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-
16
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
2014 Year in Review
January 2015