Bibcode
Goldman, B.; Pitann, J.; Zapatero Osorio, M. R.; Bailer-Jones, C. A. L.; Béjar, V. J. S.; Caballero, J. A.; Henning, Th.
Bibliographical reference
Astronomy and Astrophysics, Volume 502, Issue 3, 2009, pp.929-936
Advertised on:
8
2009
Journal
Citations
26
Refereed citations
22
Description
Context: Ultra-cool dwarfs of the L spectral type (T_eff = 1400-2200 K)
are known to have dusty atmospheres. Asymmetries of the dwarf surface
may arise from rotationally-induced flattening and dust-cloud coverage,
and may result in non-zero linear polarisation through dust scattering.
Aims: We aim to study the heterogeneity of ultra-cool dwarfs'
atmospheres and the grain-size effects on the polarisation degree in a
sample of nine late M, L and early T dwarfs. Methods: We obtain
linear polarimetric imaging measurements using FORS1 at the Very Large
Telescope, in the Bessel I filter, and for a subset in the Bessel R and
the Gunn z filters. Results: We measure a polarisation degree of
(0.31±0.06)% for LHS102BC. We fail to detect linear polarisation
in the rest of our sample, with upper-limits on the polarisation degree
of each object of 0.09% to 0.76% (95% of confidence level), depending on
the targets and the bands. For those targets we do not find evidence of
large-scale cloud horizontal structure in our data. Together with
previous surveys, our results set the fraction of ultra-cool dwarfs with
detected linear polarisation to 30+10_-6% (1-σ errors).
From the whole sample of well-measured objects with errors smaller than
0.1%, the fraction of ultra-cool dwarfs with polarisation degree larger
than 0.3% is smaller than 16% (95% confidence level). Conclusions: For three brown dwarfs, our observations indicate
polarisation degrees different (at the 3-σ level) than previously
reported, giving hints of possible variations. Our results fail to
correlate with the current model predictions for ultra-cool dwarf
polarisation for a flattening-induced polarisation, or with the
variability studies for a polarisation induced by an heterogeneous cloud
cover. This stresses the intricacy of each of those tasks, but may arise
as well from complex and dynamic atmospheric processes.
Based on observations collected at the European Observatory, Paranal,
Chile,
under programmes 075.C-0653(A) and 077.C-0819(A).
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Our goal is to study the processes that lead to the formation of low mass stars, brown dwarfs and planets and to characterize the physical properties of these objects in various evolutionary stages. Low mass stars and brown dwarfs are likely the most numerous type of objects in our Galaxy but due to their low intrinsic luminosity they are not so
Rafael
Rebolo López