Spectroscopic Properties of Cool Stars in the Sloan Digital Sky Survey: An Analysis of Magnetic Activity and a Search for Subdwarfs

Abstract

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