High-contrast 3.8μm imaging of the brown dwarf/planet-mass companion to GJ 758

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