An Indication of Extended Chromospheres in Line Width-Luminosity Relations

Abstract Measurements of Mg II k, Ca II K, and Hα line widths in late-type stars are compiled together with stellar basic parameters and the net radiative losses of their chromospheres estimated from the Mg II emission line intensities. We found that in giants and supergiants the Mg II k1 width corrected for activity and abundance difference (W1*) shows a different power dependence on the stellar surface gravity from that in dwarfs. The widths of Mg II k and Ca II K lines are linearly correlated even in these giants and supergiants and suggests that the density-dependent effects of the partial frequency redistribution are not the cause of the change in the power index in W1*(k). These facts indicate that the extended chromosphere is a universal phenomenon in the low-gravity late-type stars. The width of Hα shows a nearly constant velocity field in the chromosphere irrespective of gravity, providing that in hybrid stars a small portion of the systematic outflow velocity is superposed on the intrinsic width.