Helium variation due to self–pollution among Globular Cluster stars

Consequences on the horizontal branch morphology

¹ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio, Italy
² Istituto di Astrofisica Spaziale e Fisica Cosmica CNR, via Fosso del Cavaliere, 00133 Roma, Italy e-mail: caloi@rm.iasf.cnr.it
³ INAF, Osservatorio Astronomico di Padova, vicolo dell' Osservatorio, 5, 35122 Padova, Italy e-mail: gratton@mostro.pd.astro.it

Corresponding author: F. D'Antona, dantona@coma.mporzio.astro.it

Received: 19 June 2002
Accepted: 23 August 2002

Abstract

It is becoming clear that “self–pollution” by the ejecta of massive asymptotic giant branch stars has an important role in the early chemical evolution of globular cluster stars, producing CNO abundance spreads which are observed also at the surface of unevolved stars. Observing that the ejecta which are CNO processed must also be helium enriched, we have modelled stellar evolution of globular cluster stars by taking into account this possible helium enhancement with respect to the primordial value. We show that the differences between the main evolutionary phases (main sequence, turn–off and red giants) are small enough that it would be very difficult to detect them observationally. However, the difference in the evolving mass may play a role in the morphology of the horizontal branch, and in particular in the formation of blue tails, in those globular clusters which show strong CNO abundance variations, such as M13 and NGC 6752.