Two Luminous Post-AGB Stars in the Galactic Globular Cluster M19. (arXiv:2101.04657v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bond_H/0/1/0/all/0/1">Howard E. Bond</a> (1,2), <a href="http://arxiv.org/find/astro-ph/1/au:+Davis_B/0/1/0/all/0/1">Brian D. Davis</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Siegel_M/0/1/0/all/0/1">Michael H. Siegel</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Ciardullo_R/0/1/0/all/0/1">Robin Ciardullo</a> (1,3) ((1) Pennsylvania State University, (2) Space Telescope Science Institute, (3) Institute for Gravitation & the Cosmos, Penn State)
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
$M_V=-3.39pm0.09$ 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.
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
$M_V=-3.39pm0.09$ 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.
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