16220

We wish to use 5 orbits of HST-WFC3 time to observe an eclipse spectrum of the only known transiting brown dwarf that has all of its physical properties independently measured at high precision comma and that receives a negligible amount of external irradiation. Brown dwarfs evolve in both radius and temperature as they age comma changing their atmospheric spectral emission with time. Understanding this evolution is important because it informs us about their interior structure comma which ultimately helps constrain where the dividing line is between brown dwarfs that form through d_commastar-liked_comma Jeans-collapse or d_commaplanet-liked_comma accretion. However comma a fundamental problem in brown dwarf evolution and atmosphere models has been our inability to directly measure their basic physical properties comma and no one has yet observed the atmosphere of a non-irradiated brown dwarf where we know all of them. The discovery of our target system finally allows us to conduct just this sort of observation using HST-WFC3 -- in conjuntion with archival Spitzer eclipse observations at 3.6um and 4.5um. Measuring the emission spectrum will allow us to perform a set of tests on brown dwarf evolution and atmospheres models with an an over-constrained physical system with no major free parameters. The results will provide a one-of-a-kind observational anchor for brown dwarfs.