MAGNETIC FIELD TOPOLOGY IN LOW-MASS STARS: SPECTROPOLARIMETRIC OBSERVATIONS OF M DWARFS

Abstract

(ABRIDGED) We report here our mapping of the magnetic field topology of the\nM4 dwarf G 164-31 (or Gl 490B), which is expected to be fully convective, based\non time series data collected from 20 hours of observations spread over 3\nsuccessive nights with the ESPaDOnS spectropolarimeter. Our tomographic imaging\ntechnique applied to time series of rotationally modulated circularly polarized\nprofiles reveals an axisymmetric large-scale poloidal magnetic field on the M4\ndwarf. We then apply a synthetic spectrum fitting technique for measuring the\naverage magnetic flux on the star. The flux measured in G 164-31 is Bf =\n3.2+-0.4 kG, which is significantly greater than the average value of 0.68 kG\ndetermined from the imaging technique. The difference indicates that a\nsignificant fraction of the stellar magnetic energy is stored in small-scale\nstructures at the surface of G 164-31. Our H_alpha emission light curve shows\nevidence for rotational modulation suggesting the presence of localized\nstructure in the chromosphere of this M dwarf. The radius of the M4 dwarf\nderived from the rotational period and the projected equatorial velocity is at\nleast 30% larger than that predicted from theoretical models. We argue that\nthis discrepancy is likely primarily due to the young nature of G 164-31 rather\nthan primarily due to magnetic field effects, indicating that age is an\nimportant factor which should be considered in the interpretation of this\nobservational result. We also report here our polarimetric observations of five\nother M dwarfs with spectral types from M0 to M4.5, three of them showing\nstrong Zeeman signatures.\n