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 2) 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., F. 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. Occultations 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
October2013
pressure values that prevented detectability in the latest observations.
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 (unusually) the science instrument. Photometric
measurements were recorded several times
per second.
One of the challenges
of these observations is
that the catalogued positions of many of the
stars are not sufficiently precise to predict an
occultation with certainty. The team uses
observations from the
Canada-France-Hawaii
Telescope (CFHT) Legacy Survey to make
the predictions of upcoming occultations, and then they only
observe events that have the highest probability of being successful. Despite these
efforts, to date no occultation event of the
large