Abstract We have carried out a search for above-horizontal-branch (AHB) stars—objects lying above the horizontal branch (HB) and blueward of the asymptotic giant branch (AGB) in the color–magnitude diagram—in 97 Galactic and seven Magellanic Cloud globular clusters (GCs). We selected AHB candidates based on photometry in the uBVI system, which is optimized for detection of low-surface-gravity stars with large Balmer jumps, in the color range −0.05 ≤ ( B − V ) 0 ≤1.0. We then used Gaia astrometry and Gaussian-mixture modeling to confirm cluster membership and remove field interlopers. Our final catalog contains 438 AHB stars, classified and interpreted in the context of post-HB evolution as follows: (1) AHB1: 280 stars fainter than M V = −0.8, evolving redward from the blue HB (BHB) toward the base of the AGB. (2) Post-AGB (PAGB): 13 stars brighter than M V ≃ −2.75, departing from the top of the AGB and evolving rapidly blueward. (3) AHB2: 145 stars, with absolute magnitudes between those of the AHB1 and PAGB groups. This last category includes a mixture of objects leaving the extreme BHB and evolving toward the AGB, and brighter ones moving back from the AGB toward higher temperatures. Among the AHB1 stars are 59 RR Lyrae interlopers, observed by chance in our survey near maximum light. PAGB and AHB2 stars (including W Virginis Cepheids) overwhelmingly belong to GCs containing BHB stars, in accordance with predictions of post-HB evolutionary tracks. We suggest that most W Vir variables are evolving toward lower temperatures and are in their first crossings of the instability strip. Nonvariable yellow PAGB stars show promise as a Population II standard candle for distance measurement.
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Yellow Post-asymptotic-giant-branch Stars as Standard Candles. I. Calibration of the Luminosity Function in Galactic Globular Clusters
Abstract We use the results of a survey for low-surface-gravity stars in Galactic (and LMC) globular clusters to show that “yellow” post-asymptotic-branch (yPAGB) stars are likely to be excellent extragalactic standard candles, capable of producing distances to early-type galaxies that are accurate to a couple of percent. We show that the mean bolometric magnitude of the 10 yPAGB stars in globular clusters is 〈 M bol 〉 = −3.38 ± 0.03, a value that is ∼0.2 mag brighter than that predicted from the latest post-horizontal-branch evolutionary tracks. More importantly, we show that the observed dispersion in the distribution is only 0.10 mag, i.e., better than the scatter for individual Cepheids. We describe the physics that can produce such a small dispersion and show that, if one restricts surveys to the color range 0.0 ≲ ( B − V ) 0 ≲ 0.5, then samples of nonvariable yPAGB stars can be identified quite easily with a minimum of contamination. The extremely bright absolute V magnitudes of these stars (〈 M V 〉 = −3.37) make them, by far, the visually brightest objects in old stellar populations and ideal Population II standard candles for measurements out to ∼10 Mpc with current instrumentation. A Hubble Space Telescope survey in the halos of galaxies in the M81 and Sculptor groups could therefore serve as an effective cross-check on both the Cepheid and tip-of-the-red-giant-branch distance scales.
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- Award ID(s):
- 2206090
- NSF-PAR ID:
- 10428505
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 930
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 145
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac674d
