Silicate evolution in brown dwarf disks

B. Riaz

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    18 Citations (Scopus)
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    Abstract

    We present a compositional analysis of the 10 mu m silicate spectra for brown dwarf disks in the Taurus and Upper Scorpius (UppSco) star-forming regions, using archival Spitzer/Infrared Spectrograph observations. A variety in the silicate features is observed, ranging from a narrow profile with a peak at 9.8 mu m, to nearly flat, low-contrast features. 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. The median crystalline mass fraction for the Taurus brown dwarfs is found to be 20%, a factor of similar to 2 higher than the median reported for the higher mass stars in Taurus. The large-grain mass fractions are found to increase with an increasing strength in the X-ray emission, while the opposite trend is observed for the crystalline mass fractions. A small 5% of the Taurus brown dwarfs are still found to be dominated by pristine interstellar medium-like dust, with an amorphous submicron grain mass fraction of similar to 87%. 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 have occurred in the disks, resulting in a high fraction of amorphous submicron grains in the disk surface. Among the UppSco brown dwarfs, only usd161939 has a signal-to-noise ratio high enough to properly model its silicate spectrum. We find a 74% small amorphous grain and a similar to 26% crystalline mass fraction for this object.

    Original languageEnglish
    Pages (from-to)571-586
    Number of pages16
    JournalThe Astrophysical Journal
    Volume701
    Issue number1
    DOIs
    Publication statusPublished - 10 Aug 2009

    Keywords

    • circumstellar matter
    • open clusters and associations: individual (Taurus, Upper Scorpius)
    • stars: abundances
    • stars: low-mass, brown dwarfs
    • SCORPIUS OB ASSOCIATION
    • TAURUS MOLECULAR CLOUD
    • YOUNG SUBSTELLAR OBJECTS
    • LOW-MASS STARS
    • PROTOPLANETARY DISKS
    • GRAIN-GROWTH
    • CRYSTALLINE SILICATES
    • CIRCUMSTELLAR DISKS
    • DUST GRAINS
    • MINERALOGY

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