$\alpha$ from $\Delta S$ analysis of RR Lyrae stars in the bar of the LMC

Involved people at OAB: Bragaglia, Clementini, Di Fabrizio.

The slope of the luminosity-metallicity relation followed by RR Lyr's can be directly measured using a population of RR Lyrae variables all at the same distance, and spanning a suitable range in metal abundance. RR Lyrae's in the Large Magellanic Cloud (LMC) bar play a key role in this respect, since they can be considered at the same distance from us, are very numerous, and span about 1 dex in metallicity. The slope of the luminosity-metallicity relation for RR Lyrae stars is being investigated in collaboration with Carretta and Gratton (Padova Obs.).

In December 2001 we have obtained low resolution spectroscopy with FORS1@VLT for 100 RR Lyrae at minimum light and 350 clump stars in the bar of the LMC. Examples of some reduced spectra are shown in Figure 4. These spectroscopic data will allow us to measure metallicities accurate to $\sim$ 0.15-0.20 dex for both RR Lyr's (from the $\Delta S$ index, Preston 1959) and clump stars. The RR Lyrae metal abundances coupled with high quality photometry by Clementini et al. 2001a (see Section 1.4.3) will permit us to define $\alpha$ to better than 0.05 mag/dex, thus providing a clearcut choice between the steep and mild slopes proposed for the RR Lyrae luminosity-metallicity relation.

Figure 4: Spectra of RR Lyrae's and clump stars in the bar of the LMC, obtained with FORS1@VLT.

The metal distribution for a conspicuous number of LMC red clump stars will be used to address the controversial issue of the red clump absolute magnitude dependence on chemical composition, which so far limits its use as a distance indicator.