At the Bottom of the Main Sequence: Activity and magnetic fields beyond the threshold to complete convection

Abstract

The bottom of the main sequence hosts objects with fundamentally different properties. At masses of about 0.3 M${}_{\odot }$, stars become fully convective and at about 0.08 M${}_{\odot }$ the hydrogen-burning main sequence ends; less massive objects are brown dwarfs. While stars and brown dwarfs experience very different evolutions, their inner structure has relatively little impact on the atmospheres. The generation of magnetic fields and activity is obviously connected to the threshold between partial and complete convection, because dynamo mechanisms involving a layer of shear like the solar $\alpha \Omega$-dynamo must cease. Hence a change in stellar activity can be expected there. Observations of stellar activity do not confirm a rapid break in activity at the convection boundary, but the fraction of active stars and rapid rotators is higher on the fully convective side. I summarize the current picture of stellar activity and magnetic field measurements at the bottom of the main sequence and present recent results on rotational braking beyond.

25 pages, 7 figures, Ludwig Biermann Lecture given at the annual meeting of the German Astronomical Society, 2007, v2

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• A Spitzer View of the Young Open Cluster NGC 2264
• The Initial Mass Function and Young Brown Dwarf Candidates in NGC 2264. IV. The Initial Mass Function and Star Formation History
• Accretion discs as regulators of stellar angular momentum evolution in the ONC and Taurus-Auriga
• HST search for M subdwarf binaries
• Magnetic fields from low mass stars to brown dwarfs