Two luminous post-AGB stars in the galactic globular cluster M19

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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 MV = - 3.39 ± 0.09 for the M19 star. This is in close agreement with the MV values found for yellow PAGB stars in the GCs ω Cen, NGC 5986, and M79, indicating a very narrow luminosity function. These objects are 4 mag 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 LOdot;= 3.24 and 3.22, respectively.

Original languageEnglish (US)
Article number9pp
JournalAstronomical Journal
Volume161
Issue number3
DOIs
StatePublished - Mar 1 2021

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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