Properties of RR Lyrae stars in the Inner Regions of the Large Magellanic Cloud

Abstract

We present the radial velocities, metallicities and the K-band magnitudes of 74 RR Lyrae stars in the inner regions of the LMC. The intermediate resolution spectra and infrared images were obtained with FORS1 at the ESO VLT and with the SOFI infrared imager at the ESO NTT. The best 43 RR Lyrae with measured velocities yield an observed velocity dispersion of 61+-7 km/s. We obtain a true LMC RR Lyrae velocity dispersion of 53 km/s, which is higher than the velocity dispersion of any other LMC population previously measured. This is the first empirical evidence for a kinematically hot, metal-poor halo in the LMC as discussed in Minniti et al. (2003). Using Layden's (1994) modification of the Delta S method we measured the metallicity for 23 of our stars. The mean value is [Fe/H] =-1.46+-0.09 dex. The absolute magnitudes M_V and M_K of RR Lyrae stars are linear functions of metallicity. In the V band, our data agree with the Olech et al. (2003) relation, in the K band the slope is flatter. The average apparent V luminosity of 70 RR Lyrae stars is <V> = 19.45+-0.04 and the average K luminosity of 37 RR Lyrae stars is <K>=18.20+-0.06. There is no obvious relation between apparent V magnitude and LogP, while the RR Lyrae K band magnitudes show a well defined linear trend with LogP. Using the Bono et al. (2001) and Bono et al. (2003) theoretical Near-Infrared Period-Luminosity-Metallicity relations we calculate the LMC distance modulus 18.48+-0.08.