Open clusters or their remnants : B and V photometry of NGC 1901 and NGC 1252

Abstract

Photometry in the B and V bands is presented for the southern stellar groups NGC1901 and NGC1252. NGC1901 is often described as an open cluster while NGC 1252 consists of a concentration of about 20 stars centered ≈ 20' north of the original New General Catalogue coordinates, and at the southwest edge of the large region previously assigned to this object in the literature. NGC1901 has a clear main sequence and shares similarities with the Hyades. We derive a reddening value E(B−V ) = 0.04, a distance from the Sun dʘ = 0.45 kpc (Z = −0.23 kpc) and an age 0.6 ± 0.1 Gyr. NGC1901 is conclusively a physical system, dynamically comparable to or more evolved than the Hyades. The colour-magnitude diagram of NGC1252 suggests a turnoff and main sequence, and a total of 12 probable members. We simulated the Galactic field colour-magnitude diagram in the same direction and found it to be a poor match to NGC1252, suggesting that NGC1252 is not a eld fluctuation. Isochrone fitting to the probable members is consistent with E(B − V ) = 0.02, dʘ = 0.64 kpc (Z = −0.46 kpc) and an age 3 ± 1 Gyr. NGC1252 cannot be ruled out as a physical group with the available data. If so, evidence is found that it is not a classical open cluster, but rather an open cluster remnant.