1998/01/29 by John E. Gizis · 2 citations
Physics and Astronomy · #Astro and Planetary Science #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/300325
16 pages including 1 figure, AASTeX, to appear in May 1998 A.J
arxiv created 1998/01/29 · openalex publication_date 1998/05/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
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 L X / L bol , the activity level of Gl 781A is no more than a factor of 2.5 subluminous with respect to near–solar-metallicity stars.