Surface convection and red-giant radius measurements (L. Piau et al.)

Vol. 526
In section 7. Stellar structure and evolution

Surface convection and red-giant radius measurements

by L. Piau, P. Kervella, S. Dib, and P. Hauschildt, A&A 526, A100 alt

The authors model a carefully selected sample of red giants for which absolute angular diameters, asteroseismic spectra, and distances have been measured, to test the effect of different models and parametrizations of envelope convection using a stellar evolution code with updated opacities. Two different forms of convection, mixing length (MLT) computed using the PHOENIX model atmosphere code, and full spectrum of turbulence (FST), using ATLAS12. In both cases, the results are checked against the Solar calibration. They then compare the radii and the location of the red edge and base of the red giant branch with the evolution code and a population model. They find that the field RGB requires two different populations, differing in age by about 5 Gyr depending on metallicity. The mixing length parameter they find is smaller than the canonical solar value in both cases. The paper demonstrates the strong potential of a unified approach to stellar modeling presented by the availability of absolute stellar measurements to test theories of convection and population synthesis.