Mg II chromospheric radiative loss rates in cool active and quiet stars

The Mg II 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 Mg II 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 Mg II k line luminosity and stellar absolute magnitude, performing linear fits to the points. The ratio of Mg II 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 Mg II k line surface flux-metallicity connection. The Mg II k emission level turns out to be not dependent on metallicity. Finally, the Mg II 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 RS CVn binaries and BY Dra 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