Core collapse and horizontal-branch morphology in galactic globular
  clusters
  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.
