In this paper, we report a new estimate of the absolute proper motion (PM) of the globular cluster NGC 5139 (omega Cen) as part of the HST large program GO-14118+ 14662. We analyzed a field 17 arcmin ...southwest of the center of omega Cen and computed PMs with epoch spans of similar to 15.1 years. We employed 45 background galaxies to link our relative PMs to an absolute reference-frame system. The absolute PM of the cluster in our field is (mu(alpha) cos delta, mu(delta))=(-3.341. 0.028, -6.557 +/- 0.043) mas yr(-1). Upon correction for the effects of viewing perspective and the known cluster rotation, this implies that for the cluster center of mass (mu(alpha) cos delta, mu(delta))=(-3.238. 0.028, -6.716 +/- 0.043) mas yr(-1). This measurement is direct and independent, has the highest random and systematic accuracy to date, and will provide an external verification for the upcoming Gaia Data Release 2. It also differs from most reported PMs for omega Cen in the literature by more than 5 sigma, but consistency checks compared to other recent catalogs yield excellent agreement. We computed the corresponding Galactocentric velocity, calculated the implied orbit of omega Cen in two different Galactic potentials, and compared these orbits to the orbits implied by one of the PM measurements available in the literature. We find a larger (by about 500 pc) perigalactic distance for omega Cen with our new PM measurement, suggesting a larger survival expectancy for the cluster in the Galaxy.
Metal-poor globular clusters (GCs) exhibit intriguing Al-Mg anti-correlations and possible Si-Al correlations, which are important clues to decipher the multiple-population phenomenon. NGC 5053 is ...one of the most metal-poor GCs in the nearby Universe, and has been suggested to be associated with the Sagittarius (Sgr) dwarf galaxy, due to its similarity in location and radial velocity with one of the Sgr arms. In this work, we simulate the orbit of NGC 5053, and argue against a physical connection between Sgr and NGC 5053. On the other hand, the Mg, Al, and Si spectral lines, which are difficult to detect in the optical spectra of NGC 5053 stars, have been detected in the near-infrared APOGEE spectra. We use three different sets of stellar parameters and codes to derive the Mg, Al, and Si abundances. Regardless of which method is adopted, we see a large Al variation, and a substantial Si spread. Along with NGC 5053, metal-poor GCs exhibit different Mg, Al, and Si variations. Moreover, NGC 5053 has the lowest cluster mass among the GCs that have been identified to exhibit an observable Si spread until now.
