High Chromospheric Activity in M Subdwarfs

We present spectroscopic observations of two halo M subdwarfs which have Halpha emission lines. We show that in both cases close companions are the mostlikely cause of the chromospheric activity in these old, metal-poor stars. Weargue that Gl 781 A's unseen companion is most likely a cool helium whitedwarf. Gl 455 is a near-equal-mass M subdwarf (sdM) system. Gl 781 A is rapidlyrotating with v sin i = 30 km/s. The properties of the chromospheres and X-raycoronae of these systems are compared to M dwarfs with emission (dMe). TheX-ray hardness ratios and optical chromospheric lines emission ratios areconsistent with those seen in dMe stars. Comparison to active near-solarmetallicity stars indicates that despite their low metallicity ([m/H] = -1/2),the sdMe stars are roughly as active in both X-rays and chromospheric emission.Measured by L_X/L_bol, the activity level of Gl 781 A is no more than a factorof 2.5 subluminous with respect to near-solar metallicity stars.Comment: 16 pages including 1 figure, AASTeX, to appear in May 1998 A.