Hot Chromospheres on Cool and Ultracool Dwarfs

Sara Schmidt

Ohio State

Cool (M) and ultracool (late-M and L) dwarfs include both the least massive stars and warmest brown dwarfs. While their photospheres are characterized by cool (or ultracool) temperatures, the presence of strong surface magnetic fields on many of these objects results in hot chromospheres. Quiescent chromospheres are traced by the presence and strength of Halpha emission. Using spectroscopic data from the Sloan Digital Sky Survey, I will examine the trends in Halpha emission as a function of both age and surface temperature. The same data can also be used to understand the changes in chromospheric temperature and surface coverage over changing spectral types. Strong surface magnetic fields and hot chromospheres are also linked to dramatic flares, traced both by blue/UV continuum and atomic emission. I will discuss the growing numbers of flares observed on ultracool dwarfs and compare them to the better-understood flares on cool M dwarfs.