Stellar Envelope Convection calibrated by Radiation Hydrodynamics Simulations: Influence on Globular Clusters Isochrones

Abstract

One of the largest sources of uncertainty in the computation of globular cluster isochrones and hence in the age determination of globular clusters is the lack of a rigorous description of convection. Therefore, we calibrated the superadiabatic temperature gradient in the envelope of metal-poor low-mass stars according to the results from a new grid of 2D hydrodynamical models, which cover the Main Sequence and the lower Red Giant Branch of globular cluster stars. In practice, we still use for computing the evolutionary stellar models the traditional mixing length formalism, but we fix the mixing length parameter in order to reproduce the run of the entropy of the deeper adiabatic region of the stellar envelopes with effective temperature and gravity as obtained from the hydro-models. The detailed behaviour of the calibrated mixing length depends in a non-trivial way on the effective temperature, gravity and metallicity of the star. Nevertheless, the resulting isochrones for the relevant age range of galactic globular clusters have only small differences with respect to isochrones computed adopting a constant solar calibrated value of the mixing length. Accordingly, the age of globular clusters is reduced by 0.2 Gyr at most.

9 pages, 3 figures Accepted for publication in ApJ Letters

References in corpus (3)

• The Age Of Globular Clusters In Light Of Hipparcos: Resolving the Age Problem?
• Absolute Ages of Globular Clusters and the Age of the Universe
• The age of the oldest globular clusters

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• Particle-physics constraints from the globular cluster M5: Neutrino Dipole Moments
• Coping with loss: Stability of mass transfer from post-main sequence donor stars
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• 3D hydrodynamical CO5BOLD model atmospheres of red giant stars: I. Atmospheric structure of a giant located near the RGB tip
• Calibrating the Mixing Length Parameter for a Red Giant Envelope
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