Core Helium Burning Evolution at 5M⊙

Abstract

Observations of the red giants in NGC 1866 in the Large Magellanic Cloud, made by Arp and Thackeray provide a strict test for core helium burning models of about 5M⊙. The main observational features to be reproduced theoretically are the luminosity of the red giants, the range in effective temperature that they occupy and the ratio of the number of giants near the blue end of this range to the number at the bottom of the red giant branch. Meyer-Hofmeister attempted a theoretical fit to these observations using the initial (X, Y, Z) composition (0.602, 0.354, 0.044) and assuming the stars to have formed uniformly over a period of 1.5 × 107 years about a mean age of 5.75 × 107 years. Although she was able to fit the luminosity and effective temperature spread of the red giants, her theory predicted far too many stars at the bottom of the red giant branch relative to the number of bluer giants.