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

Abstract

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

General Model for Light Curves of Chromospherically Active Binary Stars∗

“…The MLC minimum and shape remain nearly unchanged. Epochs Ac and Aa coincide with MLC maxima and minima in Figure 13(g) during the second part of the data (Gorza & Heard 1971;Özeren et al 1999;Koen & Eyer 2002;Cardini 2005, P orb , t 0 , e, Sp-type of component A).…”

The basal chromospheric Mg ii h+k flux of evolved stars: probing the energy dissipation of giant chromospheres

“…1). Since the radiative cooling is still dominated by hydrogen, there is little, if any, dependence of the Mg ii line flux on metallicity (Cardini 2005).…”

Mg II h + k Flux—Rotational Period Correlation for G-type Stars

“…This excess can be plausibly explained by the different integration processes. Since Cardini (2005) did not report the IUE images that were used, it is possible that they not coincide with ours, as may be the case of HD2151 for which our flux is 80% larger.…”

“…In order to test the consistency of our derived fluxes, in Figure 2 we compare our measurements of the Mg II k line flux with those reported by Cardini (2005) (for the 23 objects in common). Whilst the correlation is clear, our fluxes are higher by approximately 25% (about 0.1 dex) on average.…”