After years of patient orbital monitoring, there is now a large sample of very low-mass stars and brown dwarfs with precise (~5%) dynamical masses. These binaries represent the gold standard for testing substellar theoretical models. Work to date has identified problems with the model-predicted broad-band colors, effective temperatures, and possibly even luminosity evolution with age. However, our ability to test models is currently limited by how well the individual components of these highly prized binaries are characterized. To solve this problem, we propose to use NICMOS and STIS to characterize this first large sample of ultracool binaries with well-determined dynamical masses. We will use NICMOS multi-band photometry to measure the SEDs of the binary components and thereby precisely estimate their spectral types and effective temperatures. We will use STIS to obtain resolved spectroscopy of the Li I doublet at 6708 A for a subset of three binaries whose masses lie very near the theoretical mass limit for lithium burning. The STIS data will provide the first ever resolved lithium measurements for brown dwarfs of known mass, enabling a direct probe of substellar interiors. Our proposed HST observations to characterize the components of these binaries is much less daunting in comparison to the years of orbital monitoring needed to yield dynamical masses, but these HST data are equally vital for robust tests of theory.

STIS medium-resolution spectroscopy of 3 brown dwarf binaries. The highest resolution grating (G750M) will be used to measure lithium absorption at 6708 A. Two of the targets are relatively bright late-M dwarf binaries (3 orbits each), and one is a fainter L dwarf binary (4 orbits). Two of our targets are near (2.6" and 5") very bright stars, so their coordinates are specified as precise offsets from these stars. Rather than specify orientation constraints to align the binaries along the slit, we have specified time constraints excluding any dates when it would not be possible to do so. This is because the binaries' position angles are changing significantly over Cycle 18. We will need to specify the ORIENT angle once our observations are actually scheduled. COOL STARS 12 false 0.0 0.0 false false false false SUPPORTED STAR BROWN DWARF J2000 00 27 55.9300 +22 19 32.80 J2000 SDSS i = 14.5 mag STAR BROWN DWARF Offset measured from Keck AO images on 2010 May 22 UT: 4.9703+-0.0026" separation at PA = 37.07+-0.02 deg. This target is a companion to the offset star, so it shares the offset star's proper motion and parallax. 14 54 29.2300 +16 06 4.00 J2000 SDSS i = 14.6 mag STAR BROWN DWARF Offset measured from Keck AO images on 2010 Mar 22 UT: 2.631+-0.005" separation at PA = 102.91+-0.05 deg. This target is a companion to the offset star, so it shares the offset star's proper motion and parallax. 14 50 15.8100 +23 54 42.60 J2000 SDSS i = 18.3 mag STAR G V-IV J2000 00 00 0.0000 +00 00 0.00 J2000 STAR M V-IV J2000 00 00 0.0000 +00 00 0.00 J2000 STIS/CCD Spectroscopic No No G750M 6768 STIS/CCD Spectroscopic No No G750M 6768 STIS/CCD Spectroscopic No No G750M 6768 POS-TARG DITHER 0.6 LINE false false 2 90.0 STIS-ALONG-SLIT 1 false The scheduling data was generated by CASM 19.0. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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