Bridging the Brown Dwarf/Jupiter Temperature Gap with a Very Cold Brown Dwarf
Residing at the extremes of low mass, luminosity and temperature, brown dwarfs serve as laboratories for understanding gas-giant extrasolar planets. Still, until a few months ago, the coolest brown dwarf known was ~4 times warmer than Jupiter. We have now identified the nearby T9.5 dwarf CFBDSIR J1458+10 as a 0.11" physical binary. As established by our near-IR parallax to the system, the very blue secondary component is the coldest and least luminous object outside the solar system directly imaged. With an estimated temperature of ~350-400 K, it is the coolest known brown dwarf by ~150 K and the least luminous by a factor of 4-5. As such, CFBDSIR J1458+10B provides a gateway for measuring the properties of substellar objects at previously unexplored extremes.
We propose to use HST to obtain far-red and near-IR medium-band photometry of CFBDSIR J1458+10B and to measure its 0.8-1.6 micron spectral energy distribution. Theoretical models predict this wavelength range to be highly sensitive to completely new physical processes not yet seen in brown dwarfs, including the formation of photospheric water clouds and the disappearance of the very broad potassium line that dominates the far-red spectra of T dwarfs. The impact of these changes on the emergent spectrum, however, depends on very uncertain input physics. Our observations will sensitively probe these processes, with the A and B components providing a pure temperature probe at constant metallicity and age.
This program will obtain photometry in 12 narrow to wide bandpasses spanning the optical to NIR for the ~400 K object CFBDS 1458+10B using ACS/WFC and WFC3/UVIS+IR (14 orbits). Observations of 2 other ultracold T dwarfs in the same bandpasses will prove the PSF reference (2 orbits each). Observations are grouped into visits in a logical fashion (i.e., by target and instrument), though some blocks of orbits had to be broken up into multiple visits in order to avoid exceeding the 5-contiguous-orbit limit. We intentionally split the ACS F814W and F850LP orbits into 2 separate visits to occur at the beginning and end of Cycle 19 in order to measure the absolute orbital motion of the binary, and therby its mass ratio.
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