Osservatorio Astronomico di Palermo Giuseppe S. Vaiana

STAR CLUSTERS AND STELLAR POPULATIONS

One of the main stellar research field at OAPa concerns star clusters, i.e. groups of stars born from the same gas cloud and thus sharing characteristics such as age and chemical composition. X-ray, optical infrared spectral observations allow to selected the young cluster members and to study the physical properties of stars and their atmospherest. OAPA researchers carry out X-ray, photometric and spectroscopic studies by means of space and ground facilities, to which they have access upon approval of observation proposals made at an international and competitive scale. The most commonly used telescopes are, from space, Chandra, XMM-Newton, Spitzer and Corot and, from ground, at the European Southern Observatory (ESO) in Chile - La Silla and Paranal - and at the Telescopio Nazionale Galileo (Galileo National Telescope) in the Canary Islands, at the Observatory of Roque de los Muchachos.

The mass, age and temperature of stars can be determined by optical photometry, whereas infrared photometry highlights the cold material constituting the gas disks which surround the youngest stars, whose study is essential to understanding stellar formation processes. Disk evolution and lifetime depend on the environmental conditions at star birth and influence the formation of planetary systems. To understand these processes, INAF-OAPa researchers have started several systematic multiwavelength studies of the sky regions where new stars have recently formed.

By optical high-resolution spectroscopy atmospheres can be examined in detail, focusing on chemical abundances, stellar gravity, rotation, atmospheric motions and turbulences. Spectroscopy also allows the study of magnetic fields on the stellar chromosphere and surface, phenomena which in active stars can be more intense than in the Sun. By analysing the spectrum of very young stars, and in particular by observing calcium and hydrogen emission lines, it is possible to study the increase of matter coming out of the source generating them, a crucial occurrence for the subsequent evolution of the star.

Young stars trace the recent star-formation history in the solar neighbourhood. Over the last decades, the cross-correlation between X-ray observations, optical spectroscopy and kinematic data have suggested the presence of a young (< 109 yr) population of cool stars in the solar neighbourhood. It is less clear, however, whether these stars form an excess population that requires a recent episode of star formation. Stellar rotation is one of the best proxies for young main sequence stars, since their photometric and spectroscopic properties do not differ significantly from those of the oldest field stars. Since stellar rotation decreases with stellar age for main-sequence stars, the distribution of rotational periods will roughly reflect their age distribution. The OAPA researchers took advantage of the large number of precise light curves of main-sequence stars, obtained with CoRoT, to measure stellar rotation period in a representative sample, thus enabling a statistical analysis of the period as function of spectral type (or stellar mass), with implications on age distribution.

OAPa researchers have a long record in studying the galactic stellar population by means of X-ray surveys. Comparing observed X-ray stellar counts and logN-logS distributions to a galactic model allows us to infer the spatial distribution of stellar populations in the Galaxy and the recent Galactic star formation rate. This is currently being investigated on the basis of the XMM/COSMOS data, an X-ray mosaic spanning a two square-degree region on the sky for a total exposure time of 1.4Msec.