R. Korakitis

The design and performance of the Gaia photometric system

The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outlined.

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.

Towards a library of synthetic galaxy spectra and preliminary results of classification and parametrization of unresolved galaxies for Gaia. II

Aims. This paper is the second in a series, implementing a classification system for Gaia observations of unresolved galaxies. Our goals are to determine spectral classes and estimate intrinsic astrophysical parameters via synthetic templates. Here we describe (1) a new extended library of synthetic galaxy spectra; (2) its comparison with various observations; and (3) first results of classification and parametrization experiments using simulated Gaia spectrophotometry of this library. Methods. Using the PEGASE.2 code, based on galaxy evolution models that take account of metallicity evolution, extinction correction, and emission lines (with stellar spectra based on the BaSeL library), we improved our first library and extended it to cover the domain of most of the SDSS catalogue. Our classification and regression models were support vector machines (SVMs). Results. We produce an extended library of 28 885 synthetic galaxy spectra at zero redshift covering four general Hubble types of galaxies, over the wavelength range between 250 and 1050 nm at a sampling of 1 nm or less. The library is also produced for 4 random values of redshift in the range of 0–0.2. It is computed on a random grid of four key astrophysical parameters (infall timescale and 3 parameters defining the SFR) and, depending on the galaxy type, on two values of the age of the galaxy. The synthetic library was compared and found to be in good agreement with various observations. The first results from the SVM classifiers and parametrizers are promising, indicating that Hubble types can be reliably predicted and several parameters estimated with low bias and variance.

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.

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.

Classification and Parametrization of Unresolved Galaxies with Gaia

The ESA satellite mission Gaia will acquire spectrophotometric observations of several million unresolved galaxies during its five years of operation. Our objective is to design and implement a classification system for these data. For this purpose 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 4 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. Using simulated Gaia photometry of this library we train and test the perfor...

The role of central density in the evolution and formation of LMC clusters. observational evidence

Abstract The central density ϱO in M [odot] pc−3 of a stellar system is known to be one of the principal parameters determining its dynamical history and disruption time. The central density of young populous clusters seems also to be a predominant parameter for mass segregation effects. The theoretical definition of bound or unbound stellar systems, based on their central density and the corresponding observational constraints regarding their classification (in stellar associations and open or globular clusters) are discussed. We also present our results on the spatial distribution of the star clusters in the LMC and SMC according to their central density, and the possible explanations for cluster formation in galaxies, which can be derived from these distributions.

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.