The Bayesian analysis revealed GU Psc A, an
M3 star, to have a high probability of belonging to the 70-130 million-year-old AB Doradus Moving Group (ABDMG). The radial velocity estimated by the analysis agrees with
the value we measured on our optical and
near-infrared (NIR) spectra, which further
confirm the membership. These radial-velocity data were obtained with the Phoenix
spectrometer at Gemini South, CRIRES at the
VLT, and ESPaDOnS at CFHT.
The star’s X-ray luminosity is also consistent
with that of ABDMG members. Furthermore, the many youth indicators we studied
all agreed with GU Psc A being part of the
ABDMG. A parallax measurement, ongoing
through the Cerro Tololo Inter-American
Observatory Parallax Investigation project,
will allow reinforcing this membership even
further. Meanwhile, as mentioned, we have
adopted a distance of 48 ± 5 pc, for the star,
as estimated by the Bayesian analysis.
GU Psc b: A Peculiar System?
As expected for a cool, planetary-mass object, GU Psc b was detected in the GMOS z
observations. It is so faint in the optical, however, that the planet remained undetected in
i band in a deep follow-up observation with
GMOS. These results suggest a very red, i - z
> 3.5, a color that is typical to a young planetary-mass object (and also ultracool field
brown dwarfs and high-redshift quasars).
GU Psc b was the only such object detected
among the 90 stars surveyed with GMOS.
Follow-up observations were obtained with
NIR cameras at Observatoire du Mont-Mégantic (CPAPIR) and at CFHT (WIRCam). Two
sets of WIRCam J-band observations, spaced
one year apart, allowed us to measure the
proper motion of GU Psc b and to show that
the suspected planet was indeed co-moving
with GU Psc A. The Wide-field Infrared Survey
Explorer (WISE) also observed GU Psc’s field
January 2015
in the mid-infrared. All these photometric
observations confirmed we had a planetarymass companion; they were also of great use
to calibrate the NIR spectrum, subsequently
obtained at Gemini North using the Gemini
Near-Infrared Spectrograph (GNIRS).
A comparison of the GNIRS spectrum both
with standard objects and two models — a
“Low-temperature cloud” atmosphere model provided by Caroline Morley and Didier
Saumon (University of California Santa Cruz
and Los Alamos National Laboratory), and a
“BT-Settl” model by France Allard and Derek
Homeier (Centre de Recherche Astrophysique de Lyon) — yielded precious information, including the spectral type (T3.5 1)
±
and a temperature between 1000-1100 K.
In addition, the comparison showed indicators of low surface gravity (mainly the very
strong K band), which is, compatible with
the star’s young age.
2014 Year in Review
GeminiFocus
Figure 2.
Composite image of
GU Psc b and its host
star. The i (blue) and
z (green) discovery
observations from
GMOS on Gemini South
are shown with the
follow-up J-band (red)
image taken at CFHT.
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