The horizontal-branch stars in globular clusters. II. The second parameter phenomenon

Young-Wook Lee, Pierre Demarque, Robert Zinn

Research output: Contribution to journalArticle

318 Citations (Scopus)

Abstract

Using synthetic horizontal-branch models, we have investigated the origin of the systematic variation in horizontal-branch (HB) morphology with galactocentric distance (RG) among globular clusters. The variations in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed properties of the RR Lyrae variables or the observed main-sequence turnoffs in the clusters. There is also no clear evidence that the trend with RG is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age required to explain this effect are not in conflict with any observations. A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar [Fe/H] reveals reasonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertainty is the need for ad hoc hypotheses in the modeling of the HB morphologies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as ∼5 Gyr) among the halo globular dusters. Our results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and we estimate this difference to be ∼2 Gyr.

Original languageEnglish
Pages (from-to)248-265
Number of pages18
JournalAstrophysical Journal
Volume423
Issue number1
DOIs
Publication statusPublished - 1994 Mar 1

Fingerprint

horizontal branch stars
globular clusters
halos
parameter
trends
modeling
estimates
effect

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Lee, Young-Wook ; Demarque, Pierre ; Zinn, Robert. / The horizontal-branch stars in globular clusters. II. The second parameter phenomenon. In: Astrophysical Journal. 1994 ; Vol. 423, No. 1. pp. 248-265.
@article{fb89836724e74c2b84c27579fa1bad0b,
title = "The horizontal-branch stars in globular clusters. II. The second parameter phenomenon",
abstract = "Using synthetic horizontal-branch models, we have investigated the origin of the systematic variation in horizontal-branch (HB) morphology with galactocentric distance (RG) among globular clusters. The variations in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed properties of the RR Lyrae variables or the observed main-sequence turnoffs in the clusters. There is also no clear evidence that the trend with RG is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age required to explain this effect are not in conflict with any observations. A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar [Fe/H] reveals reasonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertainty is the need for ad hoc hypotheses in the modeling of the HB morphologies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as ∼5 Gyr) among the halo globular dusters. Our results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and we estimate this difference to be ∼2 Gyr.",
author = "Young-Wook Lee and Pierre Demarque and Robert Zinn",
year = "1994",
month = "3",
day = "1",
doi = "10.1086/173803",
language = "English",
volume = "423",
pages = "248--265",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "IOP Publishing Ltd.",
number = "1",

}

The horizontal-branch stars in globular clusters. II. The second parameter phenomenon. / Lee, Young-Wook; Demarque, Pierre; Zinn, Robert.

In: Astrophysical Journal, Vol. 423, No. 1, 01.03.1994, p. 248-265.

Research output: Contribution to journalArticle

TY - JOUR

T1 - The horizontal-branch stars in globular clusters. II. The second parameter phenomenon

AU - Lee, Young-Wook

AU - Demarque, Pierre

AU - Zinn, Robert

PY - 1994/3/1

Y1 - 1994/3/1

N2 - Using synthetic horizontal-branch models, we have investigated the origin of the systematic variation in horizontal-branch (HB) morphology with galactocentric distance (RG) among globular clusters. The variations in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed properties of the RR Lyrae variables or the observed main-sequence turnoffs in the clusters. There is also no clear evidence that the trend with RG is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age required to explain this effect are not in conflict with any observations. A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar [Fe/H] reveals reasonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertainty is the need for ad hoc hypotheses in the modeling of the HB morphologies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as ∼5 Gyr) among the halo globular dusters. Our results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and we estimate this difference to be ∼2 Gyr.

AB - Using synthetic horizontal-branch models, we have investigated the origin of the systematic variation in horizontal-branch (HB) morphology with galactocentric distance (RG) among globular clusters. The variations in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed properties of the RR Lyrae variables or the observed main-sequence turnoffs in the clusters. There is also no clear evidence that the trend with RG is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age required to explain this effect are not in conflict with any observations. A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar [Fe/H] reveals reasonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertainty is the need for ad hoc hypotheses in the modeling of the HB morphologies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as ∼5 Gyr) among the halo globular dusters. Our results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and we estimate this difference to be ∼2 Gyr.

UR - http://www.scopus.com/inward/record.url?scp=12044253069&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=12044253069&partnerID=8YFLogxK

U2 - 10.1086/173803

DO - 10.1086/173803

M3 - Article

AN - SCOPUS:12044253069

VL - 423

SP - 248

EP - 265

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1

ER -