The helium spread in the globular cluster 47 Tuc

Spectroscopy has shown the presence of CN band dichotomy and Na-O anticorrelations for 50-70 per cent of the investigated samples in the cluster 47 Tuc, otherwise considered a `normal' prototype of high-metallicity clusters from its photometric analysis. These anomalies are also found in main-sequence stars, suggesting that a consistent second generation is present in 47 Tuc. Very recently, the reanalysis of a large number of archival Hubble Space Telescope (HST) data of the cluster core has been able to provide evidence of the presence of structures in the subgiant branch: it has a brighter component with a spread in magnitude of ~0.06mag and a second one, consisting of about 10 per cent of the stars, a little fainter (by ~0.05mag). These data also show that the main sequence of the cluster has an intrinsic spread in colour, which can be interpreted as due to a small spread in helium abundance. In this work we examine in detail whether the horizontal-branch morphology and subgiant structure provide further independent indications that a real - although very small - helium spread is present in the cluster. We reanalyse the HST archival data for the horizontal branch of 47 Tuc, obtaining a sample of ~500 stars with very small photometric errors, and build a population synthesis based on new models to show that its particular morphology can be better explained by taking into account a spread in helium abundance of 2 per cent in mass. The same variation in helium is able to explain the spread in luminosity of the subgiant branch, while a small part of the second generation is characterized by a small C+N+O increase and provides an explanation for the fainter subgiant branch. We conclude that three photometric features concur to form the paradigm that a small but real helium spread is present in a cluster that has no spectacular evidence for multiple populations like those shown by other massive clusters. This work thus shows that multiple populations in globular clusters are increasingly confirmed to be ubiquitous.