2.4. The Red Clump/Horizontal Branch (RC/HB)
Red Clump (RC) stars and their lower mass cousins, Horizontal Branch (HB) stars are core helium-burning stars, and their luminosity varies depending upon age, metallicity and mass loss (e.g. Caputo et al. 1995). The extent in luminosity of the RC can be used to estimate the age of the population that produced it, as shown in the upper panel of Figure 2. This age measure is independent of absolute magnitude and hence distance, and indeed these properties can be used to determine an accurate distance measure on the basis of the RC (e.g. Cole 1998).
Figure 2. In the top panel is plotted the magnitude of the upper and lower edge of the RC versus age, in Gyr, from Caputo et al. (1995). This shows the variation in the extent in MV of a RC with age, for a metallicity of Z = 0.0004. It can be clearly see that this extent is strong function of the age of the stellar population. Also plotted is MV of the zero age HB against age. In the bottom panel are plotted the results of running a series of Monte-Carlo simulations using stellar evolution models at Z = 0.0004 (from, Fagotto et al. 1994) and counting the number of RC and RGB stars in the same part of the diagram, and thus we determine the expected ratio of RC/RGB stars versus age.
The classical RC and RGB appear in a population at about the same time (~ 0.9-1.5 Gyr, depending on model details), where the RGB are the progenitors of the RC stars. The lifetime of a star on the RGB, tRGB, is a decreasing function of Mstar, but the lifetime in the RC, tRC is roughly constant. Hence the ratio, tRC / tRGB, is a decreasing function of the age of the dominant stellar population in a galaxy, and the ratio of the numbers of stars in the RC, and the HB to the number of RGB is sensitive to the SFH of the galaxy (Tolstoy et al. 1998; Han et al. 1997). Thus, the higher the ratio, N(RC) / N(RGB), the younger the dominant stellar population in a galaxy, as shown in the lower panel of Figure 2.
The presence of a large HB population on the other hand (indicated by a high N(HB) / N(RGB) or even N(HB) / N(MS), is caused by a predominantly much older (> 10 Gyr) stellar population in a galaxy. The HB is the brightest indicator of very lowest mass (hence oldest) stellar populations in a galaxy.