A Helium Spread among the Main‐Sequence Stars in NGC 2808

We studied the color distribution of the main sequence of the GlobularCluster NGC 2808, based on new deep HST-WFPC2 photometry of a field in theuncrowded outskirts of the cluster. The color distribution of main sequencestars is wider than expected for a single stellar population, given our(carefully determined) measurement errors. About 20% of the sample stars aremuch bluer than expected and are most plausibly explained as a populationhaving a much larger helium abundance than the bulk of the main sequence. Weestimate that the helium mass fraction of these stars is Y ~ 0.4. NGC 2808 mayhave suffered self-enrichment, with different stellar populations born from theejecta of the intermediate mass asymptotic giant branch (AGB) stars of thefirst generation. In addition to the Y=0.40 stars, roughly 30% of the starsshould have Y distributed between 0.26-0.29 while 50% have primordial Y, toexplain also the peculiar horizontal branch morphology. Three main stages ofstar formation are identified, the first with primordial helium content Y ~0.24, the second one born from the winds of the most massive AGBs of the firststellar generation (6-7msun), having Y ~ 0.4, and a third one born from thematter ejected from less massive AGBs (~ 3.5-4.5msun) with Y ~ 0.26-0.29. For along hiatus of time (several 10^7yr) between the second and third generation,star formation might have been inhibited by the explosion of late Supernovae IIderiving from binary evolution.Comment: accepted for publication on "The Astrophysical Journal