A&A 396, L35-L38 (2002)
DOI: 10.1051/0004-6361:20021657
Letter
Optical linear polarimetry of ultra cool dwarfs
F. Ménard1, X. Delfosse1 and J.-L. Monin1, 21 Laboratoire d'Astrophysique, Observatoire de Grenoble, CNRS/UJF UMR 5571, 414 rue de la piscine, BP 53, 38041 Grenoble Cedex 9, France
2 Institut Universitaire de France
(Received 14 October 2002 / Accepted 14 November 2002)
Abstract
We present optical linear polarimetry of 8 ultra cool field dwarfs,
with spectral types ranging from M9 to L8. The linear polarisation,
P,
of each dwarf we measured is
. Three dwarfs have
polarisations compatible with zero, two are marginal detections, and
three have significant polarisation. Due to their small distance, an
insterstellar origin for the detected polarisation can be safely ruled
out. Our detections confirm that dust is present in the atmosphere of
these brown dwarfs and that the scattering geometry is not
symmetric. Possibilities for asymmetry include the dwarfs rotating
rapidly and being oblate, or the cloud coverage in the atmosphere
being inhomogenous.
Key words: stars: low-mass, brown dwarfs -- polarization -- dust -- scattering -- stars: atmospheres
Offprint request: F. Ménard, menard@obs.ujf-grenoble.fr
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