Tatiana Muraveva, Michele Bellazzini, Alessia Garofalo, Gisella Clementini, Lorenzo Monti, Maria Letizia Valentini

The Sagittarius (Sgr) dwarf spheroidal galaxy is one of the most prominent satellites of the Milky Way (MW). It is currently undergoing tidal disruption, forming an extensive stellar stream that provides key insights into the assembly history of the MW halo. In this study we analyzed RR Lyrae stars (RRLs) in the Sgr stream provided in Gaia Data Release 3 (DR3), for which new estimates of photometric metallicities are available in the literature, and accurate distances were calculated using the reddening-free period-Wesenheit-metallicity ($PWZ$) relation. We determine the mean metallicity of RRLs in the Sgr stream to be ${\rm [Fe/H]}=-1.62 \pm 0.01$ dex. We measure a metallicity gradient as a function of stripping time from the Sgr progenitor of $0.05 \pm 0.02$ dex/Gyr, indicating that the metal-poor RRLs were stripped earlier during the accretion process. The far arm is found to be the most metal-poor structure of the Sgr stream, with a mean metallicity of ${\rm [Fe/H]}=-1.98 \pm 0.37$ dex, significantly lower than that of the leading ($-1.69\pm0.31$ dex) and trailing ($-1.64 \pm 0.28$ dex) arms. Our findings show that the RRLs in the far arm of the Sgr stream exhibit a bimodal metallicity distribution with peaks at [Fe/H]=$-2.4$ dex and $-1.7$ dex. The main body of the stream is the most metal-rich structure, with a mean metallicity of ${\rm [Fe/H]}=-1.58 \pm 0.31$ dex and a radial gradient of $-0.008 \pm 0.005$ dex/kpc. We find almost negligible metallicity gradients of $(-0.2 \pm 0.3)\times 10^{-3}$ dex/deg in the trailing arm and $(-1.0 \pm 0.5)\times 10^{-3}$ dex/deg in the leading arm, in agreement with previous studies. Finally, we investigate the bifurcation of the Sgr stream and conclude that the metallicity difference between the faint and bright branches is not confirmed based on the RRLs in our sample.