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.
Bibliographical noteFunding Information:
Support for this work was provided by the National Research Foundation of Korea to the Center for Galaxy Evolution Research, and also by the KASI-Yonsei Joint Research Program for the Frontiers of Astronomy and Space Science and the DRC program of Korea Research Council of Fundamental Science and Technology (FY 2012). This work received partial financial support by INAF−PRIN/2010 (PI G. Clementini) and INAF−PRIN/2011 (PI M. Marconi). We wish to thank the referee for comments and questions that helped us clarify several important points.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science