Magnetic Field Topology in Low-Mass Stars: Spectropolarimetric Observations of M Dwarfs

TL;DR

This study maps the magnetic field topology of the fully convective M4 dwarf G 164-31, revealing a large-scale poloidal magnetic field and significant small-scale magnetic energy, with implications for stellar structure and evolution.

Contribution

It provides the first detailed magnetic topology of a fully convective M dwarf using spectropolarimetric imaging, and compares magnetic flux measurements from different techniques.

Findings

G 164-31 has an axisymmetric poloidal magnetic field.

The magnetic flux measured by spectrum fitting is higher than that from imaging.

The star's radius is larger than theoretical predictions, likely due to youth.

Abstract

(ABRIDGED) We report here our mapping of the magnetic field topology of the M4 dwarf G 164-31 (or Gl 490B), which is expected to be fully convective, based on time series data collected from 20 hours of observations spread over 3 successive nights with the ESPaDOnS spectropolarimeter. Our tomographic imaging technique applied to time series of rotationally modulated circularly polarized profiles reveals an axisymmetric large-scale poloidal magnetic field on the M4 dwarf. We then apply a synthetic spectrum fitting technique for measuring the average magnetic flux on the star. The flux measured in G 164-31 is Bf = 3.2+-0.4 kG, which is significantly greater than the average value of 0.68 kG determined from the imaging technique. The difference indicates that a significant fraction of the stellar magnetic energy is stored in small-scale structures at the surface of G 164-31. Our H_alpha emission light curve shows evidence for rotational modulation suggesting the presence of localized structure in the chromosphere of this M dwarf. The radius of the M4 dwarf derived from the rotational period and the projected equatorial velocity is at least 30% larger than that predicted from theoretical models. We argue that this discrepancy is likely primarily due to the young nature of G 164-31 rather than primarily due to magnetic field effects, indicating that age is an important factor which should be considered in the interpretation of this observational result. We also report here our polarimetric observations of five other M dwarfs with spectral types from M0 to M4.5, three of them showing strong Zeeman signatures.