The Lyman alpha emission in W Ursae Majoris.

Abstract

The paper presents results of reductions of six low resolution IUE-SWP spectra properly exposed at Ly-alpha during one orbital revolution of WUMa, the prototype for late-type contact binaries. Two different methods have been used to eliminate the geocoronal contribution to the emission line. The effect of the interstellar H I absorption has been modelled for rotational profiles at different orbital phases, using different estimates of the hydrogen column density in the direction and distance of WUMa (less than 10 to the 19th atoms/sq cm). The stellar emission seems to follow variations of the bolometric brightness so that the absorption-corrected average over the orbit is f(Ly-alpha)/f(bol) = (4.2 + or 0.6) x 10 to the -5th, a value about 9 times larger than for the sun. This result indicates a fairly uniform chromosphere around the common surface of the system.