HIGH-CONTRAST 3.8 μm IMAGING OF THE BROWN DWARF/PLANET-MASS COMPANION TO GJ 758

We present L'-band (3.8 mu m) MMT/Clio high-contrast imaging data for the nearby star GJ 758, which was recently reported by Thalmann et al. to have one-possibly two-faint comoving companions (GJ 758B and C, respectively). GJ 758B is detected in two distinct data sets. Additionally, we report a possible detection of the object identified by Thalmann et al. as GJ 758C in our more sensitive data set, though it is likely a residual speckle. However, if it is the same object as that reported by Thalmann et al. it cannot be a companion in a bound orbit. GJ 758B has an H - L' color redder than nearly all known L-T8 dwarfs. Based on comparisons with the COND evolutionary models, GJ 758B has T(e) similar to 560 K(-90K)(+150K) and a mass ranging from similar to 10-20 M(J) if it is similar to 1 Gyr old to similar to 25-40 M(J) if it is 8.7 Gyr old. GJ 758B is likely in a highly eccentric orbit, e similar to 0.73(-0.21)(+ 0.12), with a semimajor axis of similar to 44AU(-14AU)(+32AU). Though GJ 758B is sometimes discussed within the context of exoplanet direct imaging, its mass is likely greater than the deuterium-burning limit and its formation may resemble that of binary stars rather than that of Jovian-mass planets.