Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs

Astrophysics, abstract
astro-ph/0304226

From: Adam Burrows <burrows@zenith.as.arizona.edu>

Date: Fri, 11 Apr 2003 21:20:00 GMT   (227kb)

Authors:
Adam Burrows,
David Sudarsky,
Jonathan Lunine

Comments: Includes 14 figures, most in color; submitted to the Astrophysical
Journal

We explore the spectral and atmospheric properties of brown dwarfs cooler
than the latest known T dwarfs. Our focus is on the yet-to-be-discovered
free-floating brown dwarfs in the eff range from $sim$800 K to $sim$130 K
and with masses from 25 to 1 mj. This study is in anticipation of the new
characterization capabilities enabled by the launch of SIRTF and the eventual
launch of JWST. We provide spectra from $sim$0.4 mic to 30 mic, highlight
the evolution and mass dependence of the dominant H$_2$O, CH$_4$, and NH$_3$
molecular bands, consider the formation and effects of water-ice clouds, and
compare our theoretical flux densities with the sensitivities of the
instruments on board SIRTF and JWST. The latter can be used to determine the
detection ranges from space of cool brown dwarfs. In the process, we determine
the reversal point of the blueward trend in the near-infrared colors with
decreasing eff, the effs at which water and ammonia clouds appear, the
strengths of gas-phase ammonia and methane bands, the masses and ages of the
objects for which the neutral alkali metal lines are muted, and the increasing
role as eff decreases of the mid-infrared fluxes longward of 4 mic. These
changes suggest physical reasons to expect the emergence of at least one new
stellar class beyond the T dwarfs. Our spectral models populate, with cooler
brown dwarfs having progressively more planet-like features, the theoretical
gap between the known T dwarfs and the known giant planets. Such objects likely
inhabit the galaxy, but their numbers are as yet unknown.

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