Ruprecht 1: a small, moderately young open cluster in the third galactic quadrant

Abstract

We present CCD observations in the Washington system C and T1 passbands down to T1 ∼ 18.5 in the field of Ruprecht 1, a poorly studied open cluster located in the third Galactic quadrant. We measured T1 magnitudes and C − T1 colors for a total of 862 stars distributed throughout an area of 13.60 × 13.60 . The cluster turned out to be very small; its linear radius being 2.6 ± 0.2 pc, as estimated from star counts in appropriate-sized boxes distributed throughout the entire field observed. By fitting the zero-age main sequence to the T1 vs. C − T1 color-magnitude diagram, we derive EB−V = 0.25 ± 0.05, independently from the cluster’s metallicity. Our analysis suggests that Ruprecht 1 is moderately young. In fact, adopting the theoretical metal contents Z = 0.02 and 0.008, which provide the best global fits, we derive heliocentric distances of d = 1.9 ± 0.4 kpc and 1.5 ± 0.3 kpc and ages of 200 ± 47 Myr and 251 ± 58 Myr in each case.