[Paper Review] Evolution of two stellar populations in globular clusters II. Effects of primordial gas expulsion

T. Decressin, Holger Baumgardt+2 more|Queensland's institutional digital repository (The University of Queensland)|Mar 30, 2010

Stellar, planetary, and galactic studiesPhysics and Astronomy68 references57 citations

TL;DR

This paper investigates how primordial gas expulsion in globular clusters can dynamically enrich the second generation of stars, showing that gas expulsion preferentially ejects first-generation stars while retaining more centrally concentrated second-generation stars. With a star formation efficiency of ~0.33, the model reproduces observed second-generation fractions of up to 60% in present-day clusters.

ABSTRACT

We investigate the early evolution of two distinct populations of low-mass stars in globular clusters under the influence of primordial gas expulsion driven by supernovae to study if this process can increase the fraction of second generation stars at the level required by observations. We analyse N-body models that take into account the effect of primordial gas expulsion. We divide the stars into two populations which mimic the chemical and dynamical properties of stars in globular clusters so that second generation stars start with a more centrally concentrated distribution. The main effect of gas expulsion is to eject preferentially first generation stars while second generation stars remain bound to the cluster. In the most favourable cases second generation stars can account for 60% of the bound stars we see today. We also find that at the end of the gas expulsion phase, the radial distribution of the two populations is still different, so that long-term evolution will further increase the fraction of second generation stars. The large fraction of chemically anomalous stars is readily explainable as a second generation of stars formed out of the slow winds of rapidly rotating massive stars if globular clusters suffer explosive residual gas expulsion for a star formation efficiency of about 0.33.

Motivation & Objective

• To determine whether primordial gas expulsion can explain the high observed fraction of second-generation stars in globular clusters.
• To investigate how radial segregation of second-generation stars during formation affects their retention after gas expulsion.
• To constrain initial cluster properties—such as star formation efficiency, initial half-mass radius, and gas expulsion timescale—that reproduce observed second-generation fractions.
• To assess whether gas expulsion can account for the observed O-Na and Mg-Al anticorrelations in clusters with high orbital inclinations or long orbital periods.
• To evaluate the viability of the fast-rotating massive star (WFRMS) scenario for chemical self-enrichment in the context of dynamical evolution.

Proposed method

• N-body simulations incorporating primordial gas expulsion driven by supernovae, with two dynamically and chemically distinct stellar populations.
• First-generation stars are assigned a more extended radial distribution; second-generation stars are initialized with a more concentrated distribution to mimic observed kinematic and chemical segregation.
• Gas expulsion is modeled as a sudden mass loss with a timescale comparable to the cluster crossing time, simulating supernova-driven outflows.
• The simulations track the survival and radial distribution of both populations post-expulsion, focusing on the final fraction of second-generation stars bound to the cluster.
• Initial cluster parameters such as star formation efficiency (SFE), initial half-mass radius, and tidal field strength are varied to match observational constraints.
• The model assumes that slow winds from fast-rotating massive stars enrich the interstellar medium, which then forms second-generation stars with distinct chemical signatures.

Experimental results

Research questions

• RQ1 Can primordial gas expulsion selectively eject first-generation stars while retaining a high fraction of second-generation stars?
• RQ2 What initial cluster conditions (e.g., SFE, initial concentration, expulsion timescale) are required to reproduce the observed 60% second-generation star fraction?
• RQ3 How does radial segregation of second-generation stars during formation influence their survival after gas expulsion?
• RQ4 Can gas expulsion explain the observed correlation between orbital parameters (e.g., inclination, period) and the extent of O-Na and Mg-Al anticorrelations?
• RQ5 Is the slow wind from fast-rotating massive stars viable as a source of chemical enrichment if gas expulsion is included in the dynamical evolution?

Key findings

• Primordial gas expulsion preferentially ejects first-generation stars due to their more extended distribution, while second-generation stars—initially more centrally concentrated—remain bound to the cluster.
• In the most favorable conditions, second-generation stars can constitute up to 60% of the bound stars in the cluster today, matching observational estimates.
• The final fraction of second-generation stars is highest for star formation efficiencies around 0.33, consistent with observed cluster masses and initial conditions.
• Initial cluster half-mass radii are constrained to 1–3 pc (up to 4–5 pc for the most massive clusters), with total proto-GC cloud masses of several ×10⁶ M☉, reaching up to 9×10⁶ M☉ for clusters like NGC 6752.
• The radial distribution of the two populations remains distinct even after gas expulsion, indicating that long-term dynamical evolution further enhances the second-generation fraction.
• The combination of gas expulsion and tidal stripping during long-term evolution can reproduce the observed second-generation star fractions in present-day globular clusters.