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1.
本文根据球状星团所特有的金属丰度特征,利用星族综合方法,探讨了球状星团诸恒星的形成史。研究表明,这些恒星不可能通过恒星形成率和初始质量函数均不随时间变化的单一恒星形成模式产生。原初云通过恒星演化而得到金属丰度污染的过程和多数恒星的形成过程必须分为两个不同的阶段。 球状星团得到金属丰度污染的过程中,若恒星形成具有通常的初始质量函数,则其恒星形成率必须较低,且初始质量函数不能太陡,从而使污染过程中只形成数量较少的低质量恒星,以保证单个球状星团内金属丰度的均匀性。另一种可能性是污染阶段有非常特殊的初始质量函数,只形成大质量的恒星,从而除提供适量污染外不留下任何痕迹。 多数恒星应是在原初云不同部位得到适当污染后通过局域的短暂的爆发性恒星形成(星暴过程)产生。本文进一步探讨了在Fan和Rees球状星团形成模型的框架下,两相介质中温云块相互碰撞造成星暴过程的可能性。  相似文献   

2.
I review the long-term survival chances of young massive star clusters (YMCs), hallmarks of intense starburst episodes often associated with violent galaxy interactions. In particular, I address the key question as to whether at least some of these YMCs can be considered proto-globular clusters (GCs). In the absence of significant external perturbations, the key factor determining a cluster’s long-term survival chances is the shape of its stellar initial mass function. I conclude that there is an increasing body of evidence that GC formation appears to be continuing until today; their long-term evolution crucially depends on their environmental conditions, however.  相似文献   

3.
Star formation happens in a clustered way which is why the star cluster population of a particular galaxy is closely related to the star formation history of this galaxy. From the probabilistic nature of a mass function follows that the mass of the most massive cluster of a complete population, M max, has a distribution with the total mass of the population as a parameter. The total mass of the population is connected to the star formation rate (SFR) by the length of a formation epoch.
Since due to evolutionary effects only massive star clusters are observable up to high ages, it is convenient to use this M max(SFR) relation for the reconstruction of a star formation history. The age distribution of the most massive clusters can therefore be used to constrain the star formation history of a galaxy. The method, including an assessment of the inherent uncertainties, is introduced with this contribution, while following papers will apply this method to a number of galaxies.  相似文献   

4.
We present a multiwavelength study of the formation of massive stellar clusters, their emergence from cocoons of gas and dust, and their feedback on surrounding matter. Using data that span from radio to optical wavelengths, including Spitzer and Hubble Space Telescope ACS observations, we examine the population of young star clusters in the central starburst region of the irregular Wolf–Rayet galaxy IC4662. We model the radio-to-infrared (IR) spectral energy distributions of embedded clusters to determine the properties of their Hii regions and dust cocoons (sizes, masses, densities, temperatures), and use near-IR and optical data with mid-IR spectroscopy to constrain the properties of the embedded clusters themselves (mass, age, extinction, excitation, abundance). The two massive star-formation regions in IC4662 are excited by stellar populations with ages of ～4 Myr and masses of ～3×105 M⊙ (assuming a Kroupa initial mass function). They have high excitation and subsolar abundances, and they may actually be comprised of several massive clusters rather than the single monolithic massive compact objects known as ‘super star clusters’ (SSCs). Mid-IR spectra reveal that these clusters have very high extinction values, A V ～20–25 mag, and that the dust in IC4662 is well mixed with the emitting gas, not in a foreground screen.  相似文献   

5.
Photoionizing feedback in star cluster formation   总被引：1，自引：0，他引：1  
We present the first ever hydrodynamic calculations of star cluster formation that incorporate the effect of feedback from ionizing radiation. In our simulations, the ionizing source forms in the cluster core at the intersection of several dense filaments of inflowing gas. We show that these filaments collimate ionized outflows and suggest such an environmental origin for at least some observed outflows in regions of massive star formation. Our simulations show both positive feedback (i.e. promotion of star formation in neutral gas compressed by expanding H  ii regions) and negative feedback (i.e. suppression of the accretion flow in to the central regions). We show that the volume filling factor of ionized gas is very different in our simulations from the result from the case where the central source interacted with an azimuthally smoothed gas density distribution. As expected, gas density is the key parameter in determining whether or not clusters are unbound by photoionizing radiation. Nevertheless, we find – on account of the acceleration of a small fraction of the gas to high velocities in the outflows – that the deposition in the gas of an energy that exceeds the binding energy of the cluster is not a sufficient criterion for unbinding the bulk of the cluster mass.  相似文献   

6.
It is well known that galaxies accumulating large quantities of gas undergo violent bursts of star formation. This is believed to be due to tidal interactions of galaxies leading to the infall of gas into their central regions. Bursts of star formation in this scenario are transitory phenomena and can be induced only by external sources.However, in some cases there is no direct evidence of tidal interactions in starburst galaxies.We discuss another possibility of bursting phenomena in galaxies connected with nonlinear feedback processes in mass-exchange between components of star-forming region. We consider a three-component model including cold clouds, warm gas and massive stars and take into account the delay processes in the transformation of hot gas ejected by massive stars and evaporated from cold phase, into the warm phase. Self-regulating mechanism of phase transition of small clouds into warm gas due to heating radiation of massive stars is also taken into account.The analysis of stability of the system shows that it could be unstable even in case of a small efficiency in the birth of massive stars. The evolution of unstable nonlinear perturbations leads to the development of self-sustained nonlinear oscillations of star formation.  相似文献   

7.
We present new near-IR and mid-IR images of relatively isolated massive star forming regions with a special emphasis on the candidate precursors to ultra-compact HII regions. These images reveal compact, almost circularly symmetric young stellar clusters, dominated by a single luminous massive member. The clusters indicate flattened structures and dynamically unrelaxed states. In 4 of these young clusters we have discovered a nearly centrally located, prominent dark region visible at 2μm. 10μm images show examples where the dark region is associated with embedded luminous members of the cluster, as well as cases with no central 10μm source. The dark patches without embedded sources could be dense cores surviving inside the (proto?)clusters or left over wreckage from cluster formation. These images support current theoritical simulations of cluster formation.  相似文献   

8.
We present a model for the formation of massive ( M ≳10 M⊙) stars through accretion-induced collisions in the cores of embedded dense stellar clusters. This model circumvents the problem of accreting on to a star whose luminosity is sufficient to reverse the infall of gas. Instead, the central core of the cluster accretes from the surrounding gas, thereby decreasing its radius until collisions between individual components become sufficient. These components are, in general, intermediate-mass stars that have formed through accretion on to low-mass protostars. Once a sufficiently massive star has formed to expel the remaining gas, the cluster expands in accordance with this loss of mass, halting further collisions. This process implies a critical stellar density for the formation of massive stars, and a high rate of binaries formed by tidal capture.  相似文献   

9.
We review the long-term survival chances of young massive star clusters (YMCs), hallmarks of intense starburst episodes often associated with violent galaxy interactions. We address the key question as to whether at least some of these YMCs can be considered proto- globular clusters (GCs), in which case these would be expected to evolve into counterparts of the ubiquitous old GCs believed to be among the oldest galactic building blocks. In the absence of significant external perturbations, the key factor determining a cluster's long-term survival chances is the shape of its stellar initial mass function (IMF). It is, however, not straightforward to assess the IMF shape in unresolved extragalactic YMCs. We discuss in detail the promise of using high-resolution spectroscopy to make progress towards this goal, as well as the numerous pitfalls associated with this approach. We also discuss the latest progress in worldwide efforts to better understand the evolution of entire cluster systems, the disruption processes they are affected by, and whether we can use recently gained insights to determine the nature of at least some of the YMCs observed in extragalactic starbursts as proto-GCs. We conclude that there is an increasing body of evidence that GC formation appears to be continuing until today; their long-term evolution crucially depends on their environmental conditions, however.  相似文献   

10.
Luminous infrared galaxies (LIRGs) provide insights into star formation (SF) and nuclear activity (AGN) under extreme conditions. We are carrying out a multi-wavelength (X-rays, ultraviolet through mid-infrared, and radio) program to obtain high angular resolution observations of a volume-limited sample of local LIRGs. The typical distances to these LIRGs (D=35–75 Mpc) allow us to identify star clusters and H II regions on scales of tens to hundreds of parsecs. We present here recent results on properties of the massive star-forming regions and star clusters in two LIRGs in our sample, Arp 299 and NGC7469.  相似文献   

11.
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   

12.
The galaxy population in rich local galaxy clusters shows a ratio of one quarter elliptical galaxies, two quarters S0 galaxies, and one quarter spiral galaxies. Observations of clusters at redshift 0.5 show a perspicuously different ratio, the dominant galaxy type are spiral galaxies with a fraction of two quarters while the number of S0 galaxies decreases to a fraction of one quarter (Dressler et al. 1997). This shows an evolution of the galaxy population in clusters since redshift 0.5 and it has been suspected that galaxy transformation processes during the infall into a cluster are responsible for this change. These could be merging, starburst or ram-pressure stripping. We use our evolutionary synthesis models to describe various possible effects of those interactions on the star formation of spiral galaxies infalling into clusters. We study the effects of starbursts of various strengths as well as of the truncation of star formation at various epochs on the color and luminosity evolution of model galaxies of various spectral types. As a first application we present the comparison of our models with observed properties of the local S0 galaxy population to constrain possible S0 formation mechanisms in clusters. Application to other types of galaxies is planned for the future. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

13.
We have constructed a dynamo model for the magnetic field in spiral galaxies that takes into account the differences in star formation rates in different galaxies. The difficulty in constructing the model is that the star formation rate does not enter directly into the equations of magnetohydrodynamics, which include only the root-mean-square velocity of the interstellar gas, its density, and the half-thickness of the ionized gas disk. We propose a parametrization of these quantities that relates them to the star formation rate and investigate our model in terms of the so-called no-z approximation, which neglects the details of the magnetic field structure in a direction perpendicular to the galactic disk. The influence of the star formation rate on the galactic dynamo is a threshold one. This influence is small at moderate star formation rates and significant only at very high star formation rates. If the starburst intensity reaches some critical level (exceeding that in the Milky Way by an order of magnitude), then the large-scale magnetic field is destroyed and it is restored only after completion of the starburst. We provide a list of galaxies that exhibit a fairly high star formation rate and that can be interesting to study their magnetic fields.  相似文献   

14.
With the help of a statistical parameter derived from optical spectra, we show that the current star formation rate of a galaxy, falling into a cluster along a supercluster filament, is likely to undergo a sudden enhancement before the galaxy reaches the virial radius of the cluster. From a sample of 52 supercluster-scale filaments of galaxies joining a pair of rich clusters of galaxies within the two-degree Field Redshift Survey region, we find a significant enhancement of star formation, within a narrow range between ∼2 and  3  h −170 Mpc  of the centre of the cluster into which the galaxy is falling. This burst of star formation is almost exclusively seen in the fainter dwarf galaxies  ( M B ≥−20)  . The relative position of the peak does not depend on whether the galaxy is a member of a group or not, but non-group galaxies have on average a higher rate of star formation immediately before falling into a cluster. From the various trends, we conclude that the predominant process responsible for this rapid burst is the close interaction with other galaxies falling into the cluster along the same filament, if the interaction occurs before the gas reservoir of the galaxy gets stripped off due to the interaction with the intracluster medium.  相似文献   

15.
The initial condition of the formation of massive stars is still unclear at present. In particular, it is still debatable whether or not massive stars are formed in the cluster center. Some people considered from the viewpoint of time scale and thought that the mass segregation phenomena in embedded clusters means that the massive stars can only be born in the cluster center. In this paper we used the Monte Carlo method to make numerical simulation of the dynamical evolution of embedded clusters and the result is compared with the observations. It is assumed that at the initial time massive stars are randomly distributed. It was found that, due to the random motions of massive stars, temporary mass segregation may exist at certain times in the course of evolution of a given embedded cluster, and this phenomenon may be very prominent in some of them. It is pointed out that massive star formation in the center is not the only explanation for mass segregation in embedded clusters. In addition, dynamical friction from the gas can effectively reduce the time scale of the dynamical mass segregation. In consequence, the probability of temporary mass segregation is increased.  相似文献   

16.
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17.
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   

18.
Two independent sets of arguments lead us to conclude that the progenitors of superintense bursts (with an energy yield larger than that for ordinary supernovae by one or two orders of magnitude) are born in massive dense star clusters, but generally flare up only after they have left the cluster; these are the same objects that are the progenitors of gamma-ray bursts (GRBs). Each of the giant stellar arcs which are grouped into multiple systems of stellar complexes in the LMC and NGC 6946 could only be produced by a single powerful energy release near its center. The progenitors of these systems of arc-shaped stellar complexes must have had a common source nearby, and it could only be a massive star cluster. Such clusters are actually known near both systems. On the other hand, calculations of the dynamical evolution of star clusters show that close binary systems of compact objects are formed in the dense central parts of the clusters and are then ejected from them during triple encounters. Mergers of the components of such systems are believed to be responsible for GRBs. Since their progenitors are ejected from the cluster before merging, the arc-shaped stellar complexes produced by GRBs are observed near (but not around) the parent clusters. If a considerable fraction of the GRB progenitors are formed as a result star encounters in massive star clusters, and if the GRBs themselves trigger star formation near the parent clusters, then observations of GRBs in star-forming regions are consistent with their origin during mergers of pairs of compact objects.  相似文献   

19.
We examine the evolutionary status of luminous, star-forming galaxies in intermediate-redshift clusters by considering their star formation rates (SFRs) and the chemical and ionization properties of their interstellar emitting gas. Our sample consists of 17 massive, star-forming, mostly disc galaxies with   MB ≲−20  , in clusters with redshifts in the range  0.31 ≲ z ≲ 0.59  , with a median of  〈 z 〉= 0.42  . We compare these galaxies with the identically selected and analysed intermediate-redshift field sample of Mouhcine et al., and with local galaxies from the Nearby Field Galaxy Survey of Jansen et al.
From our optical spectra, we measure the equivalent widths of  [O  ii ]λ3727, Hβ  and [O  iii ]λ5007 emission lines to determine diagnostic line ratios, oxygen abundances and extinction-corrected SFRs. The star-forming galaxies in intermediate-redshift clusters display emission-line equivalent widths which are, on average, significantly smaller than measured for field galaxies at comparable redshifts. However, a contrasting fraction of our cluster galaxies have equivalent widths similar to the highest observed in the field. This tentatively suggests a bimodality in the SFRs per unit luminosity for galaxies in distant clusters. We find no evidence for further bimodalities, or differences between our cluster and field samples, when examining additional diagnostics and the oxygen abundances of our galaxies. This maybe because no such differences exist, perhaps because the cluster galaxies which still display signs of star formation have recently arrived from the field. In order to examine this topic with more certainty, and to further investigate the way in which any disparity varies as a function of cluster properties, larger spectroscopic samples are needed.  相似文献   

20.
Mergers of massive gas-rich galaxies trigger violent starbursts that - over timescales of > 100 Myr and regions > 10 kpc - form massive and compact star clusters comparable in mass and radii to Galactic globular clusters. The star formation efficiency is higher by 1–2 orders of magnitude in these bursts than in undisturbed spirals, irregulars or even BCDs. We ask the question if star formation in these extreme regimes is just a scaled-up version of the normal star formation mode of if the formation of globular clusters reveals fundamentally different conditions. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

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