BVI Photometric Study of the Old Open Cluster Ruprecht 6
  We present a BVI optical photometric study of the old open cluster Ruprecht 6
using the data obtained with the SMARTS 1.0 m telescope at the CTIO, Chile. Its
color-magnitude diagrams show the clear existence of the main-sequence stars,
of which turn-off point is located around V ~ 18.45 mag and B-V ~ 0.85 mag.
Three red clump (RC) stars are identified at V = 16.00 mag, I = 14.41 mag and
B-V = 1.35 mag. From the mean Ks-band magnitude of RC stars (Ks=12.39 +- 0.21
mag) in Ruprecht 6 from 2MASS photometry and the known absolute magnitudes of
the RC stars (M_Ks = -1.595 +- 0.025 mag), we obtain the distance modulus to
Ruprecht 6 (m-M)_0 = 13.84 +- 0.21 mag (d=5.86 +- 0.60 kpc). From the (J-K_s)
and (B-V) colors of the RC stars, comparison of the (B-V) and (V-I) colors of
the bright stars in Ruprecht 6 with those of the intrinsic colors of dwarf and
giant stars, and the PARSEC isochrone fittings, we derive the reddening values
of E(B-V) = 0.42 mag and E(V-I) = 0.60 mag. Using the PARSEC isochrone fittings
onto the color-magnitude diagrams, we estimate the age and metallicity to be:
log (t) =9.50 +- 0.10 (t =3.16 +- 0.82 Gyr) and [Fe/H] = -0.42 +- 0.04 dex. We
present the Galactocentric radial metallicity gradient analysis for old (age >
1 Gyr) open clusters of Dias et al. catalog, which likely follow a single
relation of [Fe/H] =(-0.034 +- 0.007) R_GC + (0.190 +- 0.080) (rms = 0.201) for
the whole radial range or dual relation of [Fe/H] =(-0.077 +- 0.017) R_GC +
(0.609 +- 0.161) (rms = 0.152) and constant ([Fe/H] ~ -0.3 dex) value, inside
and outside of R_GC ~ 12 kpc, respectively. The metallicity and Galactocentric
radius (13.28 +- 0.54 kpc) of Ruprecht 6 obtained in this study seem to be
consistent with both of the relations.
