Transition Region Emission from Very Low Mass Stars

Abstract

We present results from our Cycle 10 Hubble Space Telescope (HST) program to search for transition region emission in very low mass main-sequence stars in the spectral range M7-M9. Our program is aimed at (1) detecting emission and (2) distinguishing between flaring and quiescent origin for the emission. We have obtained HST/Space Telescope Imaging Spectrograph time series observations of three active, very low mass stars (VB 8, VB 10, and LHS 2065), which show persistent activity in transition region lines including Si IV, C IV, and He II. Emission in transition region lines appears to be variable between exposures but is always observed. A strong flare was also observed in one 10 minute exposure on VB 10. Our results indicate that active, very low mass stars maintain a persistent quiescent chromosphere and transition region that is similar to those observed in active earlier type M dwarfs, in contrast to suggestions that these low-mass main-sequence stars exhibit only relatively strong flares and no quiescent emission.