Lower chromospheric activity in low activity M dwarfs

Abstract

Four low activity dM(e) stars Gl 105B, Gl 643, Gl 813 and Gl 821 were observed spectroscopically in the optical and ultraviolet regions. Despite relatively long integrations, only one of the four had detectable Ca II K&H emission, Gl 821, which had an observed flux of 9 10^-15^ erg/cm2/s, giving a surface flux of only 0.8 10^4^ erg/cm2/s. Similarly, the IUE observations implied very weak Mg II emission. In one of these stars, Gl 813, the Mg II h&k surface flux was less than 0.4 10^4^ erg/cm2/s, which is a factor of two less than the previous known weakest dM(e) star, Gl 105B. Using radiative transfer techniques the Ca II H&K fluxes have been derived for a range of model atmospheres. The zero Hα flux can be produced by a range of models, this line is therefore a rather poor constraint for the lower atmosphere. On the other hand, the Ca II K flux can be dramatically effected by simply varying the temperature minimum. Comparing our calculations to the observational data, we find that the Ca II H&K surface fluxes cannot be reproduced with T_min_ 3000K. However, decreasing the temperature minimum by less than 400K produces Ca II H&K fluxes of the order of 10^3^ erg/cm2/s, in very good agreement with the weakest dM(e) stars.