Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004iaus..219..275b&link_type=abstract
Stars as suns : activity, evolution and planets, Proceedings of the 219th symposium of the International Astronomical Union held
Astronomy and Astrophysics
Astronomy
1
Scientific paper
The rotation-activity paradigm has proven very effective in solar-type stars explaining both their activity and rotational histories. The basic ingredients are rotationally-driven dynamos and conversion of magnetic energy into heating. Things change near the bottom of the main sequence. A saturation velocity persists cooler than M3 suggesting that the remaining dynamo is not purely turbulent (even though the stars are fully convective and cannot support a solar-type dynamo). At M9 the levels of stellar activity plummet. This is probably due to a decoupling of the field from atmospheric motions given extreme neutrality of the atmospheres. It is no longer forced into dissipative configurations and quiescent chromospheres and coronae disappear. The lack of magnetic braking leaves these stellar and substellar objects very rapid rotators. Thus the rotation-activity relation is REVERSED at the low mass limit. Flaring persists in very cool objects suggesting some field is still present. We don't know how these flares are generated (they exhibit odd ratios of X-ray to radio flux). A further mystery is the elevated activity of very young cool objects. Clearly we still have much to learn in this area.
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