Spectroscopic Confirmation of Two Luminous Post-AGB Stars in the
  Globular Cluster M19
  The visually brightest stars in globular clusters (GCs) are the ones evolving
off the asymptotic giant branch (AGB) and passing through spectral types F and
A--the "yellow" post-AGB (yPAGB) stars. yPAGB stars are potentially excellent
"Population II" standard candles for measuring extragalactic distances. A
recent survey of the Galactic GC system, using uBVI photometry to identify
stars of low surface gravities with large Balmer discontinuities, discovered a
candidate luminous yPAGB star in the GC M19 (NGC 6273), designated ZNG 4. The
same survey also identified a bright, hotter candidate blue PAGB star, ZNG 2,
lying near the top of the white-dwarf cooling sequence. Both PAGB candidates
have proper motions and parallaxes in the recent Gaia Early Data Release 3
consistent with cluster membership, but they still lacked spectroscopic
verification. Here we present moderate-resolution spectra of both stars,
confirming them as low-gravity objects that are extremely likely to be cluster
members. Through comparison with a library of synthetic spectra, we made
approximate estimates of the stars' atmospheric parameters. We find that the
yPAGB star ZNG 4 has an effective temperature of Teff ~ 6500 K, a surface
gravity of log g ~ 1, and a metallicity of [Fe/H] ~ -1.5, similar to that of
the host cluster. The blue PAGB star ZNG 2 has Teff ~ 18000 K, log g ~ 3, and
an apparently low metallicity in the range of [Fe/H] ~ -2.0 to -2.5. Both stars
are bright (V=12.5 and 13.3, respectively). We urge high-dispersion
spectroscopic follow-up to determine detailed atmospheric parameters and
chemical compositions, and to monitor radial velocities.
