Spectroscopic properties of cool stars in the Sloan Digital Sky Survey: An analysis of magnetic activity and a search for subdwarfs

We present a spectroscopic analysis of nearly 8000 late-type dwarfs in the Sloan Digital Sky Survey. Using the Halpha emission line as an activity indicator, we investigate the fraction of active stars as a function of spectral type and find a peak near type M8, confirming previous results. In contrast to past findings, we find that not all M7 - M8 stars are active. We show that this may be a selection effect of the distance distributions of previous samples, since the active stars appear to be concentrated near the Galactic plane. We also examine the activity strength ( ratio of the luminosity emitted in Halpha to the bolometric luminosity) for each star and find that the mean activity strength is constant over the range M0-M5 and declines at later types. The decline begins at a slightly earlier spectral type than previously found. We explore the effect that activity has on the broadband photometric colors and find no significant differences between active and inactive stars. We also carry out a search for subdwarfs using spectroscopic metallicity indicators and find 60 subdwarf candidates. Several of these candidates are near the extreme subdwarf boundary. The spectroscopic subdwarf candidates are redder by similar to 0.2 mag in g - r compared with disk dwarfs at the same r - i color.