Showing papers by "I. Bellas-Velidis published in 2012"

A semi-empirical library of galaxy spectra for Gaia classification based on SDSS data and PÉGASE models

Abstract: Aims. This paper is the third in a series implementing a classificat ion system for Gaia observations of unresolved galaxies. The system makes use of template galaxy spectra in order to determine spectral classes and estimate intrinsic astrophysical param eters. In previous work we used synthetic galaxy spectra produced by P ´ EGASE.2 code to simulate Gaia observations and to test the performance of Support Vector Machine (SVM) classifiers and parametrizers . Here we produce a semi-empirical library of galaxy spectra by fitting SDSS spectra with the previously produced synthetic libraries. We present (1) the semi-empirical library of galaxy spectra, (2) a comparison between the observed and synthetic spectra, and (3) first results of classification and parametrization expe riments with simulated Gaia spectrophotometry of this library. Methods. We useχ 2 -fitting to fit SDSS galaxy spectra with the synthetic library in order to construct a semi-empirical library of galaxy spectra in which (1) the real spectra are extended by the synthetic ones in order to cover the full wavelength range of Gaia, and (2) astrophysical parameters are assigned to the SDSS spectra by the best fitting synthetic spectrum. The SVM models were trained with and applied to semi-empirical spectra. Tests were performed for the classification of spectral types and the estima tion of the most significant galaxy parameters (in particular redshift, mas s to light ratio and star formation history). Results. We produce a semi-empirical library of 33 670 galaxy spectra covering the wavelength range 250 to 1050 nm at a sampling of 1 nm or less. Using the results of the fitting of the SDSS spec tra with our synthetic library, we investigate the range of t he input model parameters that produces spectra which are in good agreement with observations. In general the results are very good for the majority of the synthetic spectra of early type, spiral and i rregular galaxies, while they reveal problems in the models used to produce Quenched Star Forming Galaxies (QSFGs). The results of the SVM classification and regression models for this library are quite accurate for the prediction of the spectral type and the esti mation of the redshift parameter, while they are quite poor for the cases of the most significant parameters used to produce the syntheti c models (i.e. the star formation histories).

International observational campaigns of the last two eclipses in EE Cephei: 2003 and 2008/9

Abstract: Context. EE Cep is an unusual long-period (5.6 yr) eclipsing binary discovered during the mid-twentieth century. It undergoes almost-grey eclipses that vary in terms of both depth and duration at different epochs. The system consists of a Be type star and a dark dusty disk around an invisible companion. EE Cep together with the widely studied e Aur are the only two known cases of long-period eclipsing binaries with a dark, dusty disk component responsible for periodic obscurations.Aims. Two observational campaigns were carried out during the eclipses of EE Cep in 2003 and 2008/9 to verify whether the eclipsing body in the system is indeed a dark disk and to understand the observed changes in the depths and durations of the eclipses.Methods. Multicolour photometric data and spectroscopic observations performed at both low and high resolutions were collected with several dozen instruments located in Europe and North America. We numerically modelled the variations in brightness and colour during the eclipses. We tested models with different disk structure, taking into consideration the inhomogeneous surface brightness of the Be star. We considered the possibility of disk precession.Results. The complete set of observational data collected during the last three eclipses are made available to the astronomical community. The 2003 and 2008/9 eclipses of EE Cep were very shallow. The latter is the shallowest among all observed. The very high quality photometric data illustrate in detail the colour evolution during the eclipses for the first time. Two blue maxima in the colour indices were detected during these two eclipses, one before and one after the photometric minimum. The first (stronger) blue maximum is simultaneous with a “bump” that is very clear in all the UBV (RI )C light curves. A temporary increase in the I -band brightness at the orbital phase ~0.2 was observed after each of the last three eclipses. Variations in the spectral line profiles seem to be recurrent during each cycle. The Na i lines always show at least three absorption components during the eclipse minimum and strong absorption is superimposed on the Hα emission. Conclusions. These observations confirm that the eclipsing object in EE Cep system is indeed a dark, dusty disk around a low luminosity object. The primary appears to be a rapidly rotating Be star that is strongly darkened at the equator and brightened at the poles. Some of the conclusions of this work require verification in future studies: (i) a complex, possibly multi-ring structure of the disk in EE Cep; (ii) our explanation of the “bump” observed during the last two eclipses in terms of the different times of obscuration of the hot polar regions of the Be star by the disk; and (iii) our suggested period of the disk precession (~11–12 P orb ) and predicted depth of about 2 for the forthcoming eclipse in 2014.

International observational campaigns of the last two eclipses in EE Cephei: 2003 and 2008/9

Abstract: Two observational campaigns were carried out during the eclipses of EE Cep in 2003 and 2008/9 to verify whether the eclipsing body in the system is indeed a dark disk and to understand the observed changes in the depth and durations of the eclipses. Multicolour photometric data and spectroscopic observations at both low and high resolution were collected. We numerically modelled the variations in brightness and colour during the eclipses. We tested models with different disk structure. We considered the possibility of disk precession. The complete set of observational data collected during the last three eclipses are made available to the astronomical community. Two blue maxima in the colour indices were detected during these two eclipses, one before and one after the photometric minimum. The first (stronger) blue maximum is simultaneous with a "bump" that is very clear in all the UBVRI light curves. Variations in the spectral line profiles seem to be recurrent during each cycle. NaI lines always show at least three absorption components during the eclipse minimum and strong absorption is superimposed on the H_alpha emission. These observations confirm that the eclipsing object in EE Cep system is indeed a dark, dusty disk around a low luminosity object. The primary appears to be a rapidly rotating Be star that is strongly darkened at the equator and brightened at the poles. Some of the conclusions of this work require verification in future studies: (i) a complex, possibly multi-ring structure of the disk in EE Cep; (ii) our explanation of the "bump" observed during the last two eclipses in terms of the different times of obscuration of the hot polar regions of the Be star by the disk; and (iii) our suggested period of the disk precession (~11-12 P_orb) and predicted depth of about 2 mag the forthcoming eclipse in 2014.

Indication of Mass Segregation in LMC Star Clusters

Abstract: 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.