GeminiFocus July 2014 | Page 8

Figure 3. The spectrum of GU Psc b, obtained at Gemini North with GNIRS, along with the GMOS z (~ 0.9 mm) and WISE W1 (~ 3.4 mm) and W2 (4.6 mm) photometric points. Also shown are the best-fitting, lowtemperature cloud models of Morley et al., 2012, and the BT-Settl models of Allard et al., 2012. The spectrum also revealed a few clues hinting that the companion itself might indeed be a binary object. First, it is quite similar to the spectrum of J1021-0304, a known brown dwarf binary (T1/T5). Second, the spectrum shows an over-luminosity around 1.6 microns in the H band — a region usually already over-luminous, as modelers like Saumon and others have previously noticed. These tentative indications encouraged us to obtain, through collaborators Christopher Gelino and Charles Beichman (both Caltech NExScI), high-resolution H and K observations with the Laser Guide Star Adaptive Optics system and the near-infrared camera NIRC2 at Keck II. These reveal only one point source, which constrains strongly the eventual binarity of GU Psc b; namely, we can exclude the presence of a companion brighter than a typical T7-T8 down to about 2 AU. To evaluate GU Psc b’s physical properties, we used two evolutionary models: one by I. Baraffe et. al., 2003; and the other by Saumon and M. Marley, 2008. We determined its luminosity using observed SED and atmo- 6 GeminiFocus spheric models. The results suggest a mass between 9 and 13 Jupiters at the age of ABDMG (70-130 million years; Myr). Many Questions, Many Answers, Some Mysteries The very large distance between this planetary-mass companion and its host star raises many questions. For instance, how could such a massive companion end up so far from GU Psc A after ~100 Myr? It certainly seems unlikely that GU Psc b formed “as a planet,” — i.e. in the protoplanetary disk surrounding the star, through either core accretion or disk instability. Could this world be in the process of being ejected? Maybe, but we know of other supposedly gravitationally bounded systems that have similar binding energies (ratio of the masses of star and companion over the distance between the two). Most likely, GU Psc A and its companion formed in the fragmentation of a collapsing molecular cloud, similar to the way a binary July 2014