RR Lyrae stars in the Sagittarius dwarf galaxy: Period analysis

Abstract

We carried out a period analysis on ∼3700 RR Lyrae stars on two Schmidt fields centred on (l, b) = (3.1°, -7.1°) and (6.6°, -10.8°) respectively, covering an area of ∼50 deg2. These stars are distributed almost evenly between the Sagittarius dwarf galaxy (Sgr) and the Milky Way. For Sgr members, the average periods are 〈Pab〉 = 0.574d and 〈Pc〉 = 0.322d for RRab and RRc stars respectively. This places Sgr in the long-period tail of the Oosterhoff I group. We report the detection of 53 double-mode RR Lyrae stars (RRd) within our sample. The magnitude of 40 of these stars is consistent with membership in Sgr whereas 13 RRds are located within our Galaxy. We also found 13 RR Lyraes (5 in Sgr and 8 in the Galaxy) exhibiting two closely spaced frequencies, most probably related to non-radial pulsations. The period distribution of the RR Lyrae variables in Sgr is compared to those of other Milky Way satellites. We find a remarkable similarity between the RR Lyrae populations in Sgr and the Large Magellanic Cloud (LMC), suggesting that these galaxies have similar horizontal branch morphologies. This may indicate that Sgr and the LMC started their formation under similar conditions. Using various photometric indicators, we estimate the metallicity of the RR Lyrae stars in Sgr and find 〈[Fe/H]〉 ≃ -1.6 dex with a dispersion of ∼±0.5 dex around this value and a minor but significant population at ≲-2.0 dex. We do not find evidence for a spatial metallicity gradient in the RR Lyrae population of Sgr. From the spatial distribution of RR Lyraes, we find that the main body of Sgr contains ∼4200 RRab stars. Assuming that population gradients are negligible in Sgr, we find Mv(Sgr) ≃ -13.9-0.6+0.4 mag for the main body. If Sgr has been stripped of 50% of its mass through Galactic tides, as assumed by some models, it would imply a total absolute magnitude of ∼-14.7 mag for this galaxy. Such a luminosity would be consistent with the empirical metallicity/luminosity relation for dwarf spheroidal galaxies***.