Super-Helium-rich Populations and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters

By Young-Wook Lee, S. Joo, Sangil Han, C. Chung, C. Ree, Y. Sohn, Yong-Cheol Kim, Suk-Jin Yoon, S. Yi, P. Demarque. 2005

180 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/db9ced402f5c7d46b3d175a9cb9495281588a4de

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In Astronomy and Astrophysics

1. R. Gratton
2. A. Bragaglia
3. E. Carretta
4. F. D’Antona
5. G. Piotto
6. L. Bedin
7. A. Renzini
8. Santi Cassisi

Citations in the corpus

• The fraction of second generation stars in globular clusters from the analysis of the horizontal branch (2008, methodology): “While a gap at Teff∼ 10 4K should probably be attributed to the operation of diffusion (Glaspey et al. 1989; Grundahl et al. 1999; Caloi 1999), other gaps may have to do with discontinuities in the helium content (D’Antona et al. 2005; Lee et al. 2005).”
• Origin of multiple stellar populations in globular clusters and their helium enrichment (2008, background): “For these stars we have no direct hint on the helium abundance, but values as high as Y ∼ 0.40 have been proposed (Lee et al. 2005; Sollima et al. 2005).”
• Formation and dynamical evolution of multiple stellar generations in globular clusters (2008, background): “…MS photometry, mainly because the moderately helium rich isochrones merge with the stan- dard isochrones in the turnoff region (D’Antona et al. 2002; Salaris et al. 2006), but also because, in general, these SG stars do not have an unique value of Y (e.g. Lee et al. 2005; Caloi & D’Antona 2007).”
• Photometric signatures of multiple stellar populations in galactic globular clusters (2011, background): “…and definitely confirmed on the basis of an accurate WFC3 photometry by Bellini et al. (2010).ω Cen is also known for hosting at least four subgiant (Lee et al. 2005; Sollima et al. 2005; Villanova et al. 2007; Bellini et al. 2010) and red giant branches (see Johnson et al. 2009, for a recent…”
• The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation (2007, background): “At the moment, the only explanation of the photometric and spectroscopic properties of the double MS that is at all plausible is that the bluer branch of the MS has an unusually high helium content (B04; Norris 2004; P05; Lee et al. 2005).”
• Scaled solar tracks and isochrones in a large region of the Z-Y plane I. From the ZAMS to the TP-A (2008, background): “In NGC 2808 a helium enhanced population could explain the MS spread and the HB morphology ( Lee et al. 2005, D’Antona et al. 2005).”
• CHEMICAL ABUNDANCES FOR 855 GIANTS IN THE GLOBULAR CLUSTER OMEGA CENTAURI (NGC 5139) (2010, background): “Lee et al. (2005a) found that if the intermediate metallicity and most metal–rich stars are in fact He–rich then these stars will populate the “extreme” horizontal branch.”
• On the self-enrichment scenario of galactic globular clusters: constraints on the IMF (2006, methodology): “Claims of supporting evidence for the presence of helium-rich populations in GCs were recently obtained through the detailed analysis of various color-magnitude diagram features (e.g., Lee et al. 2005 and references therein).”
• Multiple populations in globular clusters (2012, background): “Finally, He-rich HB stars are expected to be bluer: the colours of HB stars have been examined by many authors (see e.g. D’Antona et al. 2005; Lee et al. 2005; Gratton et al. 2010, see also next Section).”
• Massive AGB models of low metallicity: the implications for the self-enrichment scenario in metal-poor globular clusters (2009, methodology): “This idea has been elaborated in many subsequent works (e.g. D’Antona & Caloi 2004, Lee et al. 2005, Caloi & D’Antona 2005, 2007, Busso et al. 2007, D’Antona & Caloi 2008).”
• The Origin of the Double Main Sequence in ω Centauri: Helium Enrichment due to Gas Fueling from Its Ancient Host Galaxy? (2005, unknown): “A number of recent investigations have suggested that the above scenario (3) (hereafter referred to as the “helium pollution scenario” and abbreviated to “HEPS” for convenience) is the most promising among the four (e.g., Lee et al. 2005; Norris 2004; Piotto et al. 2005).”
• The second and third parameters of the horizontal branch in globular clusters (2010, background): “We note however that these small spreads in He abundance (which are usually justified on the basis of chemical evolution models) have difficulties in reproducing the observed spread in masses along the HB and the splitting of the MS of NGC 2808 (Lee et al. 2005, Piotto et al. 2007).”