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Star Clusters (star + cluster)

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Physics and Astronomy	100%

Selected Abstracts

Gas dynamics of the central few parsec region of NGC 1068 fuelled by the evolving nuclear star cluster

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
M. Schartmann
ABSTRACT Recently, high-resolution observations with the help of the near-infrared adaptive optics integral field spectrograph Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the Very Large Telescope proved the existence of massive and young nuclear star clusters in the centres of a sample of Seyfert galaxies. With the help of three-dimensional high-resolution hydrodynamical simulations with the Pluto code, we follow the evolution of such clusters, especially focusing on stellar mass loss feeding gas into the ambient interstellar medium and driving turbulence. This leads to a vertically wide distributed clumpy or filamentary inflow of gas on large scales (tens of parsec), whereas a turbulent and very dense disc builds up on the parsec scale. In order to capture the relevant physics in the inner region, we treat this disc separately by viscously evolving the radial surface density distribution. This enables us to link the tens of parsec-scale region (accessible via SINFONI observations) to the (sub-)parsec-scale region (observable with the mid-infrared interferometer instrument and via water maser emission). Thereby, this procedure provides us with an ideal testbed for data comparison. In this work, we concentrate on the effects of a parametrized turbulent viscosity to generate angular momentum and mass transfer in the disc and additionally take star formation into account. Most of the input parameters are constrained by available observations of the nearby Seyfert 2 galaxy NGC 1068, and we discuss parameter studies for the free parameters. At the current age of its nuclear starburst of 250 Myr, our simulations yield disc sizes of the order of 0.8,0.9 pc, gas masses of 106 M, and mass transfer rates of 0.025 M, yr,1 through the inner rim of the disc. This shows that our large-scale torus model is able to approximately account for the disc size as inferred from interferometric observations in the mid-infrared and compares well to the extent and mass of a rotating disc structure as inferred from water maser observations. Several other observational constraints are discussed as well. [source]

A spectroscopic survey of EC4, an extended cluster in Andromeda's halo,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
M. L. M. Collins
ABSTRACT We present a spectroscopic survey of candidate red giant branch stars in the extended star cluster, EC4, discovered in the halo of M31 from our Canada,France,Hawaii Telescope/MegaCam survey, overlapping the tidal streams, Streams ,Cp' and ,Cr'. These observations used the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope to obtain spectra around the Caii triplet region with ,1.3 Å resolution. Six stars lying on the red giant branch within two core radii of the centre of EC4 are found to have an average vr=,287.9+1.9,2.4 km s,1 and ,v,corr = 2.7+4.2,2.7 km s,1, taking instrumental errors into account. The resulting mass-to-light ratio for EC4 is M/L = 6.7+15,6.7 M,/L,, a value that is consistent with a globular cluster within the 1, errors we derive. From the summed spectra of our member stars, we find EC4 to be metal-poor, with [Fe/H]=,1.6 ± 0.15. We discuss several formation and evolution scenarios which could account for our kinematic and metallicity constraints on EC4, and conclude that EC4 is most comparable with an extended globular cluster. We also compare the kinematics and metallicity of EC4 with Streams ,Cp' and ,Cr', and find that EC4 bears a striking resemblance to Stream ,Cp' in terms of velocity, and that the two structures are identical in terms of both their spectroscopic and photometric metallicities. From this, we conclude that EC4 is likely related to Stream ,Cp'. [source]

Quantitative analysis of clumps in the tidal tails of star clusters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
A. Just
ABSTRACT 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. [source]

The mass function of , Centauri down to 0.15 M,,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
A. Sollima
ABSTRACT By means of deep FORS1/VLT and ACS/Hubble Space Telescope observations of a wide area in the stellar system , Cen we measured the luminosity function of main-sequence stars down to R= 22.6 and IF814W= 24.5. The luminosity functions obtained have been converted into mass functions and compared with analytical initial mass functions (IMFs) available in the literature. The mass function obtained, reaching M, 0.15 M,, can be well reproduced by a broken power law with indices ,=,2.3 for M > 0.5 M, and ,=,0.8 for M < 0.5 M,. Since the stellar populations of , Cen have been proved to be actually unaffected by dynamical evolution processes, the mass function measured in this stellar system should represent the best approximation of the IMF of a star cluster. The comparison with the MF measured in other Galactic globular clusters suggests that possible primordial differences in the slope of the low-mass end of their MF could exist. [source]

Long-lived disc accretion in the , Chamaeleontis pre-main sequence star cluster

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
Warrick A. Lawson
ABSTRACT High-resolution spectroscopic study of late-type members of the ,9-Myr-old , Chamaeleontis star cluster shows that four stars, RECX 5, 9, 11 and ECHA J0843.3,7905, have broad H, profiles indicative of ballistic accretion of material from circumstellar discs first identified by virtue of their infrared (IR) excess emission. Quantitative analysis of the profiles finds accretion in , Cha stars at rates comparable to that derived by Muzerolle et al. for members of the similarly aged TW Hydrae Association (TWA); rates 1,3 orders of magnitude lower than in younger classical T Tauri (CTT) stars. Together these studies indicate that the fraction of long-lived inner discs can be significantly higher than that inferred from study of younger pre-main sequence (PMS) populations, which suggest a disc lifetime of <6 Myr. The detection of long-lived discs may have implications for the formation of planetary systems. If slow accretion processes are the dominant formation mechanism for Jovian planets then long-lived discs may be ideal sites to search for evidence for protoplanets. [source]

Star cluster ecology , V. Dissection of an open star cluster: spectroscopy

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
Simon F. Portegies Zwart
ABSTRACT We have modelled in detail the evolution of rich open star clusters such as NGC 2516, NGC 2287, Pleiades, Praesepe, Hyades, NGC 2660 and 3680, using simulations that include stellar dynamics as well as the effects of stellar evolution. The dynamics is modelled via direct N -body integration, while the evolution of single stars and binaries is followed through the use of fitting formulae and recipes. The feedback of stellar and binary evolution on the dynamical evolution of the stellar system is taken into account self-consistently. Our model clusters dissolve in the tidal field of the Galaxy in a time-span of the order of a billion years. The rate of mass loss is rather constant, ,1 M, per million years. The binary fraction at first is nearly constant in time, then increases slowly near the end of a cluster's lifetime. For clusters which are more than about 108 yr old the fractions of stars in the form of binaries, giants and merger products in the inner few core radii are considerably higher than in the outer regions, beyond the cluster's half-mass radius. When stars with masses ,2 M, escape from the cluster, they tend to do so with velocities higher than average. The stellar merger rate in our models is roughly one per 30 million years. Most mergers are the result of unstable mass transfer in close binaries (,70 per cent), but a significant minority are caused by direct encounters between single and binary stars. While most mergers occur within the cluster core, even beyond the half-mass radius stellar mergers occasionally take place. We notice a significant birth rate of X-ray binaries, most containing a white dwarf as the mass acceptor. We also find one high-mass X-ray binary with a neutron-star accretor. If formed and retained, black holes participate in many (higher-order) encounters in the cluster centre, resulting in a large variety of exotic binaries. The persistent triple and higher-order systems formed in our models by dynamical encounters between binaries and single stars are not representative for the multiple systems observed in the Galactic disc. We conclude that the majority of multiples in the disc probably formed when the stars were born, rather than through later dynamical interactions. [source]

Self-consistent simulations of star cluster formation from gas clouds under the influence of galaxy-scale tidal fields

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
Jarrod R. Hurley
ABSTRACT We present the first results of a project aimed at following the formation and long-term dynamical evolution of star clusters within the potential of a host galaxy. Here, we focus on a model evolved within a simplified potential representing the Large Magellanic Cloud. This demonstrates for the first time the self-consistent formation of a bound star cluster from a giant molecular cloud. The model cluster reproduces the density profiles and structural characteristics of observed star clusters. [source]

The ultraluminous X-ray source in M82: an intermediate-mass black hole with a giant companion

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006
A. Patruno
ABSTRACT The starburst galaxy M82, at a distance of 12 million light years, is the host of an unusually bright 2.4,16 × 1040 erg s,1 X-ray point source, which is best explained by an accreting black hole 102 to 104 times more massive than the Sun. Though the strongest candidate for a so-called intermediate-mass black hole, the only support stems from the observed luminosity and the 0.05,0.1 Hz quasi-periodicity in its signal. Interestingly, the 7,12 Myr old star cluster MGG-11 which has been associated with the X-ray source is sufficiently dense that an intermediate mass black hole could have been produced in the cluster core via collision runaway. The recently discovered 62.0 ± 2.5 d periodicity in the X-ray source X-1 further supports the hypothesis that this source is powered by a black hole several hundred times more massive than the Sun. We perform detailed binary evolution simulations with an accreting compact object of 10,5000 M, and find that the X-ray luminosity, the age of the cluster, the observed quasi-periodic oscillations and the now observed orbital period are explained best by a black hole of 200,5000 M, that accretes material from a 22,25 M, giant companion in a state of Roche-lobe contact. Interestingly, such a companion star is consistent with the expectation based on the tidal capture in a young and dense star cluster such as MGG-11, making the picture self-consistent. [source]

Gas dynamics of the central few parsec region of NGC 1068 fuelled by the evolving nuclear star cluster

Quantitative analysis of clumps in the tidal tails of star clusters

Seven young star clusters in the inner region of the Small Magellanic Cloud

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Andrés E. Piatti
ABSTRACT We present CCD photometry in the Washington system C and T1 passbands down to T1, 22 in the fields of L35, L45, L49, L50, L62, L63 and L85, seven poorly studied star clusters in the inner region of the Small Magellanic Cloud (SMC). We measured T1 magnitudes and C,T1 colours for a total of 114 826 stars distributed throughout cluster areas of 13.7 × 13.7 arcmin2 each. Cluster radii were estimated from star counts distributed throughout the entire observed fields. The seven clusters are generally characterized by a relatively small angular size and by a high field star contamination. We performed an in-depth analysis of the field star contamination of the colour,magnitude diagrams (CMDs), and statistically cleaned the cluster CMDs. Based on the best fits of isochrones computed by the Padova group to the (T1, C,T1) CMDs, we derive ages for the sample, assuming Z= 0.004, finding ages between 25 Myr and 1.2 Gyr. We then examined different relationships between positions in the SMC, age and metallicity of a larger sample of clusters including our previous work whose ages and metallicities are on the same scale used in this paper. We confirm previous results in the sense that the further a cluster is from the centre of the galaxy, the older and more metal poor it is, with some dispersion; although clusters associated with the Magellanic Bridge clearly do not obey the general trend. The number of clusters within , 2° of the SMC centre appears to have increased substantially after ,2.5 Gyr ago, hinting at a burst. [source]

Comparative study between N -body and Fokker,Planck simulations for rotating star clusters , I. Equal-mass system

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Eunhyeuk Kim
ABSTRACT We have carried out N -body simulations for rotating star clusters with equal mass and compared the results with Fokker,Planck models. These two different approaches are found to produce fairly similar results, although there are some differences with regard to the detailed aspects. We confirmed the acceleration of the core collapse of a cluster due to an initial non-zero angular momentum and found a similar evolutionary trend in the central density and velocity dispersion in both simulations. The degree of acceleration depends on the initial angular momentum. Angular momentum is being lost from the cluster due to the evaporation of stars with a large angular momentum on a relaxation time-scale. [source]

A comprehensive set of simulations studying the influence of gas expulsion on star cluster evolution

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
H. Baumgardt
ABSTRACT We have carried out a large set of N -body simulations studying the effect of residual-gas expulsion on the survival rate, and final properties of star clusters. We have varied the star formation efficiency (SFE), gas expulsion time-scale and strength of the external tidal field, obtaining a three-dimensional grid of models which can be used to predict the evolution of individual star clusters or whole star cluster systems by interpolating between our runs. The complete data of these simulations are made available on the internet. Our simulations show that cluster sizes, bound mass fraction and velocity profile are strongly influenced by the details of the gas expulsion. Although star clusters can survive SFEs as low as 10 per cent if the tidal field is weak and the gas is removed only slowly, our simulations indicate that most star clusters are destroyed or suffer dramatic loss of stars during the gas removal phase. Surviving clusters have typically expanded by a factor of 3 or 4 due to gas removal, implying that star clusters formed more concentrated than as we see them today. Maximum expansion factors seen in our runs are around 10. If gas is removed on time-scales smaller than the initial crossing time, star clusters acquire strongly radially anisotropic velocity dispersions outside their half-mass radii. Observed velocity profiles of star clusters can therefore be used as a constraint on the physics of cluster formation. [source]

A census of young stellar populations in the warm ULIRG PKS 1345+12

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
J. Rodríguez Zaurín
ABSTRACT We present a detailed investigation of the young stellar populations (YSP) in the radio-loud ultraluminous infrared galaxy (ULIRG), PKS 1345+12 (z= 0.12), based on high-resolution Hubble Space Telescope (HST) imaging and long-slit spectra taken with the William Herschel Telescope (WHT) at La Palma. While the images clearly show bright knots suggestive of super star clusters (SSCs), the spectra reveal the presence of YSP in the diffuse light across the full extent of the halo of the merging double nucleus system. Spectral synthesis modelling has been used to estimate the ages of the YSP for both the SSC and the diffuse light sampled by the spectra. For the SSC, we find ages tSSC < 6 Myr with reddenings 0.2 < E(B,V) < 0.5 and masses 106 < MYSPSSC < 107 M,. In the region to the south of the western nucleus that contains the SSC our modelling of the spectrum of the diffuse light is also consistent with a relatively young age for the YSP (,5 Myr), although older YSP ages cannot be ruled out. However, in other regions of the galaxy we find that the spectra of the diffuse light component can only be modelled with a relatively old post-starburst YSP (0.04,1.0 Gyr) or with a disc galaxy template spectrum. The results demonstrate the importance of accounting for reddening in photometric studies of SSC and highlight the dangers of focusing on the highest surface brightness regions when trying to obtain a general impression of the star formation activity in the host galaxies of ULIRGs. The case of PKS 1345+12 provides clear evidence that the star formation histories of the YSP in ULIRGs are complex. While the SSC represent the vigorous phase of star formation associated with the final stages of the merger, the YSP in the diffuse light are likely to represent star formation in one or more of the merging galaxies at an earlier stage or prior to the start of the merger. Intriguingly, our long-slit spectra show line splitting at the locations of the SSC, 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 the SSCs have been formed either in fast moving gas streams/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. [source]

The evolution of binary star clusters and the nature of NGC 2136/NGC 2137

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007
S. F. Portegies Zwart
ABSTRACT We study the evolution of bound pairs of star clusters by means of direct N -body simulations. Our simulations include mass loss by stellar evolution. The initial conditions are selected to mimic the observed binary star clusters, NGC 2136 and 2137, in the Large Magellanic Cloud. Based on their rather old ages (,100 Myr), masses, sizes and projected separation, we conclude that the cluster pair must have been born with an initial separation of 15,20 pc. Clusters with a smaller initial separation tend to merge in ,60 Myr due to loss of angular momentum from escaping stars. Clusters with a larger initial separation tend to become even more widely separated due to mass loss from the evolving stellar populations. The early orbital evolution of a binary cluster is governed by mass loss from the evolving stellar population and by loss of angular momentum from escaping stars. Mass loss by stellar winds and supernovae explosions in the first ,30 Myr causes the binary to expand and the orbit to become eccentric. The initially less massive cluster expands more quickly than the binary separation increases, and is therefore bound to initiate mass transfer to the more massive cluster. This process is quite contrary to stellar binaries in which the more massive star tends to initiate mass transfer. Since mass transfer proceeds on a thermal time-scale from the less massive to the more massive cluster, this semidetached phase is quite stable, even in an eccentric orbit until the orbital separation reaches the gyration radius of the two clusters, at which point both clusters merge to one. [source]

A systematic survey for infrared star clusters with |b| < 20° using 2MASS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
D. Froebrich
ABSTRACT We used star density maps obtained from the Two-Micron All-Sky Survey (2MASS) to obtain a sample of star clusters in the entire Galactic Plane with |b| < 20°. A total of 1788 star cluster candidates are identified in this survey. Among those are 681 previously known open clusters and 86 globular clusters. A statistical analysis indicates that our sample of 1021 new cluster candidates has a contamination of about 50 per cent. Star cluster parameters are obtained by fitting a King profile to the star density. These parameters are used to statistically identify probable new globular cluster candidates in our sample. A detailed investigation of the projected distribution of star clusters in the Galaxy demonstrates that they show a clear tendency to cluster on spatial scales in the order of 12,25 pc, a typical size for molecular clouds. [source]

Induced planet formation in stellar clusters: a parameter study of star,disc encounters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005
Ingo Thies
ABSTRACT We present a parameter study of the possibility of tidally triggered disc instability. Using a restricted N -body model that allows for a survey of an extended parameter space, we show that a passing dwarf star with a mass between 0.1 and 1 M, can probably induce gravitational instabilities (GIs) in the pre-planetary solar disc for prograde passages with minimum separations below 80,170 au for isothermal or adiabatic discs. Inclined and retrograde encounters lead to similar results but require slightly closer passages. Such encounter distances are quite likely in young moderately massive star clusters. The induced GIs may lead to enhanced planetesimal formation in the outer regions of the protoplanetary disc and could therefore be relevant for the existence of Uranus and Neptune, whose formation time-scale of about 100 Myr is inconsistent with the disc lifetimes of about a few Myr according to observational data by Haisch, Lada & Lada. The relatively small gas/solid ratio in Uranus and Neptune can be matched if the perturbing fly-by occurred after early gas depletion of the solar system, i.e. when the solar system was older than about 5 Myr. We also confirm earlier results by Heller that the observed 7° tilt of the solar equatorial plane relative to the ecliptic plane could be the consequence of such a close encounter. [source]

Star cluster ecology , V. Dissection of an open star cluster: spectroscopy

Mass segregation in young compact clusters in the Large Magellanic Cloud , III.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
Implications for the initial mass function
ABSTRACT The distribution of core radii of rich clusters in the Large Magellanic Cloud (LMC) systematically increases in both upper limit and spread with increasing cluster age. Cluster-to-cluster variations in the stellar initial mass function (IMF) have been suggested as an explanation. We discuss the implications of the observed degree of mass segregation in our sample clusters for the shape of the initial mass function. Our results are based on Hubble Space Telescope/WFPC2 observations of six rich star clusters in the LMC, selected to include three pairs of clusters of similar age, metallicity and distance from the LMC centre, and exhibiting a large spread in core radii between the clusters in each pair. All clusters show clear evidence of mass segregation: (i) their luminosity function slopes steepen with increasing cluster radius, and (ii) the brighter stars are characterized by smaller core radii. For all sample clusters, both the slope of the luminosity function in the cluster centres and the degree of mass segregation are similar to each other, within observational errors of a few tenths of power-law slope fits to the data. This implies that their initial mass functions must have been very similar, down to ,0.8,1.0 M,. We therefore rule out variations in the IMF of the individual sample clusters as the main driver of the increasing spread of cluster core radii with cluster age. [source]

Testing stellar population models with star clusters in the Large Magellanic Cloud

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002
Michael A. Beasley
Abstract We present high signal-to-noise ratio integrated spectra of 24 star clusters in the Large Magellanic Cloud (LMC), obtained using the FLAIR spectrograph at the UK Schmidt telescope. The spectra have been placed on to the Lick/IDS system in order to test the calibration of Simple Stellar Population (SSP) models. We have compared the SSP-predicted metallicities of the clusters with those from the literature, predominantly taken from the Ca-triplet spectroscopy of Olszewski et al. (1991). We find that there is good agreement between the metallicities in the range ,2.10 ,[Fe/H], 0. However, the Mg2 index (and to a lesser degree Mg b) systematically predict higher metallicities (up to +0.5 dex higher) than ,Fe,. Among the possible explanations for this are that the LMC clusters possess [,/Fe] > 0. Metallicities are presented for eleven LMC clusters which have no previous measurements. We compare SSP ages for the clusters, derived from the H,, H, and H, Lick/IDS indices, with the available literature data, and find good agreement for the vast majority. This includes six old globular clusters in our sample, which have ages consistent with their HST colour-magnitude diagram (CMD) ages and/or integrated colours. However, two globular clusters, NGC 1754 and NGC 2005, identified as old (,15 Gyr) on the basis of HST CMDs, have H, line-strengths which lead ages that are too low (,8 and ,6 Gyr respectively). These findings are inconsistent with their CMD-derived values at the 3, level. Comparison between the horizontal branch morphology and the Balmer line strengths of these clusters suggests that the presence of blue horizontal branch stars has increased their Balmer indices by up to ,1.0 Å. We conclude that the Lick/IDS indices, used in conjunction with contemporary SSP models, are able to reproduce the ages and metallicities of the LMC clusters reassuringly well. The required extrapolations of the fitting functions and stellar libraries in the models to lower ages and low metallicities do not lead to serious systematic errors. However, owing to the significant contribution of horizontal branch stars to Balmer indices, SSP model ages derived for metal-poor globular clusters are ambiguous without a priori knowledge of horizontal branch morphology. [source]

Final stages of N -body star cluster encounters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000
M. R. De Oliveira
We performed numerical simulations of star cluster encounters with Hernquist's treecode on a CRAY YMP-2E computer. We used different initial conditions (relative positions and velocities, cluster sizes, masses and concentration degrees) with the total number of particles per simulation ranging from 4608 to 20 480. Long-term interaction stages (up to 1 Gyr) when the pair coalesces into a single cluster are compared with isolated LMC clusters. Evidence is found that, when seen in a favourable plane, these resulting clusters show elliptical shapes as a result of the disruption of one of the companions. These elliptical shapes are essentially time-independent, but they do depend on the initial structural parameters of the pair components. We also analysed the fraction of stars that are ejected to the field by the interaction. We found that this fraction can be almost 50 per cent for the disrupted cluster. These simulations can represent a possible mechanism with which to explain the ellipticity observed in several star clusters in the Magellanic Clouds. [source]

High-resolution simulations of galaxy mergers: resolving globular cluster formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
F. Bournaud
ABSTRACT Massive star clusters observed in galaxy mergers are often suggested to be progenitors of globular clusters. To study this hypothesis, we performed the highest resolution simulation of a gas-rich galaxy merger so far. The formation of massive star clusters of 105 to 107 M,, triggered by the galaxy interaction, is directly resolved in this model. We show that these clusters are tightly bound structures with little net rotation, due to evolve into compact long-lived stellar systems. Massive clusters formed in galaxy mergers are thus robust candidates for progenitors of long-lived globular clusters. The simulated cluster mass spectrum is consistent with theory and observations. Tidal dwarf galaxies of 108,9 M, can form at the same time, and appear to be part of a different class of objects, being more extended and rotating. [source]

Self-consistent simulations of star cluster formation from gas clouds under the influence of galaxy-scale tidal fields

Formation of blue compact dwarf galaxies from merging and interacting gas-rich dwarfs

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
Kenji Bekki
ABSTRACT We present the results of numerical simulations which show the formation of blue compact dwarf (BCD) galaxies from merging between very gas-rich dwarfs with extended H i gas discs. We show that dwarf,dwarf merging can trigger central starbursts and form massive compact cores dominated by young stellar populations. We also show that the pre-existing old stellar components in merger precursor dwarfs can become diffuse low surface brightness components after merging. The compact cores dominated by younger stellar populations and embedded in more diffusely distributed older ones can be morphologically classified as BCDs. Since new stars can be formed from gas transferred from the outer part of the extended gas discs of merger precursors, new stars can be very metal-poor ([Fe/H] < ,1). Owing to very high gaseous pressure exceeding 105 kB (where kB is the Boltzmann constant) during merging, compact star clusters can be formed in forming BCDs. The BCDs formed from merging can still have extended H i gas discs surrounding their blue compact cores. We discuss whether tidal interaction of gas-rich dwarfs without merging can also form BCDs. [source]

Integrated spectroscopic study of 7 star clusters in the Small Magellanic Cloud

ASTRONOMISCHE NACHRICHTEN, Issue 3 2010
M.L. Talavera
Abstract We present flux-calibrated integrated spectra for 7 star clusters belonging to the Small Magellanic Cloud in the optical range (,3600,7000 Ä), obtained at CASLEO (Argentina). Three out of the 7 clusters were not previously studied so their ages and reddening values are determined for the first time in the current study. Using the equivalent widths of selected spectroscopic lines and comparing the cluster spectra with template spectra of known properties, we derive foreground interstellar reddening and age. The clusters are in the (5,300) Myr age range and their E (B , V) colour excesses were in all cases smaller than 0.12. The present data also contribute to enlarge the cluster spectral library at the metallicity level of the Small Magellanic Cloud. The buildup of such database, which also includes Galactic and Large Magellanic Cloud clusters, is a long-term project that we have been developing and which has proved to be useful in the analysis of stellar populations of extragalactic systems (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Star cluster kinematics in the GAIA era

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2008
H. Baumgardt
Abstract The GAIA astrometric satellite will measure positions, proper motions and parallaxes of millions of stars with microarcsecond accuracy. This will greatly increase our understanding of the stellar populations of the Milky Way and their dynamics. In particular, it will be possible to determine internal and space motions of a large number of open or globular clusters with an accuracy of a few km/s or better, which will bring new insights into the way star clusters evolve and how they and the Milky Way as a whole have formed. It will also be possible to look for clusters which have a common space motion and follow tidal streams from dissolving globular clusters over many orbits by kinematically selecting their members, which will constrain the form of the galactic potential. I illustrate how GAIA will improve our knowledge on the kinematics and dynamics of star clusters and what can be learned by comparing the GAIA data with realistic N -body simulations of star clusters, possible with e.g. future GPU or GRAPE computers. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]