Phase 2 to be reviewed.555646860----- Attempting Submission 3 (Mon Jul 20 01:40:37 GMT 2015) -----Dynamical Masses for Free-Floating Planetary-Mass BinariesWe propose a 3-year orbit monitoring program to measure the first dynamical masses in the planetary-mass regime (5-15 Mjup) for free-floating objects, including the first known Y dwarf binary. With projected separations of only 0.7-1.4 AU, our targets are among the tightest substellar visual binaries ever found and are amenable to orbit determinations within only a few years. When combined with our parallax determination, these data will yield dynamical masses with <10% uncertainties. Two of our targets are late-T/Y dwarf field binaries (ages ~ 1-5 Gyr) and will provide the first empirical calibration in the poorly understood temperature regime of ~350-500 K, relevant both to free-floating objects and radial-velocity exoplanets. Such extreme conditions are the frontier of current theory, e.g. model uncertainties in non-equilibrium chemistry, metallicity, and clouds yield mass estimates that currently span an order of magnitude. Our third target is a young field L dwarf (age ~ 10-100 Myr), one of the rare field objects that that serve as analogs for young directly imaged exoplanets. Our dynamical mass combined with evolutionary models will yield the first precise age and temperature estimates for such an object, thereby testing our ability to derive physical parameters from current models. Our targets have been discovered at the limits of existing facilities and thus promise to be the only viable objects in the planetary-mass regime for direct mass measurements until next-generation facilities like JWST come online.The goal of our observations is to obtain precise relative astrometry (~3--5 mas) for three free-floating, planetary-mass binaries. Our data spanning 3 cycles will allow us to determine their orbits and thereby measure their dynamical masses. ACS/WFC enables a finer pixel scale than WFC3/IR and better sensitivity than WFC3/UVIS, both of which (high SNR, and better deblending of our tight <~0.1" binaries) are key to achieving our science goals.
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