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Context. Stellar collision rates in globular clusters (GCs) do not appear tocorrelate with horizontal branch (HB) morphology, sug- gesting that dynamicsdoes not play a role in the second-parameter problem. However, core densitiesand collision rates derived from surface-brightness may be significantlyunderestimated as the surface-brightness profile of GCs is not necessarily agood indicator of the dynamical state of GC cores. Core-collapse may gounnoticed if high central densities of dark remnants are present. Aims. We testwhether GC HB morphology data supports a dynamical contribution to theso-called second-parameter effect. Methods. To remove first-parameterdependence we fitted the maximum effective temperature along the HB as afunction of metal- licity in a sample of 54 Milky Way GCs. We plotted theresiduals to the fit as a function of second-parameter candidates, namelydynamical age and total luminosity. We considered dynamical age (i.e. the ratiobetween age and half-light relaxation time) among possible second-parameters.We used a set of direct N-body simulations, including ones with dark remnantsto illustrate how core density peaks, due to core collapse, in a dynamical-agerange similar to that in which blue HBs are overabundant with respect to themetallicity expectation, especially for low-concentration initial conditions.Results. GC total luminosity shows nonlinear behavior compatible with theself-enrichment picture. However, the data are amenable to a differentinterpretation based on a dynamical origin of the second-parameter effect.Enhanced mass-stripping in the late red-giant- branch phase due to stellarinteractions in collapsing cores is a viable candidate mechanism. In thispicture, GCs with HBs bluer than expected based on metallicity are thoseundergoing core-collapse.

Core collapse and horizontal-branch morphology in galactic globular clusters