Constraints on mass loss from dMe stars: theory and observations

We show that the flux distribution of a wind from a cool star differs considerably from what is predicted by the theory for mass loss from hot stars. The differences are caused by the facts that 1) themass loss rates are lower, resulting in smaller optical depths in the wind, and 2) for winds from cool stars the temperature of thewind is higher than the temperature of the star while for winds from hot stars the reverse holds. These differences result in substantial modifications of the flux distribution and imply that care must be exercised when applying the flux predictions by e.g. Wright and Barlow (1975) to winds from cool stars. By using observational constraints we show that the mass loss from cool dwarf stars equals at most 10−12 M /yr. This is a factor hundred lower than previous estimates. At this rate the mass loss from dMe stars is of little importance for the enrichment of the interstellar medium. By solving the radiative transfer equations for stellar winds from dMe stars, we show that the inferred power-law flux distributions, based on radio, JCMT and IRAS data, cannot be reconciled with the flux distributions from a stellar wind of 10−10 M /yr as was previously assumed. The maximum allowable mass loss rate is at most a few times 10−12 M /yr which implies that the fluxes observed with JCMT, IRAS, and in the future with ISO, require a different interpretation than free-free emission from a stellar wind.