The intrinsic Ly to H ratio in M dwarf stars

Using Ly line proles generated from a grid of M dwarf model atmospheres we calculate the attenuation factor (i.e. Ly (obs)/ Ly (intrin)) due to the interstellar medium as a function of hydrogen column density. Then using se- lected model atmospheres, attenuation factors were calculated for those M dwarfs with available Ly observations. The prime motivation in this work has been to look afresh at the intrinsic Ly to H flux ratio, an important constraint in the radia- tive transfer modelling of M dwarfs. For those active dMe stars where both lines were observed, the intrinsic ratio is 3 5 (with 50% variation). The major uncertainty in this work has been the interstellar hydrogen column density, emphasizing the need for further work in this area and in particular an accurate model of its variation in all directions.