The effects of physics and chemical-abundance uncertainties on the properties of lower-mass stars that are used as standard candles

The distances to old stellar populations have traditionally been derived from three standard candles: the luminosity of the red-giant-branch tip, the absolute magnitudes of RR Lyrae stars, and the colour-magnitude-diagram loci of nearby subdwarfs. The distance-modulus uncertainties that are associated with these methods are still at the level of +/-0.10-0.15 mag. Current stellar models are able to satisfy these age-independent constraints to well within their error bars, which are mainly owing to the uncertainties in the distances of the calibrating objects and, directly or indirectly, to chemical-abundance uncertainties. The impact of varying the physics and the assumed abundances in stellar models on the aforementioned standard candles is discussed, as is the use of the giant-branch bump luminosity to constrain the distance scale.