TY - JOUR
T1 - A Census of Above-horizontal-branch Stars in Galactic Globular Clusters
AU - Davis, Brian D.
AU - Bond, Howard E.
AU - Siegel, Michael H.
AU - Ciardullo, Robin
N1 - Funding Information:
B.D.D. thanks the Zaccheus Daniel Trust for a grant that helped make this publication possible. He also thanks Eric Feigelson for guidance on statistical methods involving Gaussian-mixture models.
Funding Information:
H.E.B.'s uBVI observations, and M.H.S.'s photometric reductions, were partially supported by NASA grant NAG 5-6821 under the “UV, Visible, and Gravitational Astrophysics Research and Analysis” program, and by the Director's Discretionary Research Fund at the Space Telescope Science Institute (STScI). H.E.B. also thanks the staffs at Cerro Tololo and Kitt Peak for their support over many years.
Funding Information:
This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
Funding Information:
Based in part on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA), and the Canadian Astronomy Data Centre (CADC/NRC/CSA). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.
Funding Information:
Visiting astronomer, Cerro Tololo Inter-American Observatory and Kitt Peak National Observatory, National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation.
Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.
PY - 2022/2/1
Y1 - 2022/2/1
N2 - 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.
AB - 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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U2 - 10.3847/1538-4357/ac4224
DO - 10.3847/1538-4357/ac4224
M3 - Article
AN - SCOPUS:85125752813
SN - 0004-637X
VL - 926
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 99
ER -