Quiescent Coronae of Active Chromosphere Stars

Abstract

The EINSTEIN Observatory has for the first time provided high sensitivity X-ray measurements of quiescent coronal emission from a large sample of dwarf stars. We now have observed a sufficient number of the nearby M-dwarfs to determine an X-ray luminosity function and we have explored the activity and variability of these stars to the extent of observing, for the first time, X-ray flares with simultaneous ground-based optical and IUE ultraviolet coverage.

The M dwarfs are found to have a much higher degree of variability in X-rays than does the Sun; however, in most cases a quiescent level is definable. We will discuss the quiescent emission from these stars and the changes in quiescent level on time scales from hours to ∼ 1 year. We have determined coronal temperatures for many of these stars; they are generally hotter than the Solar corona and some of the more active dM stars have T_cor ∼ 10⁷ K.

Arguments are presented in support of the hypothesis that M-dwarf coronae are magnetically dominated, as is the Solar corona. We then examine the usefulness of loop model atmosphere calculations in elucidating the coronal heating mechanism and the ways in which observations may be used to test competing theories. The X-ray measurements can be used to predict magnetic field strengths on these stars, with testable implications.