Core collapse and horizontal-branch morphology in galactic globular clusters

TL;DR

This study investigates whether the dynamical evolution, especially core collapse, influences the horizontal-branch morphology in globular clusters, suggesting that core-collapse may explain the second-parameter effect beyond metallicity.

Contribution

The paper provides evidence linking core-collapse dynamics to HB morphology variations, highlighting the role of dark remnants and stellar interactions in shaping the second-parameter effect.

Findings

GC total luminosity shows nonlinear behavior consistent with self-enrichment.

Data suggests a dynamical origin for the second-parameter effect.

Core-collapse may cause bluer HBs due to enhanced mass-stripping in collapsing cores.

Abstract

Context. Stellar collision rates in globular clusters (GCs) do not appear to correlate with horizontal branch (HB) morphology, sug- gesting that dynamics does not play a role in the second-parameter problem. However, core densities and collision rates derived from surface-brightness may be significantly underestimated as the surface-brightness profile of GCs is not necessarily a good indicator of the dynamical state of GC cores. Core-collapse may go unnoticed if high central densities of dark remnants are present. Aims. We test whether GC HB morphology data supports a dynamical contribution to the so-called second-parameter effect. Methods. To remove first-parameter dependence we fitted the maximum effective temperature along the HB as a function of metal- licity in a sample of 54 Milky Way GCs. We plotted the residuals to the fit as a function of second-parameter candidates, namely dynamical age and total luminosity. We considered dynamical age (i.e. the ratio between age and half-light relaxation time) among possible second-parameters. We used a set of direct N-body simulations, including ones with dark remnants to illustrate how core density peaks, due to core collapse, in a dynamical-age range similar to that in which blue HBs are overabundant with respect to the metallicity expectation, especially for low-concentration initial conditions. Results. GC total luminosity shows nonlinear behavior compatible with the self-enrichment picture. However, the data are amenable to a different interpretation based on a dynamical origin of the second-parameter effect. Enhanced mass-stripping in the late red-giant- branch phase due to stellar interactions in collapsing cores is a viable candidate mechanism. In this picture, GCs with HBs bluer than expected based on metallicity are those undergoing core-collapse.