Observations of Disks around Brown Dwarfs in the TW Hydra Association with the Spitzer Infrared Spectrograph

Details Observations of Disks around Brown Dwarfs in the TW Hydra Association with the Spitzer Infrared Spectrograph Observations of Disks around Brown Dwarfs in the TW Hydra Association with the Spitzer Infrared Spectrograph

Apr 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008apj...676l.143m&link_type=abstract

The Astrophysical Journal, Volume 676, Issue 2, pp. L143-L146.

Astronomy and Astrophysics

Astronomy

19

Accretion, Accretion Disks, Stars: Planetary Systems: Protoplanetary Disks, Stars: Formation, Stars: Low-Mass, Brown Dwarfs, Stars: Pre-Main-Sequence

Scientific paper

Using SpeX at the NASA Infrared Telescope Facility and the Spitzer Infrared Spectrograph, we have obtained infrared spectra from 0.7 to 40 μm for three young brown dwarfs in the TW Hydra association (τ~10 Myr), 2MASSW J1207334-393254, 2MASSW J1139511-315921, and SSSPM J1102-3431. The spectral energy distribution for 2MASSW J1139511-315921 is consistent with a stellar photosphere for the entire wavelength range of our data, whereas the other two objects exhibit significant excess emission at λ>5 μm. We are able to reproduce the excess emission from each brown dwarf using our models of irradiated accretion disks. According to our model fits, both disks have experienced a high degree of dust settling. We also find that silicate emission at 10 and 20 μm is absent from the spectra of these disks, indicating that grains in the upper disk layers have grown to sizes larger than ~5 μm. Both of these characteristics are consistent with previous observations of decreasing silicate emission with lower stellar masses and older ages. These trends suggest that either (1) the growth of dust grains, and perhaps planetesimal formation, occurs faster in disks around brown dwarfs than in disks around stars or (2) the radii of the mid-IR-emitting regions of disks are smaller for brown dwarfs than for stars, and grains grow faster at smaller disk radii. Finally, we note the possible detection of an unexplained emission feature near 14 μm in the spectra of both of the disk-bearing brown dwarfs.
Based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory at the California Institute of Technology under NASA contract 1407.

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