Terzan 5: an alternative interpretation for the split horizontal branch

Abstract

We consider the horizontal branch (HB) of the Globular Cluster Terzan 5, recently shown to be split into two parts, the fainter one (delta M_K ~ 0.3mag) having a lower metallicity than the more luminous. Both features show that it contains at least two stellar populations. The separation in magnitude has been ascribed to an age difference of ~6 Gyr and interpreted as the result of an atypical evolutionary history for this cluster. We show that the observed HB morphology is also consistent with a model in which the bright HB is composed of second generation stars that are metal enriched and with a helium mass fraction larger (by delta Y ~ 0.07) than that of first generation stars populating the fainter part of the HB. Terzan 5 would therefore be anomalous, compared to most "normal" clusters hosting multiple populations, only because its second generation is strongly contaminated by supernova ejecta; the previously proposed prolonged period of star formation, however, is not required. The iron enrichment of the bright HB can be ascribed either to contamination from Type Ia supernova ejecta of the low-iron, helium rich, ejecta of the massive asympotic giant branch stars of the cluster, or to its mixing with gas, accreting on the cluster from the environment, that has been subject to fast metal enrichment due to its proximity with the galactic bulge. The model here proposed requires only a small age difference, of ~100Myr.

in press on The Astrophysical Journal Letters

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• The age of the young bulge-like population in the stellar system Terzan5: linking the Galactic bulge to the high-z Universe
• Formation of Multiple Populations in Globular Clusters: Another Possible Scenario
• Discovery of the Third Transient X-ray Binary in the Galactic Globular Cluster Terzan 5
• OGLE-III Detection of the Anomalous Galactic Bulge Red Giant Branch Bump: Evidence of Enhanced Helium Enrichment
• The Milky Way without X: An alternative interpretation of the double red clump in the Galactic bulge
• Separating the conjoined red clump in the Galactic Bulge: Kinematics and Abundances
• Nucleosynthesis in helium-enriched asymptotic giant branch models: Implications for Heavy Element Enrichment in Omega Centauri
• The Controversial Star-Formation History and Helium Enrichment of the Milky Way Bulge
• Sodium abundances of AGB and RGB stars in Galactic globular clusters II. Analysis and results of NGC 104, NGC 6121 and NGC 6809
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• Explaining the multiple populations in globular clusters by multiple episodes of star formation and enrichment without gas expulsion from massive star feedback
• Double Horizontal Branches in NGC 6440 and NGC 6569 unveiled by the VVV Survey
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• HERBS II: Detailed chemical compositions of Galactic bulge stars
• The near-IR counterpart of IGR J17480-2446 in Terzan 5
• A possible physical connection between helium-rich stellar populations of massive globular clusters and the UV upturn of galactic spheroids
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