5 Period-amplitude diagram

Figure 8: Panel a) period/amplitude diagram of the RR Lyrae stars in Sgr. Dots represent stars detected in SAG and triangle are double-mode RR Lyraes. Also shown are the fiducial lines of M 3 (OoI group) and M 9 (OoII group). Panel b) Metallicity estimate from the period-amplitude-metallicity relation. The dotted line represents the best Gaussian fit, whose relation is given in the upper right corner of the panel.

The period-amplitude diagram is presented in Fig. 8a. To construct this figure, we considered only the SAG field, which has a more precise photometry than DUO, resulting in a higher accuracy of the amplitudes. As will be shown in Sect. 7, the RR Lyrae population is homogeneous over the field and there is therefore no loss of generality by restricting ourself to SAG. We rescaled the amplitudes in SAG to the V amplitude from the relation $A_V=0.93(\pm 0.17) A_{4415}-0.05(\pm 0.16)$, inferred from 15 RR Lyrae stars in common with those detected by Layden & Sarajedini (2000) near M 54.

The distribution of RRab stars in Fig. 8a presents a high dispersion but is preferentially clumped around the ridge line of the globular cluster M 3, confirming that this population is related to the OoI group. However, the distribution is skewed toward the ridge line of the OoII cluster member M 9, suggesting the presence of a metal-poor sub-population within the RR Lyrae population. The RRab population is dominated by low-amplitude stars with a ratio $N(A_{V}>1.0~{\rm mag})/N(A_{V}<{\rm 1.0~mag})=0.55$. The RR Lyrae population is thus dominated by RRb stars, indicative of a red horizontal branch morphology.

Remarkably, the same features are apparent in the period-amplitude diagram of the LMC (Alcock et al. 1996; Alcock et al. 2000a), providing new evidence of the similarity of the RR Lyrae populations between these two galaxies.