  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.
