Two Luminous Post-AGB Stars in the Galactic Globular Cluster M19

Abstract

We report the discovery of a luminous "yellow" post-asymptotic-giant-branch (PAGB) star in the globular cluster (GC) M19 (NGC 6273), identified during our uBVI survey of Galactic GCs. The uBVI photometric system is optimized to detect stars with large Balmer discontinuities, indicating very low surface gravities and high luminosities. The spectral-energy distribution (SED) of the star is consistent with an effective temperature of about 6250 K and a surface gravity of $\log g=0.5$. We use Gaia data to show that the star's proper motion and radial velocity are consistent with cluster membership. One aim of our program is to test yellow PAGB stars as candidate Population II standard candles for determining extragalactic distances. We derive a visual absolute magnitude of $M_V=-3.39\pm 0.09$ for the M19 star. This is in close agreement with the $M_V$ values found for yellow PAGB stars in the GCs omega Cen, NGC 5986, and M79, indicating a very narrow luminosity function. These objects are four magnitudes brighter than RR Lyrae variables, and they can largely avoid the issues of interstellar extinction that are a problem for Population I distance indicators. We also identified a second luminous PAGB object in M19, this one a hotter "UV-bright" star. Its SED is consistent with an effective temperature of about 11,750 K and $\log g=2.0$. The two objects have nearly identical bolometric luminosities, $\log L/L_{\odot }=3.24$ and 3.22, respectively.

arXiv commentAccepted by Astronomical Journal

References in corpus (25)

• Gaia Data Release 2. Summary of the contents and survey properties
• The Gaia mission
• The Wavelength Dependence of Interstellar Extinction from 1.25 to 8.0 microns Using GLIMPSE Data
• Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way
• A catalogue of masses, structural parameters and velocity dispersion profiles of 112 Milky Way globular clusters
• New Grids of ATLAS9 Model Atmospheres
• Mean proper motions, space orbits and velocity dispersion profiles of Galactic globular clusters derived from Gaia DR2 data
• Variable Stars in Galactic Globular Clusters
• Kraken reveals itself -- the merger history of the Milky Way reconstructed with the E-MOSAICS simulations
• Near-IR properties of 24 Globular Clusters in the Galactic Bulge
• Uncloaking globular clusters in the inner Galaxy
• The Extragalactic Distance Scale
• HST Observations of Galactic Globular Cluster Cores. II. NGC 6273 and the Problem of Horizontal-Branch Gaps
• The Accreted Nuclear Clusters of the Milky Way
• A Homogenous Set of Globular Cluster Relative Distances and Reddenings
• A Chemical Composition Survey of the Iron-Complex Globular Cluster NGC 6273 (M 19)
• Modelling depletion by re-accretion of gas from a dusty disc in post-AGB stars
• The Swift UVOT Stars Survey: I. Methods and Test Clusters
• CCD Photometry of the Globlular Cluster NGC 5986 and its Post-Asymptotic-Giant-Branch and RR Lyrae Stars
• Hot UV-bright stars of galactic globular clusters
• High Resolution Optical Spectroscopy of a Newly Discovered Post-AGB Star with a Surprising Metallicity in the Globular Cluster M79
• The uBVI Photometric System. I. Motivation, Implementation, and Calibration
• The uBVI Photometric System. II. Standard Stars
• A Luminous Yellow Post-AGB Star in the Galactic Globular Cluster M79
• BD+14${}^{\circ }$3061: A Luminous Yellow Post-AGB Star in the Galactic Halo

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• A Census of Above-Horizontal-Branch Stars in Galactic Globular Clusters
• Hubble Space Telescope imaging of the extremely metal-poor globular cluster EXT8 in Messier 31
• Astrometric Membership Tests for the Zinn-Newell-Gibson "UV-Bright" Stars in Galactic Globular Clusters
• Zwicky Transient Facility and Globular Clusters: Calibration of the gr-Band Absolute Magnitudes for the Yellow Post-Asymptotic-Giant-Branch Stars
• Yellow Post-Asymptotic-Giant-Branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters