LOWER CHROMOSPHERIC ACTIVITY IN LOW ACTIVITY M-DWARFS

Four low activity dM(e) stars GI 105B, GI 643, GI 813 and GI 821 were observed spectroscopically in the optical and ultraviolet regions. Des pite relatively long integrations, only one of the four had detectable Ca Il K&H emission, GI 821, which had an observed flux of 9 10(-15) e rg cm-2 s-1, giving a surface flux of only 0.8 10(4) erg cm-2 S-1. Sim ilarly, the IUE observations implied very weak Mg II emission. In one of these stars, GI 813, the Mg II h&k surface flux was less than 0.4 1 0(4) erg cm-2 s-1, which is a factor of two less than the previous kno wn weakest dM(e) star, GI 105B. Using radiative transfer techniques th e Ca II H&K fluxes have been derived for a range of model atmospheres. The zero Ha flux can be produced by a range of models, this line is t herefore a rather poor constraint for the lower atmosphere. On the oth er hand, the Ca II K flux can be dramatically effected by simply varyi ng the temperature minimum. Comparing our calculations to the observat ional data, we find that the Ca II H&K surface fluxes cannot be reprod uced 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 cm-2.s-1, in very good agreement with the weakest dM(e) stars.