next previous
Up: RR Lyrae stars in the analysis


7 Homogeneity of the RR Lyrae population over the field

Several authors claim having detected a metallicity gradient in the stellar population of Sgr, the centre being more metal-rich than the outer regions. This finding is based on the redder morphology of the Red Giant Branch and the Horizontal Branch at the centre of Sgr relative to the outer regions (Marconi et al. 1998; Bellazzini et al. 1999b; Alard 2001). If confirmed, this feature would be in concordance with observations of other dwarf galaxies (e.g. Saviane et al. 2001 and references therein).We now question whether such a gradient is apparent in the RR Lyrae population.

  \begin{figure}
\par\includegraphics[width=6.8cm,clip]{1395f9.ps} \end{figure} Figure 9: Panel a) average period of RRab stars as a function of position. Filled circles correspond to RRab variables in Sgr and a distance (X) from M 54 projected onto the main axis (upper labels). Open circles are average periods of Galactic RRab and are plotted as a function of Galactic latitude (lower labels). Panel b) Normalized period histograms of RRab stars. The solid line correspond to RRab stars with $X<5^{\circ }$ whereas the dotted line correspond to RRabs with $X>5^{\circ }$. Panel c) Normalized histograms of X for the 250 shortest period (solid line) and longest period (dotted line) RRab variables in Sgr.

Any metallicity gradient would translate into a gradient in the mean period of RRab stars. Figure 9a presents the mean period of RRab stars in Sgr (solid line) as a function of distance from M54 projected onto the main axis[*]. Each bin size has been adapted in order to contain the same number of RRab stars. A linear least square fit through these points yields a slope of $\partial (\langle P_{\rm ab}\rangle)/\partial X=0.0002(\pm0.0004)$. From Eq. (2), it results in a metallicity difference <0.1 dex between the two extremities of the field. Clearly, there is no significant metallicity gradient in the RR Lyrae population of Sgr. For comparison, we present in Fig. 9a the average period of Galactic RRab as a function of latitude (dotted line), where the period dependence as a function of position is evident.

To test the homogeneity further, we divided the RRab catalogue into two subsamples around the median distance from M 54 and compared the period distributions (Fig. 9b). A KS test shows that the two histograms are drawn from the same parent distribution with a probability of 82$\%$, suggesting a similar horizontal branch morphology in the two subsamples.

Finally, in Fig. 9c we compare the spatial distributions of the 250 longest and shortest period RRab variables. The KS test yields a probability of 88$\%$ for the two histograms to be issued from the same parent distribution. This definitely excludes any modification of the period distribution with position. Thus, unless a metallicity gradient and the second parameter effect conspire to keep the horizontal branch morphology constant over the field, the RR Lyrae population should be homogeneous in the main body of Sgr. Furthermore, if the metallicity gradient was confirmed, this may indicate that the RR Lyrae population is not associated to the prominent red horizontal branch which is apparent in the CMDs of Sgr.


next previous
Up: RR Lyrae stars in the analysis

Copyright ESO 2001