Small-scale magnetic fields on late-type M-dwarfs
Abstract
We performed kinematic studies of the evolution of small-scale magnetic fields in the surface layers of M-dwarfs. We solved the induction equation for a prescribed velocity field, magnetic Reynolds number \ReM, and boundary conditions in a Cartesian box, representing a volume comprising the optically thin stellar atmosphere and the uppermost part of the optically thick convective envelope. The velocity field is spatially and temporally variable, and stems from detailed radiation-hydrodynamics simulations of convective flows in a proto-typical late-type M-dwarf (Teff =2800pun {K}, logg =5.0, solar chemical composition, spectral type ~M6). We find dynamo action for ReM >= 400. Growth time scales of the magnetic field are comparable to the convective turn-over time scale (~ 150pun {sec}). The convective velocity field concentrates the magnetic field in sheets and tubular structures in the inter-granular downflows. Scaling from solar conditions suggests that field strengths as high as 20\pun{kG} might be reached locally. Perhaps surprisingly, \ReM\ is of order unity in the surface layers of cooler M-dwarfs, rendering the dynamo inoperative. In all studied cases we find a rather low spatial filling factor of the magnetic field.
- Publication:
-
Astronomische Nachrichten
- Pub Date:
- July 2002
- DOI:
- 10.1002/1521-3994(200208)323:3/4<402::AID-ASNA402>3.0.CO;2-H
- arXiv:
- arXiv:astro-ph/0205458
- Bibcode:
- 2002AN....323..402D
- Keywords:
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- stars: activity;
- stars: low-mass;
- stars: magnetic fields;
- Astrophysics
- E-Print:
- 5 pages, 7 figures, submitted to Astron. Nachr. (from 1st Potsdam Thinkshop on Sunspots and Starspots)