共查询到20条相似文献，搜索用时 609 毫秒
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《New Astronomy》2016
The evolution of massive stars is only partly understood. Observational constraints can be obtained from the study of massive stars located in young massive clusters. The ESO Public Survey “VISTA Variables in the Vía Lácteá (VVV)” discovered several new clusters hosting massive stars. We present an analysis of massive stars in four of these new clusters. Our aim is to provide constraints on stellar evolution and to better understand the relation between different types of massive stars. We use the radiative transfer code CMFGEN to analyse K-band spectra of twelve stars with spectral types ranging from O and B to WN and WC. We derive the stellar parameters of all targets as well as surface abundances for a subset of them. In the Hertzsprung–Russell diagram, the Wolf–Rayet stars are more luminous or hotter than the O stars. From the log(C/N)–log(C/He) diagram, we show quantitatively that WN stars are more chemically evolved than O stars, WC stars being more evolved than WN stars. Mass loss rates among Wolf–Rayet stars are a factor of 10 larger than for O stars, in agreement with previous findings. 相似文献
3.
Svetozar A. Zhekov Francesco Palla 《Monthly notices of the Royal Astronomical Society》2007,382(3):1124-1132
Chandra grating spectra of a sample of 15 massive OB stars were analysed under the basic assumption that the X-ray emission is produced in an ensemble of shocks formed in the winds driven by these objects. Shocks develop either as a result of radiation-driven instabilities or due to confinement of the wind by a relatively strong magnetic field, and since they are radiative, a simple model of their X-ray emission was developed that allows a direct comparison with observations. According to our model, the shock structures (clumps, complete or fractional shells) eventually become 'cold' clouds in the X-ray sky of the star. As a result, it is expected that for large covering factors of the hot clumps, there is a high probability for X-ray absorption by the 'cold' clouds, resulting in blueshifted spectral lines. Our analysis has revealed that such a correlation indeed exists for the considered sample of OB stars. As to the temperature characteristics of the X-ray emission plasma, the studied OB stars fall in two groups: (i) one with plasma temperature limited to ∼0.1–0.4 keV and (ii) the other with X-rays produced in plasmas at considerably higher temperatures. We argue that the two groups correspond to different mechanisms for the origin of X-rays: in radiation-driven instability shocks and in magnetically confined wind shocks, respectively. 相似文献
4.
We summarize currently available evidence for ionization stratification in the winds of [WR] stars and point out that ionization stratification may be important to explain with radiation-driven wind theory the winds of [WR] stars, while also providing an empirical tool to investigate [WR] wind structures. 相似文献
5.
Francesca Matteucci Antonio Pipino 《Monthly notices of the Royal Astronomical Society》2005,357(2):489-496
We have studied the effects of a hypothetical initial generation containing very massive stars [ M > 100 M⊙ , pair-creation supernovae] on the chemical and photometric evolution of elliptical galaxies. To this purpose, we have computed the evolution of a typical elliptical galaxy with luminous mass of the order of 1011 M⊙ and adopted chemical evolution models already tested to reproduce the main features of ellipticals. We have tested several sets of yields for very massive zero-metallicity stars: these stars should produce quite different amounts of heavy elements than lower-mass stars. We found that the effects of Population III stars on the chemical enrichment is negligible if only one or two generations of such stars occurred, whereas they produce quite different results from the standard models if they continuously formed for a period not shorter than 0.1 Gyr. In this case, the results are at variance with the main observational constraints of ellipticals such as the average [〈α/ Fe〉* ] ratio in stars and the integrated colours. Therefore, we conclude that if Population III stars ever existed they must have been present for a very short period of time and their effects on the following evolution of the parent galaxy must have been negligible. This effect is minimum if a more realistic model with initial infall of gas rather than the classic monolithic model is adopted. Ultimately, we conclude that there is no need to invoke a generation of very massive stars in ellipticals to explain their chemical and photometric properties. 相似文献
6.
As neutron stars spin-down and contract, the deconfinement phase transition can continue to occur, resulting in energy release (so-called deconfinement heating) in case of the first-order phase transition. The thermal evolution of neutron stars is investigated to combine phase transition and the related energy release self-consistently. We find that the appearance of deconfinement heating during spin-down result in not only the cooling delay but also the increase of surface temperature of stars. For stars characterized by intermediate and weak magnetic field strength, a period of increasing surface temperature could exist. Especially, a sharp jump in surface temperature can be produced as soon as quark matter appears in the core of stars with a weak magnetic field. We think that this may serve as evidence for the existence of deconfinement quark matter. The results show that deconfinement heating facilitates the emergence of such characteristic signature during the thermal evolution process of neutron stars. 相似文献
7.
Starforming factories in galaxies produce compact clusters and loose associations of young massive stars. Fast radiation-driven winds and supernovae input their huge kinetic power into the interstellar medium in the form of highly supersonic and superalfvenic outflows. Apart from gas heating, collisionless relaxation of fast plasma outflows results in fluctuating magnetic fields and energetic particles. The energetic particles comprise a long-lived component which may contain a sizeable fraction of the kinetic energy released by the winds and supernova ejecta and thus modify the magnetohydrodynamic flows in the systems. We present a concise review of observational data and models of nonthermal emission from starburst galaxies, superbubbles, and compact clusters of massive stars. Efficient mechanisms of particle acceleration and amplification of fluctuating magnetic fields with a wide dynamical range in starburst regions are discussed. Sources of cosmic rays, neutrinos and multi-wavelength nonthermal emission associated with starburst regions including potential galactic “PeVatrons” are reviewed in the global galactic ecology context. 相似文献
8.
Dany Vanbeveren Houria Belkus Joris Van Bever Nicki Mennekens 《Astrophysics and Space Science》2009,324(2-4):271-276
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.
- 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⊙.
- 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.
9.
W.-R. Hamann 《Astrophysics and Space Science》2010,329(1-2):151-158
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf–Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with “normal” composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion. 相似文献
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H. Schlattl A. Heger H. Oberhummer T. Rauscher A. Csótó 《Astrophysics and Space Science》2004,291(1):27-56
We investigate the dependence of the carbon and oxygen production in stars on the 3α rate by varying the energy of the 0+
2-state of 12C and determine the resulting yields for a selection of low-mass, intermediate-mass and massive stars. The yields are obtained
using modern stellar evolution codes that follow the entire evolution of massive stars, including the supernova explosion,
and consider in detail the 3rd dredge-up process during the thermally pulsating asymptotic giant branch of low-mass and intermediate-mass stars. Our results
show that the C and O production in massive stars depends strongly on the initial mass, and that it is crucial to follow the
entire evolution. A rather strong C production during the He-shell flashes compared to quiescent He burning leads to a lower
sensitivity of the C and O production in low-mass and intermediate-mass stars on the 3α-rate than predicted in our previous
work. In particular, the C production of intermediate-mass stars seems to have a maximum close to the actual value of the
0+
2 energy level of 12C.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
12.
Guy S. Miller & Scott A. Grossman 《Monthly notices of the Royal Astronomical Society》1998,299(1):139-149
In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star accretion disc when the mass transfer rate is very high. By linearizing the radiation hydrodynamic equations around steady spherical outflow, we evaluate the time-dependent response of these winds to perturbations introduced at their inner boundaries. We find that although radiation-driven winds are generally stable, they act as mechanical filters that should produce quasi-periodic oscillations or peaked noise in their radiation output when perturbations force them stochastically. This behaviour may underlie the photospheric oscillations observed in some strong Type I X-ray bursts. 相似文献
13.
Gustavo A. Lanfranchi Francesca Matteucci Gabriele Cescutti 《Monthly notices of the Royal Astronomical Society》2006,365(2):477-488
By means of a detailed chemical evolution model, we follow the evolution of barium (Ba) and europium (Eu) in four Local Group Dwarf Spheroidal (dSph) galaxies, in order to set constraints on the nucleosynthesis of these elements and on the evolution of this type of galaxies compared with the Milky Way. The model, which is able to reproduce several observed abundance ratios and the present-day total mass and gas mass content of these galaxies, adopts up-to-date nucleosynthesis and takes into account the role played by supernovae (SNe) of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assuming that Ba is a neutron-capture element produced in low-mass asymptotic giant branch stars by s-process but also in massive stars (in the mass range 10–30 M⊙ ) by r-process, during the explosive event of SNe of Type II, and that Eu is a pure r-process element synthesized in massive stars also in the range of masses 10–30 M⊙ , we are able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of [Fe/H] in all four galaxies studied. We confirm also the important role played by the very low star formation (SF) efficiencies (ν= 0.005–0.5 Gyr−1 ) and by the intense galactic winds (6–13 times the star formation rate) in the evolution of these galaxies. These low SF efficiencies (compared to the one for the Milky Way disc) adopted for the dSph galaxies are the main reason for the differences between the trends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxies and in the metal-poor stars of our Galaxy. Finally, we provide predictions for Sagittarius galaxy for which data of only two stars are available. 相似文献
14.
《天文和天体物理学研究(英文版)》2014,(8)
In massive stars,convection in the interior is different from that of intermediate and small mass stars. In the main-sequence phase of small mass stars,there is a convective core and a radiative envelope,between which are the radiative intermediate layers with uneven chemical abundances. Semiconvection would occur in the intermediate layers between the convective core and the homogeneous envelope in massive stars. We treat core convective overshooting and semiconvection together as a process. We found that when decreasing overshooting,the semiconvection is more pronounced. In these two processes,we introduce one diffusive parameter D,which is different from other authors who have introduced different parameters for these two zones. The influences of the turbulent diffusion process on chemical evolution and other quantities of the stellar structure are shown in the present paper. 相似文献
15.
邓李才 《紫金山天文台台刊》2003,(2)
大质量恒星由于其高光度、短寿命和质量损失 ,对星系的积分光谱能量分布和重元素增丰起主导作用 ,从而在研究星系的形成和演化上具有特殊的意义。特别是随着天文设备的长足进展 ,我们可以回溯宇宙演化的历史 ,得到形成初期时星系的观测性质。那时 ,大质量恒星主导星系的辐射性质 ,这更加突出了对大质量恒星进一步了解的迫切性。但是大质量恒星的演化性质相对中小质量恒星而言 ,有很多不确定性。本文通过对比现有恒星模型与实测结果 ,对现有大质量恒星演化理论中存在的几个与对流和质量损失相关的问题进行了评述 ,并对从理论上 ,特别是通过数字模拟方法对这些问题进行诊断提出了展望。 相似文献
16.
This paper reviews the current state of the problem of magnetism in massive Main Sequence stars. Chemically peculiar Bp stars with enhanced silicon lines and anomalous helium lines in their spectra are shown to be the most promising targets for the observational verification of various mechanisms of the formation and subsequent evolution of magnetic fields in CP stars. A catalog of magnetic Bp stars, containing 125 objects is prepared. Applying different criteria, we compiled a variety of magnetic star samples, which were then used to analyze magnetic fields in objects of different ages. The results of this analysis show that massive stars generally have stronger fields in all the samples studied, and thus confirm earlier results based on smaller star samples. No tight relation is observed and the parameters of individual objects show a very large scatter about the mean relation. The strongest and most complex fields are found in the youngest Bp stars with ages below 30 Myr. Magnetic Bp stars generally rotate slower than normal B-type stars, except for the hottest objects with enhanced helium lines, which have normal rotation velocities. No systematic differences are found between the angular rotation velocities of Bp stars with anomalous helium and silicon lines. We discuss various criteria, which can be used to observationally test the alternative mechanisms of formation and evolution of magnetic fields in CP stars and, in particular, to quantitatively compare not only the magnitudes, but also the topology of fields in objects of different ages. 相似文献
17.
R. Kh. Hovhannessian 《Astrophysics》2004,47(4):499-504
Relationships between two interstellar reddening-free quantities, the photometric parameter Q and the Balmer jump D, are derived for main sequence O9-G0 stars. We compare our average values of the Balmer jumps for main sequence stars with a theoretical model of the Balmer jump as a function of the effective temperature for main sequence stars. The theoretical and observational data, as well as our calculations, are shown to correlate well. 相似文献
18.
The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M≥100M
⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with
heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main-Sequence stars
for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300M
⊙ galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10−6 and 10−9, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously.
For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and
semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations.
Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low-metallicity massive
stars are hotter and more compact and luminous than their metal-enriched counterparts. Due to their high temperatures, pregalactic
stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars
are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics
of the first stars have significant influence in predictions of the ionizing photon yields from the first luminous objects;
also they develop large convective cores with important helium core masses which are important for explosion calculations. 相似文献
19.
R. Silvotti A. Bonanno A. Frasca S. Bernabei R. Janulis R. Østensen S.-L. Kim B.-G. Park J. Xiaojun J. Guo Z. Liu M.D. Reed R.S. Patterson K.M. Gietzen P.J. Clark G.W. Wolf Y. Lipkin L. Formiggini E. Leibowitz T.D. Oswalt M. Rudkin K. Johnston 《Astrophysics and Space Science》2004,289(3-4):445-448
I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new 3D chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z = 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. The feedback of stars leads to turbulent motions and large-scale flows in the ISM. As one result the galactic disk is significantly enriched by chemical elements synthesized in bulge stars. 相似文献