Properties of RR Lyrae stars in the inner regions of the Large Magellanic Cloud* III. Near-infrared study

Abstract

Context. RR Lyrae variable stars are the primary Population II distance indicator. Likewise, the Large Magellanic Cloud (LMC) constitutes a key step in the extragalactic distance scale. Aims. By combining near-IR photometry and spectroscopically measured metallicities for a homogeneous sample of 50 RR Lyr stars in the LMC, we investigate the metallicity dependence of the period-luminosity relation in the near-infrared (TR), use the newly derived relations to re-derive the distance to the LMC, and compare the distance moduli obtained from RR Lyr and red clump stars. Methods. This paper presents new (single-epoch) J-band and (multi-epoch) Ks-band photometry of RR Lyr stars in 7 different LMC fields, observed with the near-IR camera SOFI at ESO's New Technology Telescope. Additional Ks-band data for another two LMC fields were taken with the ISPI infrared array at CTTO's Blanco 4m telescope. The near-IR photometry was cross-correlated with the MACHO and OGLE databases, resulting in a catalog of 62 RR Lyr stars with BVRIJKs photometry. A subsample of 50 stars also has spectroscopically measured metallicities. Results. In the deep JK color-magnitude diagrams of 7 fields, red giant branch, red clump and RR Lyr stars are detected. The majority of RR Lyr stars are located within the instability strip with near-IR colors between 0.14 ≤ (J - Ks)0 < 0.32. The period-luminosity relation only has a very mild dependence on metallicity in the K band, consistent with no dependence: MKs = 2.11(±0.17) log P + 0.05(±0.07) [Fe/H] - 1.05. In the J band the currently available data do not allow firm conclusions regarding the metallicity dependence of the period-luminosity relation. Conclusions. The distance modulus of the LMC, derived using our near-IR period-luminosity-metallicity relation for RR Lyr stars, is (m - M) 0 = 18.53 ± 0.13, in very good agreement with the distance modulus from the red clump stars, 18.46 ± 0.07. However, LMC modulus derived from the RR Lyrae stars depends on the parallax of the star RR Lyrae. © ESO 2009.