High Chromospheric Activity in M Subdwarfs

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Abstract
We present spectroscopic observations of two halo M subdwarfs that have Hα emission lines. We show that in both cases close companions are the likely cause of the chromospheric activity in these old, metal-poor stars. We argue that Gl 781A's unseen companion is most likely a cool helium white dwarf. Gl 455 is a near–equal-mass M subdwarf (sdM) system. Gl 781A is rapidly rotating, with v sin i ≈ 30 km s-1. The properties of the chromospheres and X-ray coronae of these systems are compared with M dwarfs with emission (dMe's). The X-ray hardness ratios and optical chromospheric line emission ratios are consistent with those seen in dMe stars. Comparison with active near–solar metallicity 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 LX/Lbol, the activity level of Gl 781A is no more than a factor of 2.5 subluminous with respect to near–solar-metallicity stars.
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