The Double Subgiant Branch of NGC 1851: The Role of the CNO Abundance

By Santi Cassisi, Maurizio Salaris, Adriano Pietrinferni, G. Piotto, A. Milone, L. Bedin, Joseph M. Anderson. 2007

136 citations recorded by the source corpus.

We explore the possibility that the anomalous split in the subgiant branch (SGB) of the Galactic globular cluster NGC 1851 is due to the presence of two distinct stellar populations with very different initial metal mixtures: a normal α-enhanced component, and one characterized by strong anticorrelations among the CNONa abundances, with a total CNO abundance increased by a factor of 2. We test this hypothesis taking into account various empirical constraints, and conclude that the two populations should be approximately coeval, with the same initial He content. More high-resolution spectroscopic measurements of heavy elements—and in particular of the CNO sum—for this cluster are necessary to prove (or disprove) this scenario.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/98dc5f749b5910056b27dd9b257982d181639ade

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In Astronomy and Astrophysics

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

Citations in the corpus

• Spectroscopic and photometric evidence of two stellar populations in the Galactic globular cluster NGC 6121 (M 4) (2008, background): “Cassisi et al. (2008) suggest that extreme CNO variations could be the basis of the SGB split found in NGC 1851.”
• The two metallicity groups of the globular cluster M 22: a chemical perspective (2011, background): “Cassisi et al. (2008) and Ventura et al. (2009) have suggested that a bulk CNO abundance difference can account for a similar SGB split in NGC1851 (Milone et al. 2008).”
• The second and third parameters of the horizontal branch in globular clusters (2010, methodology): “Variations in total CNO content can be most likely attributed to the contribution of thermally pulsing AGB stars (Cassisi et al. 2008), which have a rather low mass and probably do not contribute much to He abundance variations.”
• Photometric signatures of multiple stellar populations in galactic globular clusters (2011, methodology): “Cassisi et al. (2008) used these isochrones texplain the splitting of the subgiant branch (SGB) in NGC 1851 as a consequence of enhanced C+N+O abundances.”
• Formation and dynamical evolution of multiple stellar generations in globular clusters (2008, background): “…and it is present only 2 A different, borderline, situation seems to be present in NGC 1851, whose HR diagram shows a splitting of the subgiant branch (Milone et al. 2008), that can be interpreted as due to two different stellar populations differing in total CNO abundance (Cassisi et al. 2008).”
• A double stellar generation in the globular cluster NGC 6656 (M 22): Two stellar groups with different iron and s-process element abundances (2009, methodology): “The correct reproduction of the two SGBs requires an accurate determination of the NaCNO abundances, as shown by Cassisi et al. (2008) for the analogous case of NGC 1851.”
• 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.”
• THE HUBBLE SPACE TELESCOPE UV LEGACY SURVEY OF GALACTIC GLOBULAR CLUSTERS. I. OVERVIEW OF THE PROJECT AND DETECTION OF MULTIPLE STELLAR POPULATIONS (2014, methodology): “In particular the new models include variation of CNO content, which has a strong impact on age estimates (Cassisi et al. 2008, Ventura et al. 2009).”
• The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – IX. The Atlas of multiple stellar populations (2016, background): “This indicates that faint-SGBstars are either enhanced in their C+N+O overall abundance, or are older than stars on the bright SGB by∼ 1−2Gyr (Cassisi et al. 2008; Ventura et al. 2009; Marino et al. 2011).”
• Origin of multiple stellar populations in globular clusters and their helium enrichment (2008, background): “Alternatively, it has been proposed that the double SGB may be due to one of the two populations being enhanced in CNO elements by a factor of ∼2 relative to the other (Cassisi et al. 2008).”
• Multiple stellar populations in Magellanic Cloud clusters - I. An ordinary feature for intermediate age globulars in the LMC? (2008, background): “This feature can be explained either by two distinct bursts of star formation with a time separation of about 1 Gyr or by two stellar populations with distinct initial chemical composition and a much smaller age difference (Milone et al. 2008a; Cassisi et al. 2008).”
• Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta (2010, background): “Two interpretations are possible: either the stars in the lower subgiant branch are older by ∼ 1Gyr (Milone et al. 2008), or they are SG stars with a larger total C+N+O content (Cassisi et al. 2008).”
• MULTIPLE STELLAR POPULATIONS IN 47 Tucanae (2011, background): “Finally, theoretical models have proposed that the SGB split observed in some GCs could be due to two stellar groups with either an age difference of 1–2 Gyr or a different C+N+O content (Cassisi et al. 2008; Ventura et al. 2009).”
• THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED METHOD ALONG WITH COLOR–MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES (2013, background): “The reduced luminosity of the fainter SGB population is most easily explained if the stars in that sequence have higher C+N+O abundances than in the majority of the clusters stars (see Cassisi et al. 2008).”
• The fraction of second generation stars in globular clusters from the analysis of the horizontal branch (2008, background): “…periods are longer, so that the variables may belong to the SG. Recently, this latter cluster has been discovered to harbour a double subgiant branch (Milone et al. 2008), that can be interpreted as the presence of an FG and SG with different total CNO abundances and same age (Cassisi et al. 2008).”