Title

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

Authors

Authors

P. B. Ngoc; J. Lim; J. F. Donati; C. M. Johns-Krull;E. L. Martin

Comments

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Abbreviated Journal Title

Astrophys. J.

Keywords

stars: individual (G 164-31, Gl 890, LHS 473, KP Tau, Gl 896B, 2E 4498); stars: low-mass, brown dwarfs; stars: magnetic fields; techniques:; polarimetric; techniques: spectroscopic; H-ALPHA EMISSION; NEARBY M-DWARFS; ENTROPY IMAGE-RECONSTRUCTION; FULLY; CONVECTIVE STARS; X-RAY-EMISSION; T TAURI STARS; ULTRACOOL DWARFS; RADIO-EMISSION; ACTIVE STARS; CHROMOSPHERIC ACTIVITY; Astronomy & Astrophysics

Abstract

The magnetic field topology plays an important role in the understanding of stellar magnetic activity. While it is widely accepted that the dynamo action present in low-mass partially convective stars (e. g., the Sun) results in predominantly toroidal magnetic flux, the field topology in fully convective stars (masses below similar to 0.35M(circle dot)) is still under debate. 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 hr of observations spread over three 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 vertical bar B(f)vertical bar = 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 Ha 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.

Journal Title

Astrophysical Journal

Volume

704

Issue/Number

2

Publication Date

1-1-2009

Document Type

Article

Language

English

First Page

1721

Last Page

1729

WOS Identifier

WOS:000270486200054

ISSN

0004-637X

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