Astronomy and Astrophysics, volume 430, 303-311 (2005/1-4)
Mg II chromospheric radiative loss rates in cool active and quiet stars.
Abstract (from CDS):
The MgII k emission line is a good indicator of the level of chromospheric activity in late-type stars. We investigate the dependence of this activity indicator on fundamental stellar parameters. To this purpose we use IUE observations of the MgII k line in 225 late-type stars of luminosity classes I-V, with different levels of chromospheric activity. We first re-analyse the relation between MgII k line luminosity and stellar absolute magnitude, performing linear fits to the points. The ratio of MgII surface flux to total surface flux is found to be independent of stellar luminosity for evolved stars and to increase with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The MgII k emission level turns out to be not dependent on metallicity. Finally, the MgII k line surface flux-temperature relation is investigated by treating separately, for the first time, a large sample of very active and normal stars. The stellar surface fluxes in the k line of normal stars are found to be strongly dependent on the temperature and slightly dependent on the gravity, thus confirming the validity of recently proposed models. In contrast, data relative to RSCVn binaries and BYDra stars, which show very strong chromospheric activity, are not justified in the framework of a description based only on acoustic waves and uniformly distributed magnetic flux tubes so that they require more detailed models.