the disk of each system, is transferred in a spiral to the center, and is ejected in the ?10° and 170° directions as an excess angular momentum is accumulated. The interaction with the surrounding medium accelerates and collimates the rotating outflows. The residual material falls to the forming central massive body, a black hole, whose gravitational field stabilizes and accelerates the system formation process.  相似文献   

13.
Recent observations of the X-ray and EUV emission of non-supergiant B stars are summarized. As compared with O stars, the X-rays of most of the near-main-sequence B stars are soft, and the stars show a departure from theL x = 10?7 L bol relation. Using line driven wind models to provide an estimate of the density distribution, it is concluded that a major fraction of the wind emission measure is hot, whereas in shocked wind theory less than 10 percent of the wind emission measure should be hot. The X-ray observations suggest that all of the B stars are X-ray emitters with a basal X-ray luminosity of about 10?8.5 L bol . A hard component dominates the X-ray emission of τ Sco, and possible causes are discussed. For the Be stars, the X-ray emission is that which is expected from a normal B-star wind coming from the poles, as in the Wind Compressed Disk (WCD) model of Be stars. None of the stars, including theβ Cep stars, show noticeable variability in their X-rays.EUVE observations of∈ CMa B2 II, find it to be the brightest object in the EUV sky at 500 to 700 Å. It shows a Lyman continuum flux that is a factor of 30 higher than line blanketed model atmospheres. The continuum is seen on both sides of the He I 504 Å edge, and the discrepancy with model atmospheres is even greater shortward of 504 Å. TheEUVE spectra show emission lines both from high stages of ionization ( Feix to Fexvi) and from low stages (Heii and Oiii). The Heii Lymanα results from recombination following X-ray photoionization in the wind, and the Oiii resonance line is found to be present because of the Bowen fluorescence mechanism. Thus, there is and interesting coupling between the wind production by the EUV photospheric emission, the production of X-ray and line EUV emission by winds, and the production of fluorescence by recombination in the wind; all of these processes are now observable in B stars.  相似文献   

14.
A sample of O- and B-type stars with Hipparcos astrometric data, ages computed from Strömgren photometry and radial velocities, has been used to characterize the structure, age and kinematics of the Gould Belt system. The local spiral structure of our galaxy is determined from this sample, and also from a sample of Hipparcos Cepheid stars. The Gould Belt, with an orientation with respect to the galactic plane ofi G = 16-22° and ΩG =275-295°, extends up to a distance of 600 pc from the Sun. Roughly the 60-65% of the O and B stars younger than 60 Myr in the solar neighbourhood belong to this structure. Our results indicate that the kinematical behaviour of this system is complex, with an expansion motion in the solar neighbourhood (R<300 pc).In the frame of the Lin's theory, and analysing the O and B stars further than 600 pc and the Cepheids, we found a galactic spiral structure characterized by a 4-arm spiral pattern with the Sun located atψ⊙ = 350-355 ± 30° – near the Sagittarius-Carina arm– and outside the corotation circle. The angular rotation speed of the spiral pattern was found to be Ωp = 31-32 ± 4 kms-1 kpc-1.  相似文献   

15.
Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveals many compact (<0.5 pc) and massive (10–2100 M⊙) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.  相似文献   

16.
The systematic UBV observations of six variable post-AGB supergiants in 1991–1999 are presented. Their variability is analyzed. The coolest stars V1027 Cyg and V354 Lac exhibit bimodal pulsations with variable amplitudes. Apart from pulsations, the hotter stars V887 Her and IRAS 19386+0155 show light variations associated with a stellar wind. A variable stellar wind appears to be mainly responsible for the photometric variations in the still hotter stars SAO 163075 and IRAS 20572+4919. Distinct trends in the yearly mean brightness have been found in three of the six supergiants studied, with the trend amplitude being independent of the spectral range. They are interpreted as the result of dust envelopes composed of large grains with R=A V /E(B?V)≥7 becoming optically thin.  相似文献   

17.
A model is developed for the H2O maser source observed in the circumnuclear region of the galaxy NGC 4258. The maser emission originates at distances of 0.15–0.29 pc from the center in a thin, cold accretion disk (gas-dust torus) rotating around a supermassive black hole of mass 4 × 107 M ⊙. The conditions for the emergence of an inverse population of the signal 616?523 levels in ortho-H2O working molecules are simulated numerically. The complex line profile, which includes both central and high-velocity components, is calculated. A comparison of the calculations with radiointerferometric and spectrophotometric observations allowed the physical conditions in the emitting region to be determined.  相似文献   

18.
《New Astronomy Reviews》2002,46(2-7):239-245
The nearby E0 galaxy M87 in the Virgo Cluster contains one of the nearest examples of a powerful non-thermal jet source. In addition, HST spectroscopy of its nucleus give strong evidence for the presence of a ≈3×109 M⊙ central black hole (corresponding to Schwarzschild radius Rs∼0.0004 pc). These two facts together make M87 the best possible target for studying the initial jet formation and collimation process. Herein we report new 43 GHz VLBI observations of the nucleus of this galaxy, which clearly show the jet opening angle expanding rapidly as one approaches the core on scales ∼0.01 pc. We believe we have, for the first time, imaged the initial collimation region of a powerful extragalactic jet. We find significant collimation occurring on scales of 30–100 Rs, and argue this is consistent with expectations for poloidal collimation by a rotating accretion disk.  相似文献   

19.
We use dual-site radio observations of interplanetary scintillation (IPS) with extremely long baselines (ELB) to examine meridional flow characteristics of the ambient fast solar wind at plane-of-sky heliocentric distances of 24?–?85 solar radii (R ⊙). Our results demonstrate an equatorwards deviation of 3?–?4° in the bulk fast solar wind flow direction over both northern and southern solar hemispheres during different times in the declining phase of Solar Cycle 23.  相似文献   

20.
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   

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