showing an increase in the Lyman-α optical
depth from z = 4.9 toward larger redshifts.
A particular feature is that the IGM appears
nearly opaque in a region around z = 5.77,
although measurable Lyman-β and Lyman-γ
flux show that the IGM is still significantly
ionized over this high-redshift interval. In
addition, at the redshift of the host galaxy,
Chornock et al. establish an upper limit on
the neutral fraction of the IGM of 0.11.
A number of absorption lines were used to
determine the host galaxy’s redshift. Some
of these lines are useful tracers of the galaxy’s metallicity, with the expected result of
low metallicity — about one-tenth of solar
values. Assuming these lines are optically
thin, it sets a lower limit; e.g., [Si/H] -1.7.
The non-detection of some ionized sulfur
lines sets an upper limit of [S/H] - 0.5.
The complete results are published in The
Astrophysical Journal, 774: 26, 2013.
First Refereed GeMS Results:
Young Stars Leave the Nest
The first refereed astronomy paper based on
data using the Gemini Multi-conjugate adaptive optics System (GeMS) demonstrates the
effective use of young, lower-mass stars to
determine the age of a star cluster. In this
case, the infrared sensitivity and resolution
of GeMS, together with the Gemini South
Adapative Optics Imager (GSAOI), enabled
measurements of stars in the low-mass cluster Haffner 16 in the Milky Way.
In particular, photometry of faint, pre-mainsequence stars is now possible. These become essential for determining the cluster’s
age accurately because the higher-mass
stars usually used are often absent in lowmass clusters. The GeMs/GSAOI data yield an
age 10 million years (Myr). In contrast, optical measurements results in an age about 2
Myr greater for this cluster.
October2013
One of the broader interests of lead author Tim Davidge (Dominion Astrophysical
Observatory, Canada) is the origin of the
field star population — stars that have “left
the nest” of the clusters where they likely
formed. Haffner 16 is an example of a cluster in the processes of dissolving, providing
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.
GeminiFocus
Figure 2.
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.
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