2010ApJ...721L.177C

We present L'-band (3.8 µm) 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∼ 560 K{^{{+150  K}_{-90 K}}} and a mass ranging from ∼10-20 M_J if it is ∼1 Gyr old to ∼ 25-40 M_Jif it is 8.7 Gyr old. GJ 758B is likely in a highly eccentric orbit, e ∼ 0.73{^{{+0.12}_{-0.21}}}, with a semimajor axis of ∼44 AU{^{{+32  AU}_{-14  AU}}}. 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.