Photometric analysis of two eclipsing binaries in the magellanic clouds

B, V andI light curves of the eclipsing binaries HV12634 in the LMV, and HV2208 in the SMC (cf. Westet al., 1992), have been analysed with the aid of optimal curve-fitting techniques. Parameter derivation involves a successive approximations procedure. The results of this are combined with a standard mass:luminosity relation to explore tentative absolute parameters. The combined evidence shows some disparity with standard MS-like models. It also demonstrates some relative insensitivity of photometric analysis, in isolation, to elucidate clear facts on such possibly atypical, remote, hot stars. Auxilliary spectroscopic data are required to resolve remaining ambiguities.     

The Mt John programme of CCD photometry of eclipsing binaries in the magellanic clouds

This paper reviews recent progress in CCD photometry of eclipsing binary systems in the crowded fields of the Magellanic Clouds. Some preliminary findings are given. The interesting eccentric system HV2274 is noted as important for follow-up studies, particularly concerning its apsidal motion. Some further developments are anticipated.     

Integrated photometric properties of magellanic cloud star clusters

Integrated CCD BVR photometric measurements of Magellanic Cloud (MC) star clusters are described and summarized. The integrated colour-magnitude diagrams and colour-age relations for these systems imply that (a) the colour distribution of MC clusters is distinctly bi-modal (although this bi-modality is not simply explained as a distinction between young and old clusters), and (b) the MCs are probably not forming as many (if any) extremely massive clusters at the present as they did early in their lifetimes (i.e., 10 Gyr ago). Structurally, the MC clusters are seen to be intermediate between galactic globular and open clusters and their radial surface brightness profiles can generally be adequately fitted to single-mass King (1966) models (some exceptions do occur, however). Although the distribution of the derived structural parameters of young and old MC clusters are similar, they differ significantly from the distribution of structural parameters observed in intermediate-age clusters, suggesting that these structural properties do not evolve monotonically with time.Visiting astronomer at the Cerro Tololo Interamerican Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

On the thorium absorption lines in the visible spectra of supergiant stars in the magellanic clouds

We present the results of the analysis of thorium absorption lines in the spectra of metal-deficient stars: the thorium-rich Galactic halo star HD 221170 and some supergiants in the Magellanic Clouds. For all the analyzed stars, the Th II lines were identified by comparing the synthetic and observed spectra. The Th II 6044.433 Å line and the weaker Th II 6619.943 Å line were identified in the spectra of red supergiants in the Magellanic Clouds.     

The age determination of young clusters in the large magellanic cloud by means of integrated photometry

The relationt=t(Q), obtained from theoretical integrated colours (B-V)₀ and (U-B)₀ for Pop. I clusters, has been employed to date a group of 13 young clusters in the LMC. The comparison with the ages derived by the t.o.p. method shows that the theoretical integrated colour indices of clusters, with evolved stars in the mass range 5–15M , are systematically bluer than the observed ones.The analysis performed indicate that, in order to reconcile observed and computed colours, the maximum effective temperature of evolved stars should be suitably lowered. The hypothesis has been advanced that such changes can be obtained by the inclusion of the mass loss in the computation of evolutionary tracks.     

Molecular clouds and the formation of open clusters

Using analytical approximations of the mass spectra of molecular clouds and stellar systems including open clusters and associations a relation between the final star formation efficiency (final SFE) and the initial cloud mass is derived. The dependence of the SFE on cloud mass is estimated for two cases of molecular cloud destruction by stellar winds in spherical geometry. The result of growing SFE with rising cloud mass does not agree with current observations and the above relation. A decrease of the SFE is expected if the fraction of high-mass stars grows with increasing mass of the cloud.     

On gamma radiation from the magellanic clouds and galactic supernova remnants — Possible antiproton sources in the galaxy

Radiation from the Magellanic Clouds is discussed from the point of view of near future possibilities in observational γ-ray astronomy. The γ-ray fluxes expected according to the metagalactic and galactic theories of the origin of cosmic rays are compared. It is shown that the strongest test of the metagalactic hypothesis will be provided by a determination of the ratio of γ-ray fluxes from SMC and LMC. The γ-ray luminosity of a typical young supernova remnant that can generate sufficient antiprotons is estimated. It is shown that such remnants must have a short phase during which they are very powerful γ-ray emitters.     

The system of molecular clouds in the Gould Belt

Based on high-latitude molecular clouds with highly accurate distance estimates taken from the literature, we have redetermined the parameters of their spatial orientation. This systemcan be approximated by a 350 × 235 × 140 pc ellipsoid inclined by the angle i = 17° ± 2° to the Galactic plane with the longitude of the ascending node l _Ω = 337° ± 1°. Based on the radial velocities of the clouds, we have found their group velocity relative to the Sun to be (u ₀, v ₀, w ₀) = (10.6, 18.2, 6.8) ± (0.9, 1.7, 1.5) km s^?1. The trajectory of the center of the molecular cloud system in the past in a time interval of ~60 Myr has been constructed. Using data on masers associated with low-mass protostars, we have calculated the space velocities of the molecular complexes in Orion, Taurus, Perseus, and Ophiuchus. Their motion in the past is shown to be not random.     

Infrared dark clouds as precursors to star clusters

Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveals many compact (<0.5 pc) and massive (10–2100 M_⊙) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.     

Observing molecular clouds in halo dark clusters through absorption lines

In addition to Massive Astrophysical Compact Halo Objects (MACHOs), as detected by microlensing, cold molecular clouds (mainly ofH ₂) may well contribute substantially to the galactic halo dark matter. Here, we argue that the existence of such halo clouds can be inferred from the observation of absorption lines (due to heavy molecules located in the clouds themselves) towards stars of the Large Magellanic Cloud, which lie very close, within 1, to a previously microlensed one.The possibility of clusters of MACHOs has been investigated by several authors (see e.g. Carr and Lacey, 1987; Ashman, 1990; Eichler and Silk, 1992 and Wasserman and Salpeter, 1994).     

The evolution of super-metal-rich stars and the interpretation of old galactic clusters

The influence of initial helium content on the evolutionary characteristics of super-metalrich stars (Z=0.10) has been investigated. The evolution of models in the range –0.05L/L <1.0 has been followed up to the relative luminosity maximum in the subgiant branch, and under the assumptionY=0.20 orY=0.40.Comparison with previous results suggests the existence of theoretical constraints that could be adopted as metal indicators for the observed H–R diagrams of old open clusters.     

AGB evolution in the large magellanic cloud

We review observations and theoretical studies of the evolution of massive asymptotic giant branch stars in the Magellanic Clouds. Recent results support the hypothesis that AGB is truncated, probably through mass loss, before these stars can attain the theoretically predicted maximum luminosity. In particular, IRAS observations of the LMC provide little evidence for the presence of highly obscured cocoon stars.     

Gravitational collapse of cosmic gas clouds and formation of star clusters

In this paper the gravitational collapse of cosmic gas clouds and formation of star clusters has been considered. Hoyle's view of successive fragmentation has been taken as the basic mechanim in the present work. The initial masses of protostars have been estimated as the function of their distances from the centre of the cluster and the intensity of the magnetic field of the medium. It has been shown that the fragmentation process is greatly inhibited by the presence of a strong magnetic field. A model has been constructed showing how a protostar grows in mass by accretion from the surrounding medium, on the basis of the assumption that as the star moves at random in the cluster it picks up a fraction of the material through which it passes. It has been estimated that a protostar of initial mass of about 0.1M grows to one of 1.0M in a time period which ranges from a few multiples of 10⁵ to a few multiples of 10⁷ yr, depending on the parameters involved in the accretion process. The number of stars per unit mass range has also been estimated; it is found to be proportional tom ^–3.3,m being the mass of the star.     

Note on the spatial distribution of old open clusters

Using the sample of 74 old open clusters compiled by Friel (1995) the increase of the mean distance from the galactic plane with cluster age has been determined. The result is in a qualitative agreement with the hypothesis of scattering of the clusters during encounters with massive molecular clouds and by spiral arms. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)