Fully Convective M Dwarfs



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Fully Convective M Dwarfs

Ambruster and A. Brown (U. CO) obtained a 250 ksec EUVE spectrum of the least massive star in the EUVE Bright Star List, the single, dM6e flare star CN Leo. With M = 0.1 M, CN Leo is both fully convective and close to the 0.08 M brown dwarf boundary. Part of the observation was coordinated with an observation of the star by G. Stringfellow (Penn St.) using the Japanese ASCA X-ray satellite. Coronal spectroscopy with EUVE is the most efficient way to address differences in upper atmospheric heating between more massive M dwarfs with radiative cores (where activity is believed to derive from the classical dynamo), and the fully convective M dwarfs where the dynamo must operate differently because there no longer is an interface between the convection zone and the radiative core. Although fully convective M dwarfs are intrinsically faint at all wavelengths including X-ray (e.g., log L = 26.70 for CN Leo), L is a much higher percentage of L in these stars than it is for G and K dwarfs whose L may be considerably greater. Thus changes in dynamo heating due to the transition to a fully convective structure should be most visible at coronal temperatures. Reduction of the EUVE data is currently in progress. The combined EUVE and ASCA spectral data will then be used to establish the dominant temperature(s) of the outer atmosphere using Volume Emission Measure modelling. This will allow comparison of CN Leo's temperature structure and energy balance with earlier (radiative core) M dwarfs which have been observed with EUVE (including EV Lac above), in a search for clues to the nature of the magnetic dynamo in fully convective dwarfs.



Astronomy
Mon Feb 26 18:11:01 EST 1996