GeminiFocus April 2019 - Page 14

Figure 5. Results of fitting the GeMS near-infrared CMD of HP 1 using the Dartmouth Stellar Evolutionary Database (DSED) models. Left panel: CMD showing all likely member stars (grey) and those used in the fit (black). The best-fit isochrone is indicated by a thick green line; the green shading shows the uncertainty range. The red arrow indicates a change in reddening of ΔE(B − V) = 0.10 mag. Right panels: The result- ing one- and two-dimen- sional constraints for all model parameters. The contours correspond to confidence levels of 0.5σ, 1.0σ, 1.5σ, and 2.0σ. [Figure reproduced from Kerber et al., Monthly Notices of the Royal Astronomical Society, 484: 5530, 2019.] 12 low the MSTO. The study was led by Leandro Kerber of the Universidade de São Paulo and Universidade Estadual de Santa Cruz in Brazil. The team combined their GSAOI data with archival F606W (wide V) images from the HST’s Advanced Camera for Surveys to de- termine relative proper motions and select bona fide cluster members. They then fitted two different sets of model isochrones to the color-magnitude diagrams (CMDs) to deter- mine the stellar population parameters, dis- tance, and reddening. Figure 5 shows the re- sults for one set of isochrones using only the GeMS/GSAOI data; the team also performed fits to CMDs made with a combination of HST and GeMS data. The analysis indicates an age near 13 billion years, confirming that HP 1 is one of the oldest globular clusters in the Milky Way and likely formed less than a billion years after the Big Bang. The heliocentric distance of 6.6 kiloparsecs (kpc) estimated from the isochrone fitting agrees well with the distance implied by the extinction-corrected brightnesses of 11 RR Lyrae stars identified within the cluster. The team combined this distance with the GeminiFocus measured radial velocity and the absolute proper motion given by Gaia (Data Release 2) in order to constrain the cluster’s orbit. They find that HP 1 passes just 0.12 kpc from the Galactic Center at closest approach and reaches a maximum distance of about 3 kpc. It is likely that many of the cluster’s stars have been stripped away as it has re- peatedly plunged through the bulge during the course of its long history. “HP 1 is one of the surviving members of the fundamental building blocks that as- sembled our Galaxy’s inner bulge,” said Kerber. Added coauthor Mattia Libralato of the Space Telescope Science Institute, “The combination of high angular resolution and near-infrared sensitivity makes GeMS/GS- AOI an extremely powerful tool for study- ing these compact, dust-enshrouded stellar clusters.” The study appears in Monthly No- tices of the Royal Astronomical Society. John Blakeslee is the Chief Scientist at Gemini Observatory and located at Gemini South in Chile. He can be reached at: [email protected] April 2019