Lithium and oxygen in globular cluster dwarfs and the early disc accretion scenario

¹ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
e-mail: M.Salaris@ljmu.ac.uk
² INAF − Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
e-mail: cassisi@oa-teramo.inaf.it

Received: 27 February 2014
Accepted: 22 April 2014

Abstract

A new scenario – early disc accretion – has recently been proposed to explain the discovery of multiple stellar populations in Galactic globular clusters. According to this model, the existence of well defined (anti-)correlations amongst light element abundances (i.e. C, N, O, Na) in the photospheres of stars belonging to the same cluster (and the associated helium enrichment) is caused by accretion of the ejecta of short-lived interacting massive binary systems (and single fast rotating massive stars) on fully convective pre-main sequence low- and very low-mass stars, during the early stages of the cluster evolution. In a previous paper we applied this scenario to the cluster NGC 2808, showing how the knowledge of the He abundance of its triple main sequence can constrain both the He abundance of the accreted matter and the accretion efficiency. Here we have investigated the constraints provided by considering simultaneously the observed spread of lithium and oxygen (and when possible also sodium) abundances for samples of turn-off stars in NGC 6752, NGC 6121 (M4), and NGC 104 (47Tuc), and the helium abundance of their multiple main sequences. These observations provide a very powerful test of the accretion scenario, because the observed O, Li, and He abundance distributions at the turn-off can be used to constrain the composition (and mass) of the accreted matter and the timescales of the polluting stars. In the case of NGC 6752, we could not find a physically consistent solution. If early disc accretion does happen, observations point towards accretion of gas with a non-negligible Li abundance, contrary to the expectations for the ejecta of the “natural” polluters in this scenario. For M4, spectroscopic errors are too large compared to the intrinsic spread, to constrain the properties of the accreted matter. As for 47Tuc, we could find a physically consistent solution for the abundances of He and O (and Na) in the accreted gas and predict the abundances of these elements in the accreted matter – that will have to be reproduced by evolutionary calculations for the polluters and simulations of the global evolution of the intracluster gas – only if pollution happens with timescales of ~1 Myr, so polluters are objects with masses of several tens of solar masses. Accurate spectroscopic measurements of Li and other light elements in dwarf stars in a larger sample of clusters are needed to test this scenario more comprehensively.