The Na-O anticorrelation in horizontal branch stars

III. 47 Tucanae and M 5 ^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: raffaele.gratton@oapd.inaf.it
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁴ Department of Physics & Astronomy, Macquarie University, Balaclava Rd., North Ryde, Sydney, NSW 2109, Australia
⁵ Monash Centre for Astrophysics, School of Mathematical Sciences, Building 28, Monash University, VIC 3800, Australia
⁶ INAF – Osservatorio Astronomico di Teramo, Via Collurania, Teramo, Italy
⁷ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, UK

Received: 9 July 2012
Accepted: 4 October 2012

Abstract

To check the impact of the multiple population scenario for globular clusters on their horizontal branch (HB), we present an analysis of the composition of 110 red HB (RHB) stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M 5. In 47 Tuc we found tight relations between the colours of the stars and their abundances of p-capture elements. This strongly supports the idea that the He content – which is expected to be closely correlated with the abundances of p-capture elements – is the third parameter (after overall metallicity and age) that determines the colour of HB stars. However, the range in He abundance must be small (ΔY < 0.03) in 47 Tuc to reproduce our observations; this agrees with previous analyses. There is possibly a correlation between the abundances of p- and n-capture elements in 47 Tuc. If confirmed, this might suggest that asymptotic giant branch stars of moderate mass contributed to the gas from which second-generation stars formed. Considering the selection effects in our sample (which does not include stars warmer than 11 000 K and RR Lyrae variables, which were excluded because we could not obtain accurate abundances with the adopted observing procedure) is important to understand our results for M 5. In this case, we find that, as expected, RHB stars are Na-poor and O-rich, and likely belong to the primordial population. There is a clear correlation of the [Na/O] ratio and N abundance with colour along the BHB. A derivation of the He abundance for these stars yields a low value of Y = 0.22 ± 0.03. This is expected because HB stars of a putative He-rich population in this cluster should be warmer than 11 000 K, and would accordingly not have been sampled by our analysis. However, we need some additional source of scatter in the total mass loss of stars climbing up the red giant branch to reproduce our results for M 5. Finally, we found a C-star on the HB of 47 Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M 5. These stars are among the brightest and coolest HB stars.