Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21560603r&link_type=abstract
American Astronomical Society, AAS Meeting #215, #606.03; Bulletin of the American Astronomical Society, Vol. 36, p.1130
Astronomy and Astrophysics
Astronomy
Scientific paper
We present a compositional analysis of the 10µm silicate emission features for brown dwarf disks in the Taurus and Upper Scorpius (UppSco) star-forming regions, using archival Spitzer/IRS observations. We find a trend of increasing large-grain mass fractions with an increasing strength in the X-ray emission, while lower crystalline levels are observed among the weaker X-ray emitters. The median crystalline mass fraction for the Taurus brown dwarfs is found to be 20%, a factor of 2 higher than the median reported for the higher mass stars in Taurus. For most objects, we find nearly equal fractions for the large-grain and crystalline mass fractions, indicating both processes to be active in these disks. No strong correlation is observed between the mass fractions for the large and crystalline grains, and the degree to which the disk has sedimented. A small 5% of the Taurus brown dwarfs are still found to be dominated by pristine ISM-like dust, with an amorphous sub-micron grain mass fraction of 90%. For 15% of the objects, we find a negligible large-grain mass fraction, but a >60% small amorphous silicate fraction. These may be the cases where substantial grain growth and dust sedimentation has occurred in the disks, resulting in a high fraction of amorphous sub-micron grains in the disk surface. The detection rate for emission in the 10µm silicate feature shows a dependence on the age of the system, such that the rate drops from 75% in Taurus to only 23% among the older UppSco brown dwarfs.
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