An empirical calibration of the mixing-length parameter α

We present an empirical calibration of the mixing-length free parameter alpha based on a homogeneous infrared database of 28 Galactic globular clusters spanning a wide metallicity range (-2: 15 < [Fe/H] < -0.2). Empirical estimates of the red giant effective temperatures have been obtained from infrared colors. Suitable relations linking these temperatures to the cluster metallicity have been obtained and compared to theoretical predictions. An appropriate set of models for the Sun and Population II giants has been computed by using both the standard solar metallicity (Z/X)(circle dot) = 0.0275 and the most recently proposed value (Z/X)(circle dot) = 0.0177. We find that when the standard solar metallicity is adopted, a unique value of alpha = 2.17 can be used to reproduce both the solar radius and the Population II red giant temperature. Conversely, when the new solar metallicity is adopted, two different values of alpha are required: alpha = 1.86 to fit the solar radius and alpha approximate to 2.0 to fit the red giant temperatures. However, it must be noted that regardless the adopted solar reference, the alpha-parameter does not show any significant dependence on metallicity.