evidence of the transition of stars from a
cluster to the field. In particular, the authors
found that the sub-solar mass population is
deficient in Haffner 16, which they suggest
results from the cluster’s dynamic evolution, during which it lost protostars of subsolar masses.
Haffner 16 contains a large population of
pre-main-sequence stars that are still accreting material, demonstrated by their line
emission. This is unexpected given Haffner 16’s
age — usually the accretion phase ends after
only a few Myr. This extended period of mass
buildup may eventually result in somewhat
overly massive stars for their position on the
main sequence. To explain the observations,
the authors suggest that the supernovae and
strong stellar winds of massive stars that normally disrupt accretion are absent, allowing the
process to continue unabated.
For astronomers interested in all subjects,
these observations most importantly demonstrate the utility of the GeMS AO system
even in the relatively poor seeing conditions
under which these data were obtained. The
delivered image quality here (Figure 6) provides full-width at half-maximum in the Ks
band of < 0.16 arcsecond. This represents
a significant improvement over the natural
seeing, which, on the night these data were
obtained, was roughly 0.8 arcsecond — a
value worse than average at Gemini South
on Cerro Pachón.
The paper appears in The Publications of the
Astronomical Society of the Pacific. Davidge, T.
J., et al., “Haffner 16: A Young Moving Group
in the Making.” eprint arXiv:1308.5432.
Limits on Quaoar’s Atmosphere
The Kuiper Belt Object Quaoar (pronounced
Kwa-whar), located well beyond the orbit of
Pluto, can be studied through occultations
as it passes along the line of sight through
the crowed plane of the Milky Way. Occul-
tations are an effective probe because astronomers know the speeds of Solar System
bodies very precisely from their orbits, so
the duration when starlight is blocked provides a direct measurement of the size of the
occulting object. In addition, an occultation
can uncover information about the nearby
body’s atmosphere, if it exists. A rocky body
without an atmosphere will immediately
extinguish the starlight, while one with an
atmosphere will create a “fuzzy” event with
a slow dimming and eventual blocking of
the starlight.
Recent “near-misses” of Quaoar occultations provide some constraints on a possible atmosphere, as Wesley Fraser (National
Research Council Herzberg, Canada) and
collaborators rule out some pure N2 and
CO models. They find that a methane atmosphere is possible, with temperature and
pressure values that prevented detectability in the latest observations.
Figure 6.
This image of Haffner
16 illustrates that the
GeMS AO system can
successfully sharpen data
even under relatively
poor imaging conditions.
With the correction, the
point sources appeared
spread by less than 0.16
arcsecond (full-width at
half-maximum, in the Ks
band). This represents a
significant improvement
over the natural quality of
the sky, which, on the night
these data were obtained,
was roughly 0.8 arcsecond
— a value worse than
average at Gemini South
on Cerro Pachón.
The background stars are relatively faint,
and rapid photometry is required, so the acquisition camera on Gemini, normally used
to adjust the telescope pointing, became
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