A Luminous Yellow Post-Agb Star in the Galactic Globular Cluster M79*

Abstract

We report the discovery of a luminous F-type post-asymptotic-giant-branch (PAGB) star in the Galactic globular cluster (GC) M79 (NGC1904). At visual apparent and absolute magnitudes of V 12.20 = and M 3.46 V = -, this "yellow" PAGB star is by a small margin the visually brightest star known in any GC. It was identified using CCD observations in the uBVI photometric system, which is optimized to detect stars with large Balmer discontinuities, indicative of very low surface gravities. Follow-up observations with the SMARTS 1.3 and 1.5 m telescopes show that the star is not variable in light or radial velocity, and that its velocity is consistent with cluster membership. Near-and mid-infrared observations with 2MASS and WISE show no evidence for circumstellar dust. We argue that a sharp upper limit to the luminosity function exists for yellow PAGB stars in old populations, making them excellent candidates for PopulationII standard candles, which are four magnitudes brighter than RRLyrae variables. Their luminosities are consistent with the stars being in a PAGB evolutionary phase, with core masses of M 0.53  . We also detected four very hot stars lying above the horizontal branch ("AGB-manqué" stars); along with the PAGB star, they are the brightest objects in M79 in the near-ultraviolet. In the Appendix, we give periods and light curves for five variables in M79: three RRLyrae stars, a TypeII Cepheid, and a semiregular variable.

Yellow Post-asymptotic-giant-branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters

“…The process continues until the envelope mass is reduced to ∼0.005 M e , at which point the star begins its blueward traversal of the CMD and becomes a post-AGB star. Note that, as the figure shows, M79 contains two bright members that lie on post-AGB tracks, as discussed by Bond et al (2016) and D22.…”

“…Amplifying this effect is a related systematic associated with circumstellar extinction. Low-mass PAGB stars, such as BD +39°4926 (Gezer et al 2015), BD+14°3071 (Bond 2020), and the yPAGB star in M79 (Bond et al 2016), have been shown to have little to no circumstellar material. This is likely because their evolutionary timescale across the HR diagram is slow enough to allow dust produced on the AGB to disperse into space.…”

Yellow Post-Asymptotic-Giant-Branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters

“…The process continues until the envelope mass is reduced to ∼ 0.005 M , at which point the star begins its blueward traversal of the CMD and becomes a post-AGB star. Note that, as the figure shows, M79 contains two bright members that lie on post-AGB tracks, as discussed by Bond et al (2016) and D22.…”

“…Amplifying this effect is a related systematic associated with circumstellar extinction. Low-mass PAGB stars, such as BD+39 • 4926 (Gezer et al 2015), BD+14 • 3071 (Bond 2020), and the yPAGB star in M79 (Bond et al 2016) have been shown to have little to no circumstellar material. This is likely because their evolutionary timescale across the HR diagram is slow enough to allow dust produced on the AGB to disperse into space.…”

BD+14°3061: A Luminous Yellow Post-AGB Star in the Galactic Halo

“…However, because of their short evolutionary timescales, these stars are extremely rare. Only about a half-dozen are known in the Galactic GC system (e.g., Bond et al 2016, here-after B16, and references therein; Bond et al 2020 in preparation).…”