Metallicities on the Double Main Sequence of ω Centauri Imply Large Helium Enhancement

By G. Piotto, S. Villanova, L. Bedin, R. Gratton, Santi Cassisi, Y. Momany, A. Recio-Blanco, S. Lucatello, Jay Anderson, Ivan R. King and 2 more. 2004

390 citations recorded by the source corpus.

Having shown in a recent paper that the main sequence of ω Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is less metal-poor. We have carefully combined VLT's GIRAFFE spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate signal-to-noise ratio, to show clearly that the stars of the blue main sequence are less metal-poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main-sequence stars, whereas the red main-sequence stars show a 0.2 dex spread in metallicity. We use stellar structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/0d276abd282d677b80b35bdcbde47b9aea12cdbe

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In Astronomy and Astrophysics

1. Jay Anderson
2. R. Gratton
3. F. D’Antona
4. A. Bragaglia
5. E. Carretta
6. G. Piotto
7. A. Renzini
8. A. Milone

In Instrumentation

1. F. D’Antona
2. R. Gratton
3. A. Bragaglia
4. E. Carretta
5. Jay Anderson
6. G. Piotto
7. A. Milone
8. L. Bedin

Citations in the corpus

• The Hubble Space TelescopeUV Legacy Survey of Galactic Globular Clusters – V. Constraints on formation scenarios (2015, background): “This scenario faces an insurmountable difficulty with the mass budget constraint: for example, the major 2G population inω Cen has a mass of∼ 106 M⊙ and has been enriched in helium by∆Y ∼ 0.1 (Piotto et al. 2005), for which ∼ 105 M⊙ of fresh helium is required.”
• The Double Subgiant Branch of NGC 1851: The Role of the CNO Abundance (2007, methodology): “Recent accurate photometries of GCs obtained with the Advanced Camera for Surveys (ACS) on board the HST have given direct photometric evidence of multiple populations with varying He content in q Centauri (Anderson 1997; Bedin et al. 2004; Piotto et al. 2005).”
• The second and third parameters of the horizontal branch in globular clusters (2010, background): “…e.g., in NGC 2808 (D’Antona et al. 2005); (iii) Very accurate photometric data detect multiple main sequences in some GCs (Bedin et al. 2004; Piotto et al. 2007) that can only be explained by assuming large variations in the He content (Norris et al. 2004; Piotto et al. 2005; Milone et al. 2010).”
• Na-O anticorrelation and HB - VIII. Proton-capture elements and metallicities in 17 globular clusters from UVES spectra (2009, background): “Furthermore, it is the onlycluster where multiple main sequences of quite different He content (Piotto et al. 2007) have been identified so far (beside the peculiar case ofωCen, with its multi-peaked metallicity distribution, see Piotto et al. 2005).”
• Intrinsic iron spread and a new metallicity scale for globular clusters (2009, background): “Ramirez & Cohen 2002; Carretta et al. 2004; Piotto et al. 2005) and photometric (e.g. Bedin et al. 2004) observations convincingly showed that the observed pattern of chemical composition is present also among unevolved stars on the subgiant branch and the main sequence.”
• Massive AGB models of low metallicity: the implications for the self-enrichment scenario in metal-poor globular clusters (2009, background): “The presence of a fraction of stars enriched in helium has been inferred also by the presence of a blue main sequence in ωCen (Bedin et al. 2004; Norris 2004; Piotto et al. 2005), and in NGC 2808 (Piotto et al. 2007).”
• THE HUBBLE SPACE TELESCOPE UV LEGACY SURVEY OF GALACTIC GLOBULAR CLUSTERS. I. OVERVIEW OF THE PROJECT AND DETECTION OF MULTIPLE STELLAR POPULATIONS (2014, background): “In some cases the He enhancement can be quite strong, close to Y=0.40 (e.g., D’Antona and Caloi 2004, Norris 2004, Piotto et al. 2005, 2007, King et al. 2012, Bellini et al. 2013a), while in others it is still measurableut small (e.g.,∼0.01 as in the cases of NGC 6397, Milone et al. 2012b, and NGC…”
• A Triple Main Sequence in the Globular Cluster NGC 2808 (2007, background): “The more shocking discovery, however, came from a follow-up spectroscopic analysis that showed that the blue MS has twice the metal abundance of the dominant red branch of the MS (Piotto et al. 2005, P05).”
• Photometric signatures of multiple stellar populations in galactic globular clusters (2011, background): “The only explanation found so far (Norris 2004; Piotto et al. 2005) is that the bluer sequence has a higher helium mass fractionY ≈ 0.38, compared to a normal value of 0.25.”
• Young Massive Star Clusters (2010, background): “The discovery of multiple populations of main-sequence stars and giants in an increasing number of globular clusters (Piotto et al. 2005, Piotto 2008) has led to the realization that star clusters are not idealized entities with single well defined stellar populations.”
• Origin of the abundance patterns in Galactic globular clusters: constraints on dynamical and chemical properties of globular clusters (2007, background): “It should be noted that some observational features in globular clusters such as the multiple sequences in ω Cen (Bedin et al. 2004) and NGC 2808 (Piotto et al. 2007) are explained by an He enrichment of the low-mass stars up to He 0.4 (Norris 2004; Piotto et al. 2005; D’Antona et al. 2005).”
• The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, background): “This huge increase in He is one of the reasons why intermediate-mass AGB stars have been implicated in the origin of the multiple populations observed in Galactic GCs (D’Antona et al. 2002; Norris 2004; Piotto et al. 2005).”
• Spectroscopic and photometric evidence of two stellar populations in the Galactic globular cluster NGC 6121 (M 4) (2008, background): “In a sense, the findings by Pancino et al. (2000), Bedin et al. (2004), Piotto et al. (2005), Villanova et al. (2007) and Sollima et al. (2007) could simply be considered additional evidence that ω Cen is so peculiar that it might not be a GC.”
• The fraction of second generation stars in globular clusters from the analysis of the horizontal branch (2008, background): “This came after the first discovery of a peculiar blue main sequence in ω Cen (Bedin et al. 2004), interpreted again in terms of a very high helium content (Norris 2004; Piotto et al. 2005).”
• Accreted versus in situ Milky Way globular clusters (2010, background): “EHBs in GCs appear to be due to the presence of a heliumenhanced population of stars and hence are a strong indicator for the presence of multiple stellar populations (Norris 2004; Piotto et al. 2005).”
• The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation (2007, result): “Follow-up spectroscopic studies at medium resolution led to even more enigmatic results (Piotto et al. 2005, hereafter P05): contrary to any expectation from canonical stellar models, the bluer branch of the MS is more metal-rich than the red one.”
• Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta (2010, background): “The HB of M4 shows signs of bimodality (Norris 1981) but a careful representation of the data is very tricky, as it requires use of models including also the CNO abundance enhancements of the SG (see the discussion by Salaris et al. 2008, concerning the bimodal distribution in the HB of NGC 1851).”
• Asteroseismology of old open clusters with Kepler: direct estimate of the integrated red giant branch mass-loss in NGC 6791 and 6819 (2011, methodology): “…the HB morphology (D’Antona et al. 2002; D’Antona & Caloi 2004) was confirmed by the presence of multiple main sequences in some globular clusters (Piotto et al. 2005, 2007) In order to describe the mass loss along the RGB, researchers rely on empirical laws like that of Reimers (1975a,b),…”