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

Abstract

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 $> 99,9{{\ \rm per\ cent}}$) anti-correlation between the fraction of G1 red giants (NG1/Ntot) and stellar encounter rates among 51 GCs. Moreover, a Milky Way-like initial mass function (IMF) requires at least ∼50 per cent of MS stars located in the mass range [0.1 − 0.5]M⊙. Unlike cluster mass loss, stellar collisions/merging retain these G1 stars by converting them into more massive G2 ones, with mainly MMS > 0.5M⊙. This process coupled with a decreasing relative mass loss with increasing GC masses implies a smaller (NG1/Ntot) 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 (MGC > 105.3M⊙) and it is at a confidence level of 89 per cent for the 12 least massive GCs (MGC < 105.1M⊙). 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.

Stellar collisions in globular clusters: Constraints on the initial mass function of the first generation of stars

“…Another physical process that can lead to the formation of G2 stars over a short timescale, prior to gas expulsion from the clusters by the first supernovae, is stellar collisions (Sills et al 2002;Sills & Glebbeek 2010;Jiang et al 2014;Wang et al 2020). Collisions can still occur at later times, albeit at a lower rate (Kravtsov & Calderón 2021;Kravtsov et al 2022). Collisions not only set the relative fractions of the G1 and G2 stars, but they also alter the shape of the IMF of the G1 stars (Dib et al 2007;Kravtsov et al 2022).…”

“…Collisions can still occur at later times, albeit at a lower rate (Kravtsov & Calderón 2021;Kravtsov et al 2022). Collisions not only set the relative fractions of the G1 and G2 stars, but they also alter the shape of the IMF of the G1 stars (Dib et al 2007;Kravtsov et al 2022). While more work is still needed in order to understand whether stellar collisions, particularly of low-mass stars, can lead to the chemical anticorrelations that are observed for GCs, Kravtsov et al (2022) present observational evidence that supports the scenario in which stellar collisions of G1 stars in the mass range ≈ (0.1-0.5) M ⊙ can lead to the formation of G2 stars in the mass range (0.5-0.9) M ⊙ .…”

“…Collisions not only set the relative fractions of the G1 and G2 stars, but they also alter the shape of the IMF of the G1 stars (Dib et al 2007;Kravtsov et al 2022). While more work is still needed in order to understand whether stellar collisions, particularly of low-mass stars, can lead to the chemical anticorrelations that are observed for GCs, Kravtsov et al (2022) present observational evidence that supports the scenario in which stellar collisions of G1 stars in the mass range ≈ (0.1-0.5) M ⊙ can lead to the formation of G2 stars in the mass range (0.5-0.9) M ⊙ . In particular, they presented evidence for the existence of an anticorrelation between the fraction of G1 stars (N(G1)/N(tot)) and the slope of the present-day mass function of GCs in the stellar mass range (0.2-0.8) M ⊙ (α pd ).…”

“…The present-day encounter rates of the clusters are expected to scale with their primordial values when the clusters were denser (e.g., Maccarone & Peacock 2011). Using a simple collision model with a parametrized collision efficiency and a Kroupa-like IMF for the G1 stars (Kroupa 2001), Kravtsov et al (2022) showed that it is possible to reproduce the (N(G1)/N(tot)) −α pd anticorrelation both in terms of slope and absolute values. Nonetheless, the (N(G1)/N(tot)) −α pd anticorrelation shows a level of scatter that cannot be simply explained by a fixed IMF nor by observational uncertainties.…”

The galaxy-wide stellar initial mass function in the presence of cluster-to-cluster IMF variations

“…Using this prescription of the IMF where implicitly IMF variations are driven by the gas-phase metallicity of the gas and by its density, they were able to recover the observed dependence of the slope of the IMF in the high stellar mass regime on the SFR (Gunawardhana et al 2011). More recently, Dib et al (2022) showed that allowing for variations of the IMFs, particularly in the regime of stellar masses around its peak can help explain the scatter that is observed between the fraction of first generation stars and the present-day slope of the mass function in globular clusters that has been recently presented by Kravtsov et al (2022). It is probably safe to say that more work is still needed in order to better quantify the level of IMF variations within galaxies, and to relate these variations to the physical conditions of the star forming gas.…”