The red giant branch phase transition: Implications for the RGB luminosity function bump and detections of Li-rich red clump stars

¹ INAF–Osservatorio Astronomico di Teramo, via M. Maggini, 64100 Teramo, Italy
e-mail: cassisi@oa-teramo.inaf.it
² Instituto de Astrofísica de Canarias, Calle via Lactea s/n, 38205 La Laguna, Tenerife, Spain
³ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
e-mail: M.Salaris@ljmu.ac.uk

Received: 21 September 2015
Accepted: 1 October 2015

Abstract

We performed a detailed study of the evolution of the luminosity of He-ignition stage and of the red giant branch bump luminosity during the red giant branch phase transition for various metallicities. To this purpose we calculated a grid of stellar models that sample the mass range of the transition with a fine mass step equal to 0.01 M_⊙. We find that for a stellar population with a given initial chemical composition, there is a critical age (of 1.1–1.2 Gyr) around which a decrease in age of just 20–30 million years causes a drastic drop in the red giant branch tip brightness. We also find a narrow age range (a few 10⁷ yr) around the transition, characterized by the luminosity of the red giant branch bump being brighter than the luminosity of He ignition. We discuss a possible link between this occurrence and observations of Li-rich core He-burning stars.