A Red Giants’ Toy Story. II. Understanding the Red-giant Branch Bump

Abstract

The Red-Giant Branch Bump (RGBB) is one of the most noteworthy features inthe red-giant luminosity function of stellar clusters. It is caused by thepassage of the hydrogen-burning shell through the composition discontinuityleft at the point of the deepest penetration by the convective envelope. Whencrossing the discontinuity the usual trend in increasing luminosity reversesfor a short time before it increases again, causing a zig-zag in theevolutionary track. In spite of its apparent simplicity the actual physicalreason behind the decrease in luminosity is not well understood and severaldifferent explanations have been offered. Here we use a recently proposedsimple toy-model for the structure of low-mass red giants, together withprevious results, to show beyond reasonable doubt that the change in luminosityat the RGBB can be traced to the change in the mean molecular weight of thelayers on top of the burning shell. And that these changes happen on a nucleartimescale. The change in the effective mean molecular weight, as the burningshell approaches the discontinuity, causes a drop in the temperature of theburning shell which is attenuated by the consequent feedback contraction of thelayers immediately below the burning shell. Our work shows that, when appliedcorrectly, including the feedback on the structure of the core together with ofthe increase in the mass of the core, shell-source homology relations do agreat quantitative job in explaining the properties of full evolutionary modelsat the RGBB.