Stellar collisions in globular clusters: the origin of multiple stellar populations

Two generations of stars, G1 and G2, typically populate Galactic globular clusters (GCs). The origin of G2 stars is unclear. We uncover two empirical dependencies between GC characteristics, which can be explained by the formation of G2 Main-Sequence (MS) stars due to collision/merging of their primordial counterparts (G1). A similar genesis of both G2 stars and peculiar objects like LMXBs and millisecond pulsars is also implied. Indeed, we find a significant (at a confidence level > 99,9%) anti-correlation between the fraction of G1 red giants (N-G1/N-tot) and stellar encounter rates among 51 GCs. Moreover, a Milky Way-like initial mass function (IMF) requires at least similar to 50 per cent of MS stars located in the mass range [0.1 - 0.5]M-circle dot. Unlike cluster mass loss, stellar collisions/merging retain these G1 stars by converting them into more massive G2 ones, with mainly M-MS > 0.5M(circle dot). This process coupled with a decreasing relative mass loss with increasing GC masses implies a smaller (N-G1/N-tot) in more massive GCs with a shallower present day MF. From data for 35 GCs, we find that such an anti-correlation is significant at 98.3 per cent confidence level (Spearman's correlation) for the 12 most massive GCs (M-GC > 10(5.3)M(circle dot)) and it is at a confidence level of 89 per cent for the 12 least massive GCs (M-GC < 10(5.1)M(circle dot)). Other fractions of G1 and G2 stars observed at the bottom of the MS as compared with the red giant branch in a few GCs are consistent with the scenario proposed.

Automated summary

Two generations of stars, G1 and G2, in Galactic globular clusters may have a close relationship, with G2 stars possibly formed through collisions/merging of G1 stars. Empirical dependencies between GC characteristics suggest a connection to this formation mechanism.