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Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters

Rich, R. Michael and Sosin, Craig and Djorgovski, S. George and Piotto, Giampaolo and King, Ivan R. and Renzini, Alvio and Phinney, E. Sterl and Dorman, Ben and Liebert, James and Meylan, Georges (1997) Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters. Astrophysical Journal, 484 (1). L25-L28. ISSN 0004-637X. http://resolver.caltech.edu/CaltechAUTHORS:20190205-142100455

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Abstract

We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441. All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity. However, NGC 6388 and 6441 also exhibit a prominent blue horizontal-branch (BHB) extension, including stars reaching as faint in V as the turnoff luminosity. This discovery demonstrates directly for the first time that a major population of hot horizontal-branch (HB) stars can exist in old, metal-rich systems. This may have important implications for the interpretation of the integrated spectra of elliptical galaxies. The cause of the phenomenon remains uncertain. We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB. The high central densities in NGC 6388 and 6441 suggest that the existence of the BHB tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy. Tidal collisions might act in various ways to enhance loss of envelope mass and therefore populate the blue side of the HB. However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology. While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue against dynamical effects playing a role.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1086/310758DOIArticle
https://arxiv.org/abs/astro-ph/9705039arXivDiscussion Paper
ORCID:
AuthorORCID
Rich, R. Michael0000-0003-0427-8387
Djorgovski, S. George0000-0002-0603-3087
Additional Information:© 1997. The American Astronomical Society. Received 1997 January 13; accepted 1997 May 5. This work was supported in part by NASA grant GO-6095 from STScI (R. M. R., S. G. D., I. R. K., C. S.), NASA grant NAG5-2756 (E. S. P.), NASA grants NAG5-700 and NAGW-4106 (B. D.), the Bressler Foundation (S. G. D.), and Agenzia Spaziale Italiana (G. P., A. R.). Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
Group:TAPIR
Funders:
Funding AgencyGrant Number
NASAGO-6095
NASANAG5-2756
NASANAG5-700
NASANAGW-4106
Bressler FoundationUNSPECIFIED
Agenzia Spaziale Italiana (ASI)UNSPECIFIED
NASANAS 5-26555
Subject Keywords:Galaxy: kinematics and dynamics; globular clusters: individual (NGC 104 47 Tucanae NGC 5927 NGC 6388 NGC 6441) ; stars: horizontal-branch
Record Number:CaltechAUTHORS:20190205-142100455
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20190205-142100455
Official Citation:R. Michael Rich et al 1997 ApJ 484 L25
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:92691
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:05 Feb 2019 23:00
Last Modified:05 Feb 2019 23:00

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