Abhijith Rajan (School of Earth and Space
Exploration, Arizona State University), led
the international team that observed 51 Eri
b using GPI spectroscopy (Figure 1) as part
of the Gemini Planet Imager Exoplanet Sur-
vey (GPIES), combined with mid-infrared
photometry at the W.M. Keck Observatory.
These data were used to determine that
the planet — a young, cool object between
2–10 Jupiter masses — is redder than brown
dwarfs seen elsewhere. The enhanced red-
dening may be the result of clouds forming
as the planet transitions from a partially- to
partly-cloudy atmosphere, with lower mean
surface temperatures. If true, 51 Eri b ap-
pears to be one of the only directly imaged
planets that is consistent with the cold-start
scenario, resulting in a low temperature, low
luminosity planet.
The full results have been accepted for pub-
lication in The Astronomical Journal. A pre-
print is available here.
Gemini South Joins HST in Joint
Proper Motion Study
Tobias Fritz (University of Virginia) and col-
leagues used the wide-field Gemini Multi-
conjugate adaptive optics System (GeMS)
at Gemini South, combined with the Gemini
South Adaptive Optics Imager (GSAOI), to
study the proper motion of stars in the Ga-
lactic halo globular cluster known as Pyxis.
These data, together with those from the
Hubble Space Telescope, allowed the team
to set a lower limit for the Milky Way’s mass
of 950 million Suns. This value is consistent
with most, but not all, previous determina-
tions.
GeMS/GSAOI was crucial to the study, be-
cause traditional ground-based telescopes
are seeing limited and need a time baseline
of more than 15 years for the types of mea-
surements required in this survey. On the
other hand, GeMS/GSAOI has better spatial
resolution and can complete the project in
five years — about the same time required
for HST. Using GeMS/GSAOI, the team mea-
sured absolute proper motions of Pyxis to a
resolution of 0.08 arcsecond (Figure 2), and
combined these data with those from ar-
chival HST images, with a resolution of ~ 0.1
arcsecond.
Lying at a distance of some 130,000 light
years, Pyxis is one of the most distant ex-
amples of a globular cluster. It is also about
2 billion years younger than other globular
clusters with the same metallicity. Together,
these characteristics imply that Pyxis was
likely formed in a massive dwarf galaxy
that the Milky Way then cannibalized. Thus,
Pyxis may have an extragalactic origin. One
mystery, however, is that the orbits of other
known massive dwarf galaxies are inconsis-
tent with the orbit of Pyxis, which is derived
from the new proper motion measurements.
The research is part
of a much larger ef-
fort now underway
to study the proper
motion of several
substructures across
the Milky Way’s halo.
It is also part of a
Large and Long pro-
gram at Gemini that
July 2017
GeminiFocus
Figure 2.
Left: GMOS-South image
of the Pyxis field, with
the center of the cluster
marked with a red star.
Right: A zoom of the
Pyxis area with GeMS-
GSAOI. The field-of-
view of GMOS is 5 x 5
arcminutes; GeMS is 85 x
85 arcseconds.
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