An IRS Study of the Coolest T Dwarfs and the L-T Transition

Details An IRS Study of the Coolest T Dwarfs and the L-T Transition An IRS Study of the Coolest T Dwarfs and the L-T Transition

Sep 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004sptz.prop.3431l&link_type=abstract

Spitzer Proposal ID #3431

Computer Science

Scientific paper

Recent 1-5 micron studies of ultracool L and T dwarfs indicate that changes in the near-infrared spectra across the L-T transition are governed by complex cloud dynamics rather than by large changes in effective temperature. These studies also suggest that nonequilibrium chemistry in late-T dwarfs significantly affects the photospheric abundances of CO, CH4, and NH3, all of which influence the spectral energy distributions from 1 to 30 micron. A focused study of L-T transition and late-T dwarfs longward of 5 micron is crucial to understanding the agents that govern the dynamic and chemical processes in the atmospheres of these brown dwarfs. We request 38.8 hours of IRS time in Cycle 1 to study 7 late-L and early-T dwarfs and 7 late-T dwarfs. The size of our L-T transition sample matches that of the GTO "irs_lt-stars" study, but our sample includes late-L dwarfs with extremely blue and extremely red near-infrared colours. Our sample of late-T dwarfs is similar in size to that of the GTO mid- to late-T program, but our sample includes a possible low-gravity T dwarf with red near-infrared colours as well as the coolest known T dwarf with an effective temperature ~700 K. By combining the IRS data with our existing 0.8-2.5 micron spectra and 0.9-4.7 micron photometry, we will investigate the effects of grain condensation, nonequilibrium chemistry, and gravity on the atmospheres of these brown dwarfs. Our comprehensive data will allow us to constrain models of the transition from dusty red L dwarfs to clear blue T dwarfs, determine the prevalence and importance of nonequilibrium chemistry, and examine gravity (and hence mass) signatures in the spectra of these brown dwarfs.

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