Calibrating RR Lyrae absolute magnitudes as a function of period shift to correct post-ZAHB evolution systematics

Abstract

Calibrating RR Lyrae absolute magnitudes as a function of period shift to correct post-ZAHB evolution systematics

RR Lyrae (RRL) stars of Oosterhoff type II (OoII) are known to be intrinsically more luminous than those of type I (OoI), by approximately 0.2 mag in the V-band (e.g. Lee et al.1990; Kunder & Chaboyer 2009). This over-luminosity of OoII~RRLs probably owes to a more advanced evolutionary state, with OoI RRL tracing the Zero Age Horizontal Branch (ZAHB) and OoII~stars having evolved off the ZAHB on their way to the Asymptotic Giant Branch (Lee et al. 1990, Clement & Shelton 1999).

Currently, most studies using RRLs as tracers of Milky Way structure use optical surveys and M-[Fe/H] relations (or, most commonly, fixed absolute magnitude values) to derive distances to RRLs. These relations, however, cannot account for the difference in luminosity as a function of the Oosterhoff type, translating into a systematic distance underestimation of ~10% for OoII stars, which amount to about a quarter of all RRL in the Galactic halo. Although the difference may be small enough to be neglected for many purposes, the fact that it is systematic could compromise the interpretation of analyses based on the distribution of RRL by Oosterhoff type as well as the detection of new substructures (e.g. tidal streams, clouds, etc.) that may contain mixed-type RRLs (as the majority of Galactic satellites and even many globular clusters do), by washing out their distance signature or causing confusion between distinct ones.

In this contribution we explore calibrating RRL absolute magnitudes as a function of period-shift, as first proposed by Kunder et al. 2009. Period-shift characterises the location in the Period-Amplitude diagram (e.g. Sandage 2006) and correlates with Oosterhoff type, having the potential to account for both the evolutionary and the metallicity dependence of the luminosity. We will explore here the performance of the absolute-magnitude-period-shift calibration for the Gaia G-band — the most widely used currently for Galactic structure purposes— using RRLs in globular clusters, and comparing against the traditional absolute-magnitude-metallicity calibration benchmark.