GeminiFocus October 2013 | Page 12

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. 12