Movatterモバイル変換

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

Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, via del Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: cardini@rm.iasf.cnr.it

Received: 9 June 2004
Accepted: 14 September 2004

Abstract

Thek emission line is a good indicator of the level ofchromospheric activity in late-type stars.We investigate the dependence of this activity indicator on fundamentalstellar parameters. To this purpose we useIUE observations of thek line in 225 late-type stars of luminosity classes I–V,with different levels of chromospheric activity.We first re-analyse the relation betweenk line luminosity and stellar absolute magnitude,performing linear fits to the points. The ratio of surface flux to total surface flux isfound to be independent of stellar luminosity for evolved starsand to increase with decreasing luminosity for dwarfs.We also analyse thek line surface flux-metallicityconnection. Thek emission level turns out to benot dependent on metallicity.Finally, thek line surface flux-temperature relationis investigated by treating separately, for the first time, a large sample ofvery active and normal stars. The stellar surface fluxes in thek line of normal stars arefound 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 frameworkof a description based only on acoustic waves and uniformly distributedmagnetic flux tubes so that they require more detailed models.