Core collapse and horizontal-branch morphology in Galactic globular clusters

M. Pasquato, G. Raimondo, E. Brocato, C. Chung, A. Moraghan, Young-Wook Lee

Research output: Contribution to journalArticle

3 Citations (Scopus)

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.

Original languageEnglish
Article numberA129
JournalAstronomy and Astrophysics
Volume554
DOIs
Publication statusPublished - 2013 Jun 24

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globular clusters
metallicity
collision rates
collision
brightness
luminosity
parameter
stripping
low concentrations
relaxation time
simulation
profiles
temperature
interactions

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Pasquato, M. ; Raimondo, G. ; Brocato, E. ; Chung, C. ; Moraghan, A. ; Lee, Young-Wook. / Core collapse and horizontal-branch morphology in Galactic globular clusters. In: Astronomy and Astrophysics. 2013 ; Vol. 554.
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Core collapse and horizontal-branch morphology in Galactic globular clusters. / Pasquato, M.; Raimondo, G.; Brocato, E.; Chung, C.; Moraghan, A.; Lee, Young-Wook.

In: Astronomy and Astrophysics, Vol. 554, A129, 24.06.2013.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Core collapse and horizontal-branch morphology in Galactic globular clusters

AU - Pasquato, M.

AU - Raimondo, G.

AU - Brocato, E.

AU - Chung, C.

AU - Moraghan, A.

AU - Lee, Young-Wook

PY - 2013/6/24

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N2 - 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.

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