Spectroscopy of the bright optical counterparts of X-ray sources in the direction of M 31. II

A recent survey of the Local Group spiral galaxy M 31 with XMM-Newton yielded a large number of X-ray sources. This is the second in a series of papers with the aim of identifying the optical counterparts of these X-ray sources. We have obtained optical spectra for 21 bright optical counterparts of 20 X-ray sources in the direction of M 31, using the 1.3-m Skinakas telescope in Crete, Greece. For 17 of the 20 X-ray sources, we have identified the optical counterpart as a normal late type star (of type F or later) in the foreground (i.e. in the Milky Way). For two more sources there were two possible optical counterparts in each case, while two more objects have X-ray properties that are not compatible with the spectral characteristics of late type non-flaring stars.Comment: Accepted for publication by Astronomy & Astrophysics (7 pages, 8 figures, and 2 tables

Spectroscopy of the brightest optical counterparts of X-ray sources in the direction of M~31 and M~33

Recent surveys of the Local Group spiral Galaxies M31 and M33 with XMM-Newton yielded a large number of X-ray sources. As part of the effort to identify and classify the objects responsible for this X-ray emission, we have obtained optical spectra of the brightest optical counterparts of the identified X-ray sources, using the 1.3m Skinakas Telescope. Most of these objects are foreground star candidates. The purpose of the present study is to confirm this identification and to explore the compatibility between the optical spectral classification and the observed X-ray properties of the sources. We have obtained optical spectra for the 14 brightest optical counterparts of X-ray sources identified by XMM-Newton in the direction of M31 and for 21 optical counterparts in the direction of M33, using the 1.3m Skinakas telescope in Crete, Greece. All of the M31 sources and all but one of the M33 sources were confirmed to be foreground stars, of spectral types between A and M. One of the stars is a late M dwarf with H-alpha emission, a flare star, also displaying strong X-ray variability. One of the M~33 sources (lying within the D25 ellipse) corresponds to a previously known background galaxy, LEDA 5899.Comment: 9 pages, 12 figures, accepted in A&

Ca II Triplet Spectroscopy of Giants in SMC Star Clusters: Abundances, Velocities and the Age-Metallicity Relation

We have obtained spectra at the Ca II triplet of individual red giants in seven SMC star clusters whose ages range from ~4 to 12 Gyr. The spectra have been used to determine mean abundances for six of the star clusters to a typical precision of 0.12 dex. When combined with existing data for other objects, the resulting SMC age-metallicity relation is generally consistent with that for a simple model of chemical evolution, scaled to the present-day SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and NGC 339), however, have abundances that ~0.5 dex lower than that expected from the mean age-metallicity relation. It is suggested that the formation of these clusters, which have ages of ~5 Gyr, may have involved the infall of uneriched gas, perhaps from the Magellanic Stream. The spectra also yield radial velocities for the seven clusters. The resulting velocity dispersion is 16 +/- 4 km/sec, consistent with those of the SMC planetary nebula and carbon star populations.Comment: 28 pages including 4 figure

The Low- and Intermediate-Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae

We present a study on the central stars (CSs) of Planetary Nebulae (PNe) observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging Spectrograph instrument on-board the HST. The stellar magnitudes have been measured using broad-band photometry, and Zanstra analysis of the nebulae provided the stellar temperatures. From the location of the CSs on the HR diagram, and by comparing the observed CSs with current models of stellar evolution, we infer the CSs masses. We examine closely the possibility of light contamination in the bandpass from an unrecognized stellar companion, and we establish strong constraints on the existence and nature of any binary companion. We find an average mass of 0.63 Msun, which is similar to the mass obtained for a sample of CSs in the LMC (0.65 Msun). However, the SMC and LMC CS mass distributions differ slightly, the SMC sample lacking an intermediate-mass stellar population (0.65 to 0.75 Msun). We discuss the significance and possible reasons for the difference between the two mass distributions. In particular, we consider the differences in the star formation history between the clouds and the mass-loss rate dependence on metallicity.Comment: 30 pages, 6 figures, 5 tables. To be published in ApJ (October 20

Estimation of the seismic hazard parameters for various sites in Greece using a probabilistic approach

International audienceThe probabilistic approach that was recently introduced by Kijko and Graham (1998,1999) was applied for the estimation of seismic hazard for various sites in Greece in terms of peak ground acceleration (PGA) and peak ground velocity (PGV). These sites represent geographically cities of Greece among which are the capital of Greece and some others with high, intermediate and low seismicity. The approach allows the use of earthquake catalogues with incomplete reported historical and complete instrumental data, the consideration of different magnitude thresholds, and the incorporation of magnitude uncertainties. One of the advantages of the method is that it does not require any determination of seismic zones. The estimated values of PGA for return periods of 476 years were grouped in 4 categories, the same ones used in the seismic hazard map for the New Seismic Code of Greece. Comparison results were quite interesting and very encouraging concerning the reliability of this probabilistic approach