Cool Stars 14 - Submitted Abstract # 1
This version created on 05 October 2006

Direct Measurements of Magnetic Fields in M and L Dwarfs from Radio
Observations


Edo Berger, Carnegie Observatories, Princeton University

The question of whether and how fully convective low-mass stars and
brown dwarfs generate and dissipate magnetic fields has important
implications for our understanding of their internal structure and the
physical conditions in their atmospheres.  Theory, as well as X-ray
and H-alpha observations, have long suggested that magnetic activity
dies down in late-M and L dwarfs.  However, radio observations carried
out in the last few years paint a surprising picture - magnetic fields
are in fact ubiquitous.  Moreover, unlike other techniques radio
observations allow us to directly infer the strength of the magnetic
fields.  In this talk I will summarize radio observations of nearly
100  late-M and L dwarfs.  I show that unlike X-ray and H-alpha
activity, the level of radio activity may in fact increase with later
spectral type.  The properties of the radio emission indicate the
presence of long-lived magnetic fields ranging from 0.1-1 kG with a
large covering fraction.  On the other hand, the unexpected strength
of the radio emission points to a shift in the generation or
dissipation process at spectral type of about M7.  I will briefly
discuss the implications for dynamo models which attempt to explain
the field generation via a turbulent dynamo.

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