BVI time-series data of the Galactic globular cluster NGC 3201.: I.: RR Lyrae stars

We present Johnson BV- and Kron-Cousins I-band time-series data collected over three consecutive nights in a region of 13 arcmin(2) centered on the Galactic globular cluster (GGC) NGC 3201. The time sampling of current CCD data allowed us to derive accurate light curves and, in turn, mean magnitudes and colors for a sample of 53 RR Lyrae stars. To overcome the thorny problem of differential reddening affecting this cluster, we derived new empirical relations connecting the intrinsic B V and V I colors of fundamental ( RR ab) RR Lyrae stars to the luminosity amplitude, the metallicity, and the pulsation period. The key features of these relations are the following: ( 1) they rely on stellar parameters, which are not affected by reddening; (2) they supply accurate estimates of intrinsic colors across the fundamental instability strip and cover a wide metallicity range; ( 3) they were derived by neglecting the RR Lyrae stars that are affected by amplitude modulation. Moreover, the zero point of the E ( B V) reddening scale was empirically checked using the large sample of RR Lyrae stars in M3 from Corwin & Carney, a GGC affected by a vanishing reddening. According to these relations we estimated individual reddenings for RR Lyrae stars in our sample and the main results we found are the following: ( 1) The mean cluster reddening based on E ( B V) color excesses is [E (B-V)] = 0.30 +/- 0.03. This estimate is slightly higher than the mean reddening evaluations available in the literature or based on the dust infrared map by Schlegel, Finkbeiner, & Davis, i.e., [E(B-V)] = 0.26 +/- 0.02. Note that the angular resolution of this map is approximate to6', whereas for current reddening map it is approximate to1'. (2) The mean cluster reddening based on E(V-I) color excesses is [E (V-I)] = 0.36 +/- 0.05. This estimate is only marginally in agreement with the mean cluster reddening obtained using the reddening map by von Braun & Mateo and derived by adopting cluster turnoff stars, i.e., [E (V-I)] = 0.25 +/- 0.07. On the other hand, current intrinsic spread among individual reddenings (approximate to0.2 mag) agrees quite well with the estimate provided by previous authors. It is noteworthy that previous mean cluster reddenings are in very good agreement with values obtained using the empirical relations for intrinsic RR Lyrae colors provided by Kovacs & Walker. ( 3) According to current individual E ( B V) and E ( V I) reddenings and theoretical predictions for horizontal-branch stars, we found that the true distance modulus for this cluster is 13.32 +/- 0.06 mag. This determination is somehow supported by the comparison between predicted and empirical pulsation amplitudes. ( 4) The comparison between present luminosity amplitudes and estimates available in the literature discloses that approximately 30% of fundamental RR Lyrae stars are affected by amplitude modulation (the Blazhko effect). This finding confirms empirical evidence originally brought out by Szeidl and by Smith.