Mary Kontizas

OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems

The method developed by Gouliermis et al. (2000, Paper I), for the detection and classification of stellar systems in the LMC, was used for the identification of stellar associations and open clusters in the central area of the LMC. This method was applied on the stellar catalog produced from a scanned 1.2m UK Schmidt Telescope Plate in U with a field of view almost 6. 5 × 6. 5, centered on the Bar of this galaxy. The survey of the identified systems is presented here followed by the results of the investigation on their spatial distribution and their structural parameters, as were estimated according to our proposed methodology in Paper I. The detected open clusters and stellar associations show to form large filamentary structures, which are often connected with the loci of HI shells. The derived mean size of the stellar associations in this survey was found to agree with the average size found previously by other authors, for stellar associations in different galaxies. This common size of about 80 pc might represent a universal scale for the star formation process, whereas the parameter correlations of the detected loose systems support the distinction between open clusters and stellar associations.

OB Stellar Associations in the Large Magellanic Cloud: Identification Method

We describe an objective method for the identification of stellar OB associations in the Large Magellanic Cloud under the assumption that they are loose, unbound stellar systems with a young OB stellar component. The method is based on star counts and spectral classification. First we detect the areas where an enhancement of star number density occurs above 3 σ of the average field density in large regions. The boundaries at 3 σ provide the size and morphology of the detected stellar concentrations. Further examination at different magnitude ranges allows us to select the systems with a bright stellar component within the detected areas. In the second step, star counts around the peak density of each detected stellar concentration provide a typical value of the projected half-mass radius, in order to calculate the central density using the appropriate mass function slope. The central density, being a crucial parameter for the bound and unbound systems, has been used as a tentative criterion for the distinction between open clusters and associations. Finally, spectral classification from objective-prism plates provides further evidence for the existence of OB-type stars in these concentrations. The faintest magnitude at which the various systems were detected is found to be independent of the presence or absence of gas and varies by up to 4 mag. An explanation for this effect is the possible existence of pre–main-sequence stars that are not visible in the optical region.

The stellar content of binary star clusters in the LMC

The bright stellar content for fifteen binary star clusters and their adjoining fields in the Large Magellanic Cloud (LMC) were studied here. Film copies of plates taken with the 1.2 U.K. Schmidt telescope were used for deriving the spectral types of the stars in the studied regions. All classified stars are brighter thanV=17.5 mag and situated in large areas around each pair and in a neighbouring field. Seven of the pairs, the brightest and most populous are young clusters (located mainly at the north part of the parent galaxy). The derived distributions of spectral types of their stars give strong evidence that each pair consists of similar stellar content with ages ∼0.6–8×107 yr.Eight more binary star clusters were studied as well, selected among the rest of the binaries in the LMC. It is found that their stars were faint for our limit of detection so the poor statistics did not allow a comparison among the two cluster members of each pair. However the bright limit of their stars implies ages >6×108 yr. Considering that these objects were randomly selected it is unlikely that all are projected pairs. So it seems that binarity in star clusters is a phenomenon (favourable in the LMC) which did not happen only once in the life of this galaxy.

Indication of Mass Segregation in LMC Star Clusters

In this contribution we present our investigation on mass segregation in selected LMC star clusters. As a diagnostic of mass segregation we use: (1) Colour-magnitude diagrams at various distance from each cluster centre; (2) Radial-density profiles at various magnitude ranges; and (3) comparison with dynamical models which provide core-radius, a measure of a cluster’s compactness.

A library of galaxy spectra for Gaia

The ESA satellite mission Gaia will acquire spectrophotometric observations of several million unresolved galaxies during its five years of operation. In order to implement a classification scheme for these observations we need to build a new library of galaxy spectra which covers the necessary parameter space. Using the evolutionary code PEGASE.2 we have produced a library of 28885 synthetic galaxy spectra at zero redshift covering four general spectral types of galaxies over the wavelength range from 250 to 1050 nm, at a sampling of 1 nm or less. The library was also reproduced for four random values of redshift in the range of 0–0.2 and it is computed on a random grid of four key astrophysical parameters (3 for SFR and 1 for timescale of the infall of gas). The synthetic library was compared with various photometric and spectroscopic observations (e.g. from SDSS) and found in good agreement with them.

Dynamics, Star Formation and Chemical Evolution in the Nearby Galaxies from Studies of Their Stellar Systems

The 2- to 4-m class telescopes have to be dedicated to telescope time-consuming projects for which our present knowledge is based on small statistical samples. Spectroscopy (high resolution, 2-D, multi-object) and narrow band photometry provide the appropriate tools for these telescopes in order to face important astrophysical problems in our Local Group of galaxies. The Magellanic Clouds and the other nearby galaxies offer excellent targets for a) Astrophysical questions such as dynamics of star clusters and galaxies, star formation, evolution of galaxies and b) serve as templates for the remote galaxies studies. We therefore propose three main areas of observing projects: 1) Dynamical studies of star clusters and associations in galaxies (isolated and interactive environments) have been of great interest lately due to the fast development of theory and the numerical simulations by huge dedicated computers. Observations of a large sample of clusters can provide information on i) the stellar content of their cores and particularly the binary population in them ii) search for coronae of light stars iii) tidal tails due to the gravitational field of the parent galaxy iv) mechanisms of cluster formation. 2) Kinematical studies of large stellar structures in galaxies such as carbon stars, planetary nebulae, stellar complexes, giant shells, spiral structures and kinematics of the nearby dwarf galaxies in order to understand star formation mechanisms in various ages and environments. 3) The Chemical evolution of galaxies, based on good metallicity and abundance determinations, is so far rather poorly investigated because of the huge amount of work needed for both obtaining the data and reducing them. Now the new sophisticated instruments and the reduction tools enable us to plan a pilot mega-project towards the understanding of galaxy evolution.

DETECTING THE VARIATION IN THE LOWER MASS LIMIT OF STELLAR ASSOCIATIONS IN GALAXIES

Properties of OB stellar associations in the LMC have been determined from digitized UK Schmidt plates. This material provides a homogeneous data set for revealing the intrinsic differences among the various associations at different locations in the parent galaxy. Isodensity contours, from star counts, and spectral classification offer very good criteria for the exact definition of the OB association, considered as the single unit system among the most recent stellar populations of a galaxy. The isodensity contours are derived for different magnitude levels, in order to trace the limits where the image of each association is detectable. It is found that the faint limits of the LMC associations, examined so far, vary by about four magnitudes, partly depending on their spatial distribution. This finding implies differences in the mass function and, possibly, the star formation mechanism in this galaxy.