The Origin of the Double Main Sequence in ω Centauri: Helium Enrichment due to Gas Fueling from Its Ancient Host Galaxy?

By Kenji Bekki, J. Norris. 2005

136 citations recorded by the source corpus.

Recent observational studies of ω Centauri by the Hubble Space Telescope have discovered a double main sequence in the color-magnitude diagrams (CMDs) of its stellar populations. These observations suggest that the stellar population with the blue main sequence (bMS) has a helium abundance much larger, by ΔY ~ 0.12, than that of the red main sequence (rMS). By using somewhat idealized models in which stars of the bMS are formed from gas ejected from stars of the rMS, we quantitatively investigate whether the helium overabundance of the bMS can result from self-enrichment from massive AGB stars, from mass loss of very massive young stars, or from Type II supernovae within ω Cen. We show that as long as the helium enrichment is due to ejecta from the rMS formed earlier than the bMS, none of these three enrichment scenarios can explain the observed properties of the bMS self-consistently for reasonable IMFs. The common, serious problem in all cases is that the observed number fraction of the bMS cannot be explained without assuming unusually top-heavy IMFs. This failure of the self-enrichment scenarios implies that most of the helium-enriched gas necessary for the formation of the bMS originated from other external sources. We thus suggest a new scenario, in which most of the second generation of stars (i.e., the bMS) in ω Cen could have formed from gas ejected from field stellar populations that surrounded ω Cen when it was the nucleus of an ancient dwarf galaxy.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/2913edf45a8fc6d0a2d2560153b65ce7202e6a36

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In Astronomy and Astrophysics

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

In Instrumentation

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

Citations in the corpus

• Evidence for two early accretion events that built the Milky Way stellar halo (2019, background): “This necessitates the existence of different channels for enrichment to account for the chemical peculiarities of ω Centauri (e.g., Bekki & Norris 2006).”
• Quantifying the coexistence of massive black holes and dense nuclear star clusters (2009, background): “The multiple stellar populations in ω Cen (e.g., Stanford et al. 2007, and references therein), and its possible origin as the nucleus of a stripped dwarf galaxy (Bekki & Norris 2006; Georgiev et al. 2009) ) as the 1σ uncertainties on the IMBH mass are consistent with a value of zero.”
• CHEMICAL ABUNDANCES FOR 855 GIANTS IN THE GLOBULAR CLUSTER OMEGA CENTAURI (NGC 5139) (2010, background): “…is that ω Cen, and perhaps several other globular clusters containing multiple populations, are the remnant cores of tidally disrupted dwarf galaxies (e.g., Dinescu et al. 1999; Majewski et al. 2000; Smith et al. 2000; Gnedin et al. 2002; McWilliam & Smecker–Hane 2005; Bekki & Norris 2006).”
• The fraction of second generation stars in globular clusters from the analysis of the horizontal branch (2008, background): “There are two possible ways of producing this result: the first possibility is that all these GCs formed within a dwarf galaxy environment (Bekki & Norris 2006; Bekki et al. 2007).”
• The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851 (2007, background): “…self-enrichment (Cotrell &DaCosta1981; Ventura et al. 2001; Ventura&D’Antona 2005; Maeder &Meynet 2006) or mergers might generate multiple populations in clusters, but such predictions remained controversial (see, e.g., Fenner et al. 2004; Bekki & Norris 2006) and had little observational basis.”
• Multiple populations in globular clusters (2012, background): “(2007): the globular clusters must have been initially a large factor (10-100) more massive than today and almost all the first generation stars must have been preferentially lost.”
• MULTIPLE STELLAR POPULATIONS IN 47 Tucanae (2011, result): “This is consistent with a similar suggestion for ω Cen by Bekki & Norris (2006).”
• Formation and dynamical evolution of multiple stellar generations in globular clusters (2008, background): “…in the envelopes of massive AGBs (D’Antona & Caloi 2004; 1 The age difference between FG and SG is in any case so small with respect to the total GC age that it is irrelevant for the turnoff evolution. c© 2008 RAS, MNRAS 000, 1–21 Bekki & Norris 2006), is too scarce to form a large SG population.”
• Origin of multiple stellar populations in globular clusters and their helium enrichment (2008, background): “We encounter here the main difficulty of the AGB scenario, as already pointed out by several authors (e.g. Bekki & Norris 2006; Karakas et al. 2006; Karakas & Lattanzio 2007), that is requiring a first generation of stars much more massive than that still harboured by the cluster.”