KS = 20.5 is the “main sequence knee”,
which may be useful to determine
cluster’s age, independent of distance
and reddening estimates.
The delivered image quality is near
Gemini’s diffraction limit, with an
average measured full-width at halfmaximum (FHWM) of 0.09”. Thus, the
team is able to distinguish individual
stars in the very crowded field, which
would overlap and contaminate each
other in seeing-limited observations
at lower spatial resolution (Figure 3).
The Deepest Ground-based
Photometry in a
Crowded Field
Turri and his collaborators have similar GeMS/GSAOI observations of additional globular clusters, which will
yield more general conclusions; these
will help to guide observing and data
analysis strategies for the next generation of extremely large telescopes.
The current work on NGC 1851 is published
in The Astrophysical Journal Letters, and an
introduction is available on the Gemini website (view here).
Paolo Turri (University of Victoria, Canada)
and colleagues have used the Gemini Multiconjugate adaptive optics System (GeMS)
with the Gemini South Adaptive Optics Imager (GSAOI) to produce the most accurate and
deepest near-infrared photometry from the
ground of a crowded field. Their Ks measurements of the Galactic globular cluster NGC
1851 reach the precision and depth of optical observations obtained using the Hubble
Space Telescope, and the resulting combined
color-magnitude diagram reveals physical
characteristics of the cluster (Figure 2).
Water Vapor in a Terrestrial
Planet Region
Specifically, the researchers detect the double
subgiant branch in the cluster’s center, which
indicates either multiple episodes of star formation or multiple populations having distinct metal composition, rather than a single
uniform population of stars. Turri et al. measure the main sequence well below its turnoff,
for 3.5 magnitudes. A feature observed around
Astronomers have detected significant water
vapor in the planet-forming region around
the young star DoAr 44. The central star and
development of its planets have not yet fully
cleared the surroundings, leaving an inner
ring and an outer disk around a gap that extends radially for 36 astronomical units (AU).
DoAr 44 is unusual compared to similar so-
October 2015
GeminiFocus
Figure 2.
This color-magnitude
diagram of globular
cluster NGC 1851’s
crowded center
combines near-infrared
observations obtained
using GeMS/GSAOI
at Gemini South and
optical data from the
Hubble Space Telescope.
Red dots mark the main
sequence turnoff and
the main sequence knee,
around Ks = 18 and 20.5
magnitudes, respectively.
Figure 3.
This small extract from
the full 83-arcsecond
field-of-view illustrates
the quality and depth of
the images from Gemini.
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