Osservatorio Astronomico di Palermo Giuseppe S. Vaiana

STELLAR MAGNETIC ACTIVITY

Late-type main-sequence stars, in particular those with masses and ages similar to the solar one, are known to share with the Sun also similar X-ray emission characteristics. In fact, the presence of hot plasma in stellar coronae is ultimately due to a magnetic dynamo mechanism triggered by the coupling of magnetic differential rotation and surface convection. Researchers at OAPA have studied for several years the dependence of the stellar X-ray emission on parameters such as the stellar rotational period, the convective turnover time, the evolutionary phase and the chemical composition (metallicity), using data taken with several space-borne satellites for UV and X-ray astrophysics (IUE, Einstein, ROSAT, EUVE, GALEX, Chandra, XMM_Newton, ASTRO-E). Research activity is still ongoing to elucidate the analogies and differences between the Sun and solar-type stars. In particular, we are involved in the search and characterization of magnetic cycles in solar-type stars by means of long-term monitoring programs of the chromospheric and coronal emission from selected nearby targets. We also examine the relation between chromospheric and coronal activity by comparing the diagnostics present in different wavebands, e.g. optical emission lines versus X-rays. Last but not least, we study the magnetic activity of very low-mass stars and brown dwarfs. These objects are distinguished from solar-like stars by the lack of a radiative zone and very cool photospheres with drastic impact onto their chromospheric (optical) and coronal (X-ray and radio) emissions.