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Magellanic Cloud (magellanic + cloud)
Kinds of Magellanic Cloud Selected AbstractsHigh-dispersion spectroscopy of two A supergiant systems in the Small Magellanic Cloud with novel propertiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010R. E. Mennickent ABSTRACT We present the results of a spectroscopic investigation of two novel variable bright blue stars in the SMC, OGLE004336.91-732637.7 (SMC-SC3) and the periodically occulted star OGLE004633.76-731204.3 (SMC-SC4), whose photometric properties were reported by Mennickent et al. (2010). High-resolution spectra in the optical and far-UV show that both objects are actually A + B type binaries. Three spectra of SMC-SC4 show radial velocity variations, consistent with the photometric period of 184.26 d found in Mennickent et al. 2010. The optical spectra of the metallic lines in both systems show combined absorption and emission components that imply that they are formed in a flattened envelope. A comparison of the radial velocity variations in SMC-SC4 and the separation of the V and R emission components in the H, emission profile indicate that this envelope, and probably also the envelope around SMC-SC3, is a circumbinary disc with a characteristic orbital radius some three times the radius of the binary system. The optical spectra of SMC-SC3 and SMC-SC4 show, respectively, He i emission lines and discrete blue absorption components (BACs) in metallic lines. The high excitations of the He i lines in the SMC-SC3 spectrum and the complicated variations of Fe ii emission and absorption components with orbital phase in the spectrum of SMC-SC4 suggests that shocks occur between the winds and various static regions of the stars' corotating binary-disc complexes. We suggest that BACs arise from wind shocks from the A star impacting the circumbinary disc and a stream of former wind-efflux from the B star accreting on to the A star. The latter picture is broadly similar to mass transfer occurring in the more evolved (but less massive) algol (B/A + K) systems, except that we envision transfer occurring in the other direction and not through the inner Lagrangian point. Accordingly, we dub these objects prototype of a small group of Magellanic Cloud wind-interacting A + B binaries. [source] Be/X-ray binary SXP6.85 undergoes large Type II outburst in the Small Magellanic CloudMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2010L. J. Townsend ABSTRACT The Small Magellanic Cloud (SMC) Be/X-ray binary pulsar SXP6.85 = XTE J0103,728 underwent a large Type II outburst beginning on 2008 August 10. The source was consistently seen for the following 20 weeks (MJD = 54688,54830). We present X-ray timing and spectroscopic analysis of the source as a part of our ongoing Rossi X-ray Timing Explorer (RXTE) monitoring campaign and INTEGRAL key programme monitoring the SMC and 47 Tuc. A comparison with the Optical Gravitational Lensing Experiment (OGLE) III light curve of the Be counterpart shows the X-ray outbursts from this source coincide with times of optical maximum. We attribute this to the circumstellar disc increasing in size, causing mass accretion on to the neutron star. Ground based infrared photometry and H, spectroscopy obtained during the outburst are used as a measure of the size of the circumstellar disc and lend support to this picture. In addition, folded RXTE light curves seem to indicate complex changes in the geometry of the accretion regions on the surface of the neutron star, which may be indicative of an inhomogeneous density distribution in the circumstellar material causing a variable accretion rate on to the neutron star. Finally, the assumed inclination of the system and H, equivalent width measurements are used to make a simplistic estimate of the size of the circumstellar disc. [source] Seven young star clusters in the inner region of the Small Magellanic CloudMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Andrés 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] AGB variables and the Mira period,luminosity relationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Patricia A. Whitelock ABSTRACT Published data for large-amplitude asymptotic giant branch variables in the Large Magellanic Cloud (LMC) are re-analysed to establish the constants for an infrared (K) period,luminosity relation of the form MK=,[log P, 2.38]+,. A slope of ,=,3.51 ± 0.20 and a zero-point of ,=,7.15 ± 0.06 are found for oxygen-rich Miras (if a distance modulus of 18.39 ± 0.05 is used for the LMC). Assuming this slope is applicable to Galactic Miras we discuss the zero-point for these stars using the revised Hipparcos parallaxes together with published very long baseline interferometry (VLBI) parallaxes for OH masers and Miras in globular clusters. These result in a mean zero-point of ,=,7.25 ± 0.07 for O-rich Galactic Miras. The zero-point for Miras in the Galactic bulge is not significantly different from this value. Carbon-rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period,luminosity relation zero-points for systems with different metallicity. [source] The poorly constrained cluster disruption time-scale in the Large Magellanic CloudMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008Genevičve Parmentier ABSTRACT We use Monte Carlo simulations, combined with homogeneously determined age and mass distributions, based on multiwavelength photometry, to constrain the cluster formation history and the rate of bound cluster disruption in the Large Magellanic Cloud (LMC) star cluster system. We evolve synthetic star cluster systems formed with a power-law initial cluster mass function (ICMF) of spectral index ,=,2 assuming different cluster disruption time-scales. For each of these cluster disruption time-scales, we derive the corresponding cluster formation rate (CFR) required to reproduce the observed cluster age distribution. We then compare, in a ,Poissonian',2 sense, model mass distributions and model two-dimensional distributions in log(mass) versus log(age) space of the detected surviving clusters to the observations. Because of the bright detection limit (MlimV,,4.7 mag) above which the observed cluster sample is complete, one cannot constrain the characteristic cluster disruption time-scale for a 104 M, cluster, tdis4[where the disruption time-scale depends on cluster mass as tdis=tdis4(Mcl/104 M,),, with ,, 0.62], to better than a lower limit, tdis4, 1 Gyr. We conclude that the CFR has been increasing steadily from 0.3 clusters Myr,1 5 Gyr ago to a present rate of (20,30) clusters Myr,1 for clusters spanning a mass range of ,100,107 M,. For older ages, the derived CFR depends sensitively on our assumption of the underlying CMF shape. If we assume a universal Gaussian ICMF, then the CFR has increased steadily over a Hubble time from ,1 cluster Gyr,1 15 Gyr ago to its present value. On the other hand, if the ICMF has always been a power law with a slope close to ,=,2, the CFR exhibits a minimum some 5 Gyr ago, which we tentatively identify with the well-known age gap in the LMC's cluster age distribution. [source] Kinematic structure in the Galactic halo at the North Galactic Pole: RR Lyrae and blue horizontal branch stars show different kinematicsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007T. D. Kinman ABSTRACT Radial velocities and proper motions (derived from the GSC-II data base) are given for 38 RR Lyrae (RRL) stars and 79 blue horizontal branch (BHB) stars in a ,200 deg2 area around the North Galactic Pole (NGP). Both heliocentric (UVW) and galactocentric (VR, V,, Vz) space motions are derived for these stars using a homogeneous distance scale consistent with (m,M)0= 18.52 for the Large Magellanic Cloud (LMC). An analysis of the 26 RRL and 52 BHB stars whose height (Z) above the plane is less than 8 kpc shows that this halo sample is not homogeneous. Our BHB sample (like that of Sirko et al.) has a zero galactic rotation (V,) and roughly isotropic velocity dispersions. The RRL sample shows a definite retrograde rotation (V,=,95 ± 29 km s,1) and non-isotropic velocity dispersions. The combined BHB and RRL sample has a retrograde galactic rotation (V) that is similar to that found by Majewski for his sample of subdwarfs in Selected Area (SA) 57. The velocity dispersion of the RRL stars that have a positive W motion is significantly smaller than the dispersion of those ,streaming down' with a negative W. Also, the ratio of RRL to BHB stars is smaller for the sample that has positive W. Our halo sample occupies 10.4 kpc3 at a mean height of 5 kpc above the Galactic plane. In this volume, one component (rich in RRL stars) shows retrograde rotation and the streaming motion that we associate with the accretion process. The other component (traced by the BHB stars) shows essentially no rotation and less evidence of streaming. These two components have horizontal branch (HB) morphologies that suggest that they may be the field star equivalents of the young and old halo globular clusters, respectively. Clearly, it is quite desirable to use more than one tracer in any kinematic analysis of the halo. [source] The evolution of binary star clusters and the nature of NGC 2136/NGC 2137MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007S. 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] An ATCA radio-continuum study of the Small Magellanic Cloud , IV.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006A multifrequency analysis of the N 66 region ABSTRACT Traditional identification of supernova remnants (SNRs) include the use of radio spectral index, optical spectral studies (including strong [S ii], [N ii], [O i], [O ii] and [O iii] lines) and X-ray co-identifications. Each of these can have significant limitations within the context of a particular SNR candidate and new identification methods are continually sought. In this paper, we explore subtraction techniques by Ye, Turtle and Kennicutt to remove thermal emission estimated from H, flux from radio-continuum images. The remaining non-thermal emission allows the identification of SNRs embedded within these H ii regions. Subtraction images of the N 66 region in the Small Magellanic Cloud (SMC) using H, wide-field optical CCD images from the Curtis Schmidt Telescope and the recent Australia Telescope Compact Array (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640 MHz) data are presented as an example. These show three SNRs (B0057 , 724, B0056 , 724 and B0056 , 725) separated from their surrounding H ii radio emission. 2.3-m dual-beam spectrograph long-slit spectra from selected regions within N 66 suggest the presence of an additional SNR with no radio or X-ray emission. Radio spectral index, [S ii]/H, ratio and archived Chandra images of N 66 combine to give a more coherent picture of this region, confirming B0057 , 724 as an SNR. The N 66 nebula complex is divided into 10 components, composed separately of these SNRs and H ii regions. [source] Consistent distances from Baade,Wesselink analyses of Cepheids and RR LyraesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2003Géza Kovács ABSTRACT By using the same algorithm in the Baade,Wesselink analyses of Galactic RR Lyrae and Cepheid variables, it is shown that, within 0.03-mag 1, statistical error, they yield the same distance modulus for the Large Magellanic Cloud. By fixing the zero-point of the colour,temperature calibration to those of the current infrared flux methods and using updated period,luminosity,colour relations, we get an average value of 18.55 for the true distance modulus of the LMC. [source] Mass segregation in young compact clusters in the Large Magellanic Cloud , III.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002Implications 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 CloudMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002Michael 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 Ĺ. 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] OGLE observations of four X-ray binary pulsars in the Small Magellanic CloudMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000M. J. Coe This paper presents analysis and interpretation of OGLE photometric data of four X-ray binary pulsar systems in the Small Magellanic Cloud (SMC): 1WGA J0054.9-7226, RX J0050.7-7316, RX J0049.1-7250 and 1SAX J0103.2-7209. In each case, the probable optical counterpart is identified on the basis of its optical colours. In the case of RX J0050.7-7316 the regular modulation of its optical light curve appears to reveal an ellipsoidal modulation with a period of 1.416 d. Using reasonable masses for the neutron star and the B star, we show that the amplitude and relative depths of the minima of the I -band light curve of RX J0050.7-7316 can be matched with an ellipsoidal model where the B star nearly fills its Roche lobe. For mass ratios in the range of 0.12 to 0.20, the corresponding best-fitting inclinations are about 55° or larger. The neutron star would be eclipsed by the B star at inclinations larger than ,60° for this particular mass ratio range. Thus RX J0050.7-7316 is a good candidate system for further study. In particular, we would need additional photometry in several colours, and most importantly, radial velocity data for the B star before we could draw more quantitative conclusions about the component masses. [source] Self-consistent simulations of star cluster formation from gas clouds under the influence of galaxy-scale tidal fieldsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008Jarrod 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] Integrated spectroscopic study of 7 star clusters in the Small Magellanic CloudASTRONOMISCHE NACHRICHTEN, Issue 3 2010M.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 formation in the LMC: Comparative CCD observations of young stellar populations in two giant molecular cloudsASTRONOMISCHE NACHRICHTEN, Issue 8 2009J. Ruppert Abstract This work deals with a CCD imaging study at optical and near-infrared wavelength oftwo giant molecular clouds (plus a control field) in the southern region of the Large Magellanic Cloud, one ofwhich shows multiple signs of star formation, whereas the other does not. The observational data from VLT FORS2 (R band) and NTT SOFI (Ks band) have been analyzed to derive luminosity functions and color-magnitude diagrams. The young stellar content of these two giant molecular clouds is compared and confirmed to be different, in the sense that the apparently "starless" cloud has so far formed only low-luminosity, low-mass stars (fainter than mKs , 16.5 mag, not seen by 2MASS), while the other cloud has formed both faint low-mass and luminous high-mass stars. The surface density excess oflow-luminosity stars (,2 per square arcmin) in the "starless" cloud with respect to the control field is about 20% whereas the excess is about a factor of 3 in the known star-forming cloud. The difference may be explained theoretically by the gravo-turbulent evolution of giant molecular clouds, one being younger and less centrally concentrated than the other (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Stars in the Large Magellanic CloudASTRONOMY & GEOPHYSICS, Issue 1 2000Article first published online: 24 DEC 200 Click HERE to view the article. [source] Dust mass-loss rates from asymptotic giant branch stars in the Fornax and Sagittarius dwarf spheroidal galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Eric Lagadec ABSTRACT To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius and Fornax dwarf spheroidal galaxies with the 10-,m camera VISIR at the Very Large Telescope. We derive mass-loss rates for 29 AGB stars in Sgr dSph and two in Fornax. The dust mass-loss rates are estimated from the K,[9] and K,[11] colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between K,[12] colour and dust mass-loss rates is observed for AGB stars from galaxies with different metallicities, i.e., the Galaxy, the Large Magellanic Clouds and the Small Magellanic Clouds. The derived dust mass-loss rates are in the range 5 × 10,10 to 3 × 10,8 M, yr,1 for the observed AGB stars in Sgr dSph and around 5 × 10,9 M, yr,1 for those in Fornax; while values obtained with the two different methods are of the same order of magnitude. The mass-loss rates for these stars are higher than the nuclear burning rates, so they will terminate their AGB phase by the depletion of their stellar mantles before their core can grow significantly. Some observed stars have lower mass-loss rates than the minimum value predicted by theoretical models. [source] Integrated-light VRI imaging photometry of globular clusters in the Magellanic CloudsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Paul Goudfrooij ABSTRACT We present accurate integrated-light photometry in Johnson/Cousins V, R and I for a sample of 28 globular clusters in the Magellanic Clouds. The majority of the clusters in our sample have reliable age and metallicity estimates available in the literature. The sample encompasses ages between 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between ,1.5 and 0.0 dex. The sample is dominated by clusters of ages between roughly 0.5 and 2 Gyr, an age range during which the bolometric luminosity of simple stellar populations is dominated by evolved red giant branch stars and thermally pulsing asymptotic giant branch (TP-AGB) stars whose theoretical colours are rather uncertain. The VRI colours presented in this paper have been used to calibrate stellar population synthesis model predictions. [source] Mira variables in the Galactic bulge with OGLE-II dataMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005Noriyuki Matsunaga ABSTRACT We have extracted a total of 1968 Mira variables from the Optical Gravitational Lensing Experiment II (OGLE-II) data base in the Galactic bulge region. Among them, 1960 are associated with 2 Micron All-sky Survey (2MASS) sources, and 1541 are further identified with Midcourse Space Exploration (MSX) point sources. Their photometric properties are compared with those of Mira variables in the Large and Small Magellanic Clouds. We have found that mass-losing stars with circumstellar matter are reddened such that the colour dependence of the absorption coefficient is similar to that of interstellar matter. We also discuss the structure of the bulge. The surface number density of the bulge Mira variables is well correlated with the 2.2-,m surface brightness obtained by the Cosmic Background Explorer (COBE) satellite. Using this relation, the total number of Mira variables in the bulge is estimated to be about 6 × 105. The log P,K relation of the Mira variables gives their space distribution which supports the well-known asymmetry of the bar-like bulge. [source] Final stages of N -body star cluster encountersMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000M. 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] The effect of stellar rotation on colour,magnitude diagrams: on the apparent presence of multiple populations in intermediate age stellar clustersMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009N. Bastian ABSTRACT A significant number of intermediate age clusters (1,2 Gyr) in the Magellanic Clouds appear to have multiple stellar populations within them, derived from bimodal or extended main-sequence turn-offs. If this is interpreted as an age spread, the multiple populations are separated by a few hundred million years, which would call into question the long-held notion that clusters are simple stellar populations. Here, we show that stellar rotation in stars with masses between 1.2 and 1.7 M, can mimic the effect of a double or multiple population, whereas in actuality only a single population exists. The two main causes of the spread near the turn-off are the effects of stellar rotation on the structure of the star and the inclination angle of the star relative to the observer. Both effects change the observed effective temperature, hence colour, and flux of the star. In order to match observations, the required rotation rates are 20,50 per cent of the critical rotation, which are consistent with observed rotation rates of similar mass stars in the Galaxy. We provide scaling relations which can be applied to non-rotating isochrones in order to mimic the effects of rotation. Finally, we note that rotation is unlikely to be the cause of the multiple stellar populations observed in old globular clusters, as low-mass stars (<1 M,) are not expected to be rapid rotators. [source] Radial velocities, dynamics of stars and nebulosities with GAIA and VLT-GIRAFFEASTRONOMISCHE NACHRICHTEN, Issue 9-10 2008C. Martayan Abstract This document is divided in two parts. The first part deals with the radial velocities (RV) distributions for B-type stars and nebulosities observed with the VLT-GIRAFFE in the Large and Small Magellanic Clouds towards the open clusters NGC2004 and NGC330. Thanks to the resolution of GIRAFFE spectra, we found that the RV distribution for the nebulosities in the LMC is bi-modal. This bi-modality can be interpreted, in term of dynamics, by the expansion of the LMC4 super-bubble. The second part deals with the GAIA space mission and the determination of the radial velocities by using Radial Velocity Spectrometer (RVS) spectra. The methods to determine the radial velocities are presented as well as preliminary results on simulated RVS spectra. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||