Capture of stars by rotating homogeneous spherical clusters

Changes of the orbit of a star passing through a homogeneous spherical cluster have been estimated. Before entering the cluster the star is supposed to move in a Keplerian parabolic orbit. When passing through the cluster the star is subject both to the attraction of the smoothed-out distribution of matter in the cluster and to dynamical friction. Due to dynamical friction the energy of the star becomes negative which leads to the elliptic-type motion of the star after leaving the cluster and to capturing the star by the cluster. The formulae for the changes of the star orbit in the cluster are given. Numerical estimation shows that open clusters transform star orbits more noticeably than do globular clusters.     

Another new small-amplitude red variable star with period of about one day in the globular cluster M4

A66 is a red star located at the lowest end of the red giant branch (near the top of the sub-giant branch) of the globular cluster M4. According to its location on the GM diagram, A66 may be a cluster member. No proper motion and radial velocity determinations are available up to date.

We discovered that A66 is a small-amplitude new variable star with a period of about one day and the peak to peak amplitude is about 0.05 mag in V.

However, if its membership can be confirmed by proper motion and radial velocity determinations, the cause of light variation may be not pulsation. This is because according to Xiong's theory, a red star at such low luminosity can not pulsate at a period longer than 0.1 day. Then other causes such as EW type eclipsing variable star or rotation of a spotted star might be responsible. Otherwise A66 is not a cluster member.     

球状星团M4中的一颗新红变星

G302=L3107是球状星团M4中的一颗红星，目前还没有自行或视线速度的测定，Greenstein和Lee分别给出此星的星等和色指数为：     

Captured older stars as the reason for apparently prolonged star formation in young star clusters

The existence of older stars within a young star cluster can be interpreted to imply that star formation occurs on time-scales longer than a free-fall time of a pre-cluster cloud core. Here, the idea is explored that these older stars are not related to the star formation process forming the young star cluster but rather that the orbits of older field stars are focused by the collapsing pre-cluster cloud core. Two effects appear: the focusing of stellar orbits leads to an enhancement of the density of field stars in the vicinity of the centre of the young star cluster; and due to the time-dependent potential of the forming cluster some of these stars can get bound gravitationally to the cluster. These stars exhibit similar kinematical properties to the newly formed stars and cannot be distinguished from them on the basis of radial velocity or proper motion surveys. Such contaminations may lead to a wrong apparent star formation history of a young cluster. In the case of the ONC, the theoretical number of gravitationally bound older low-mass field stars agrees with the number of observed older low-mass stars.     

High-Velocity Star Formation in the Large Magellanic Cloud

Light-echo measurements show that SN 1987A is 425 pc behind the LMC disk. It is continuing to move away from the disk at 18 km s-1. Thus, it has been suggested that SN 1987A was ejected from the LMC disk. However, SN 1987A is a member of a star cluster, so this entire cluster would have to have been ejected from the disk. We show that the cluster was formed in the LMC disk, with a velocity perpendicular to the disk of about 50 km s-1. Such high-velocity formation of a star cluster is unusual, having no known counterpart in the Milky Way.     

Star Formation Properties of the Galaxy Cluster Abell 2199

Through the morphological classifications for 290 member galaxies in the nearby galaxy Abell 2199, the star formation rates and their relations with their morphology and related physical properties are investigated in this paper. It is found that the typical star formation rate in galaxies of this galaxy cluster is strongly correlated with the H_α equivalent width, and the degree of discontinuity of the galaxy spectrum at 4000 Å is also strongly correlated with the stellar mass included in the galaxy. It is also found that star formation activities in these galaxies do not exhibit the obvious circumstance effect. This result indicates that this galaxy cluster is still situated at the stage of the violent dynamical evolution and far from the dynamical equilibrium.     

Numerical simulations of protostellar encounters — III. Non-coplanar disc–disc encounters

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. If protostars are formed individually within a cluster before falling together and interacting, there should be no preferred orientation for such interactions. As star formation within clusters is believed to be coeval, it is probable that, during interactions, both protostars possess massive, extended discs.   We have used an SPH code to carry out a series of simulations of non-coplanar disc–disc interactions. We find that non-coplanar interactions trigger gravitational instabilities in the discs, which may then fragment to form new companions to the existing stars. (This is different from coplanar interactions, in which most of the new companion stars form after material in the discs has been swept up into a shock layer, and this then fragments.) The original stars may also capture each other, leading to the formation of a small- N cluster. If every star undergoes a randomly oriented disc–disc interaction, then the outcome will be the birth of many new stars and substellar objects. Approximately two-thirds of the stars will end up in multiple systems.     

Star cluster formation and star formation: the role of environment and star-formation efficiencies

By analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to determine whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star-formation rate, burst strength, star-formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few billion years later.     

On the onset of runaway stellar collisions in dense star clusters – I. Dynamics of the first collision

We study the circumstances under which first collisions occur in young and dense star clusters. The initial conditions for our direct N -body simulations are chosen such that the clusters experience core collapse within a few million years, before the most massive stars have left the main sequence. It turns out that the first collision is typically driven by the most massive stars in the cluster. Upon arrival in the cluster core, by dynamical friction, massive stars tend to form binaries. The enhanced cross-section of the binary compared to a single star causes other stars to engage the binary. A collision between one of the binary components and the incoming third star is then mediated by the encounters between the binary and other cluster members. Due to the geometry of the binary–single star engagement the relative velocity at the moment of impact is substantially different than in a two-body encounter. This may have profound consequences for the further evolution of the collision product.     

A new red variable star in the globularcluster M4

G302 = L3107 is a red star located at the lowest end of the red giant branch of the globular cluster M4. According to its location on the C-M diagram, G302 may be a cluster member. No proper motion or radial velocity determinations are available up to date.

We discovered that C302 is a new variable star with a main period of 1.24719 day and a peak to peak amplitude of about 0.3 mag in V. It may have a second period of 2.38249 day with an amplitude of about 0.05 mag. If it is a cluster member, then there will be interesting implications for the the theory of stellar evolution.     

Photoionizing feedback in star cluster formation

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.     

Quantitative analysis of clumps in the tidal tails of star clusters

Tidal tails of star clusters are not homogeneous but show well-defined clumps in observations as well as in numerical simulations. Recently, an epicyclic theory for the formation of these clumps was presented. A quantitative analysis was still missing. We present a quantitative derivation of the angular momentum and energy distribution of escaping stars from a star cluster in the tidal field of the Milky Way and derive the connection to the position and width of the clumps. For the numerical realization we use star-by-star N -body simulations. We find a very good agreement of theory and models. We show that the radial offset of the tidal arms scales with the tidal radius, which is a function of cluster mass and the rotation curve at the cluster orbit. The mean radial offset is 2.77 times the tidal radius in the outer disc. Near the Galactic Centre the circumstances are more complicated, but to lowest order the theory still applies. We have also measured the Jacobi energy distribution of bound stars and showed that there is a large fraction of stars (about 35 per cent) above the critical Jacobi energy at all times, which can potentially leave the cluster. This is a hint that the mass loss is dominated by a self-regulating process of increasing Jacobi energy due to the weakening of the potential well of the star cluster, which is induced by the mass loss itself.     

A 'super' star cluster grown old: the most massive star cluster in the Local Group

We independently redetermine the reddening and age of the globular cluster (GC) 037−B327 in M31 by comparing independently obtained multicolour photometry with theoretical stellar population synthesis models. 037−B327 has long been known to have a very large reddening value, which we confirm to be   E ( B − V ) = 1.360 ± 0.013  , in good agreement with the previous results. We redetermine its most likely age at  12.4 ± 3.2 Gyr  .
037−B327 is a prime example of an unusually bright early counterpart to the ubiquitous 'super' star clusters presently observed in most high-intensity star-forming regions in the local Universe. In order to have survived for a Hubble time, we conclude that its stellar initial mass function (IMF) cannot have been top-heavy. Using this constraint, and a variety of simple stellar population (SSP) models, we determine a photometric mass of     , somewhat depending on the SSP models used, the metallicity and age adopted and the IMF representation. This mass, and its relatively small uncertainties, makes this object the most massive star cluster of any age in the Local Group. Assuming that the photometric mass estimate thus derived is fairly close to its dynamical mass, we predict that this GC has a (one-dimensional) velocity dispersion of the order of  (72 ± 13) km s⁻¹  . As a surviving 'super' star cluster, this object is of prime importance for theories aimed at describing massive star cluster evolution.     

The evolutionary state of magnetic Ap stars

The characteristics of two stars, 25 Sex and HD 21699, as additional candidates for the sample of magnetic stars belonging to superclusters, are discussed. For 25 Sex, which was already accepted as a probable member of the Hyades supercluster in a previous study, arguments supporting the view that this star indeed is a magnetic star are presented. In the case of HD 21699, the radial velocity derived from our observations is not inconsistent with membership. But from the determinations of its proper motion found in the literature, this star cannot be regarded as a probable member of the α Per cluster. On the basis of recent evolutionary models, all the well established Ap cluster members appear to be close to the end of their main-sequence life. This suggests that A stars possibly become magnetic at the end of core hydrogen burning.     

Variable Star Status of Two Stars:V204 and I-I-39 in M3

The star I-I-42 (=vZ1390),a cluster member in M3,located near the red edge of the instability strip of the horizontal branch,was discovered by Roberts and Sandage as a low amplitude variable,it was designated as V204 in the "second catalogue of variable stars in globular clusters",but its coordinates given in all versions of this catalogue are wrong since 1955. We argue that V204 is indeed a low amplitude HB variable star,located near to the red edge of the instability strip,with a period of 0.74785d and an amplitude of about 0.04mag in V . We also find that the red cluster member star I-I-39 is a low amplitude variable with a period of 1.16d and amplitude of about 0.03mag in V which might be pulsating at the second overtone.     

Merging time-scales of stellar subclumps in young star-forming regions

Recent observations and hydrodynamical simulations of star formation inside a giant molecular cloud have revealed that, within a star-forming region, stars do not form evenly distributed throughout this region, but rather in small subclumps. It is generally believed that these subclumps merge and form a young star cluster. The time-scale of this merging process is crucial for the evolution and the possible survival of the final star cluster. The key issue is whether this merging process happens faster than the time needed to remove the residual gas of the cloud. A merging time-scale shorter than the gas-removal time would enhance the survival chances of the resulting star cluster. In this paper, we show by means of numerical simulations that the time-scale of the merging is indeed very fast. Depending on the details of the initial subclump distribution, the merging may occur before the gas is expelled from the newly formed cluster via either supernovae or the winds from massive stars. Our simulations further show that the resulting merger objects have a higher effective star formation efficiency than the overall star-forming region and confirm the results that mass-segregated subclumps form mass-segregated merger objects.     

Origin and dynamical evolution of young mass-segregated clusters

A significant degree of mass segregation inconsistent with the effects of standard two-body relaxation has been observed in a number of young star clusters. In this paper we present the results of a survey of N-body simulations aimed at exploring the origin and the dynamical evolution of young mass-segregated star clusters. Our simulations show that large segregated clusters can form from the merger of small clumps that are either initially segregated or in which segregation is produced before the merger is complete; the large cluster produced at the end of the merger process inherits the progenitor clumps’ segregation. We show that, in a young mass-segregated cluster, the effect of early mass loss associated with stellar evolution is, in general, more destructive than for an unsegregated cluster with the same density profile, and leads to shorter lifetimes, a faster initial evolution towards less-concentrated structure and a faster flattening of the stellar initial mass function.     

Flare star activity in the open cluster Alpha Persei

The results from a flare star investigation in the open cluster Alpha Persei are presented. Photographic flare star monitoring and CCD photometry of the discovered 4 new flare stars are made. The flare star activity phenomenon is restricted to the classical flare stars (UV Ceti type) as classified in the GCVS. The V/V‐I diagramme of the members of the cluster with the locations of the considered flare stars is given. Most of the flare stars are probable cluster members. The flare frequency determined from the Rozhen flare star monitoring is very low–one flare event occurs for 38.5 hours effective observing time. Comparison with the flare activity of the Pleiades is made because of the small difference in the age and distance of the clusters.     

Galaxy evolution in clusters

The galaxy populations in present-day clusters are distinctly different from those of the field, indicating that environment plays a strong role in galaxy evolution. This review discusses some of the recent observations of moderate to high redshift clusters. A consistent picture of galaxy evolution in clusters appears to be emerging, which includes a population of galaxies which formed early in the cluster history, as well as field galaxies which have had their star formation truncated upon falling into the cluster potential. Galaxy interactions probably play an important role in exhausting star formation in some of these galaxies. However, there is significant variation in the populations of different cluster samples, with substantial evidence that some galaxies have their star formation terminated more gradually. This suggests that different mechanisms may dominate in different clusters, perhaps because of the recent merging history of the clusters. We also present a recent analysis of population gradients in clusters which suggests that the observed evolution in cluster populations is consistent with a scenario where changing infall rates drive the fraction of star forming galaxies in clusters, rather than a changing physical mechanism within the cluster. Thus, galaxy populations may provide a fundamental measure of the growth of large scale structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

The evolution of the galaxy red sequence in simulated clusters and groups

N -body/hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a Λ cold dark matter (ΛCDM) cosmology are used in order to follow the building-up of the colour–magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the red sequence (RS) as they get aged over times and eventually set upon a 'dead sequence' (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However, galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z ∼ 1, after which major events only involve the brightest cluster galaxies.
The emerging scenario is that the evolution of the colour–magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter haloes assembly. Galaxies progressively quenching their star formation settle to a very sharp 'red and dead' sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z ∼ 1.5) and environment.
Differently from the DS, the operatively defined RS evolves more evidently with z , the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star-forming one: this goes from z ≃ 1 in clusters down to 0.4 in normal groups.