Core collapse and horizontal-branch morphology in Galactic globular clusters

¹ Department of Astronomy & Center for Galaxy Evolution ResearchYonsei University, 120-749 Seoul, Republic of Korea
e-mail: mario.pasquato@gmail.com
² Yonsei University Observatory, 120-749 Seoul, Republic of Korea
³ INAF–Osservatorio Astronomico di Teramo, Mentore Maggini s.n.c., 64100 Teramo, Italy
⁴ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy

Received: 26 February 2013
Accepted: 3 May 2013

Abstract

Context. Stellar collision rates in globular clusters (GCs) do not appear to correlate with horizontal branch (HB) morphology, suggesting 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 metallicity 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.