Super-helium-rich populations and the origin of extreme horizontal-branch stars in globular clusters

Young Wook Lee, Seok Joo Joo, Sang I.L. Han, Chul Chung, Chang H. Ree, Young Jong Sohn, Yong Cheol Kim, Suk Jin Yoon, K. Y.I. Sukyoung, Pierre Demarque

Research output: Contribution to journalArticle

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Abstract

Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS stars well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (ΔY = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in ω Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super-helium-rich populations in some globular clusters suggests that a third parameter, other than metallicity and age, also influences the CMD morphology of these clusters.

Original languageEnglish
Pages (from-to)L57-L60
JournalAstrophysical Journal
Volume621
Issue number1 II
DOIs
Publication statusPublished - 2005 Mar 1

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horizontal branch stars
globular clusters
helium
color-magnitude diagram
main sequence stars
diagram
augmentation
minorities
metallicity
luminosity
stars

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Lee, Young Wook ; Joo, Seok Joo ; Han, Sang I.L. ; Chung, Chul ; Ree, Chang H. ; Sohn, Young Jong ; Kim, Yong Cheol ; Yoon, Suk Jin ; Sukyoung, K. Y.I. ; Demarque, Pierre. / Super-helium-rich populations and the origin of extreme horizontal-branch stars in globular clusters. In: Astrophysical Journal. 2005 ; Vol. 621, No. 1 II. pp. L57-L60.
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Super-helium-rich populations and the origin of extreme horizontal-branch stars in globular clusters. / Lee, Young Wook; Joo, Seok Joo; Han, Sang I.L.; Chung, Chul; Ree, Chang H.; Sohn, Young Jong; Kim, Yong Cheol; Yoon, Suk Jin; Sukyoung, K. Y.I.; Demarque, Pierre.

In: Astrophysical Journal, Vol. 621, No. 1 II, 01.03.2005, p. L57-L60.

Research output: Contribution to journalArticle

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AU - Sohn, Young Jong

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AU - Demarque, Pierre

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AB - Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS stars well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (ΔY = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in ω Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super-helium-rich populations in some globular clusters suggests that a third parameter, other than metallicity and age, also influences the CMD morphology of these clusters.

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