Berkeley 94 and Berkeley 96: two young clusters with different dynamical evolution★

Abstract

We have performed multiband UBVRCICJHKS photometry of two young clusters located at large Galactocentric distances in the direction of the Perseus spiral arm. The obtained distances and colour excesses amount to 3.9 ± 0.11 kpc, E(B − V) = 0.62 ± 0.05 for Berkeley 94, and 4.3 ± 0.15 kpc, E(B − V) = 0.58 ± 0.06 for Berkeley 96. The respective ages, as measured from the comparison of the upper colour–magnitude diagrams to model isochrones, amount to log10Age(yr) = 7.5 ± 0.07 and 7.0 ± 0.07, respectively. A sequence of optical pre-main-sequence (PMS) members is proposed in both clusters. In addition, samples of objects showing (H − KS) excess are found. Part of these are suggested to be PMS cluster members of lower mass than the optical candidates. The spatial distribution of these sources, the comparison to Galactic models and to the expected number of contaminating distant red galaxies, and the spectral energy distribution in particular cases support this suggestion. The spatial distributions shown by members in different mass ranges can be interpreted in terms of the results from numerical simulations. According to these, different initial conditions and evolutionary dynamical paths are suggested for the clusters. Berkeley 94 would have formed under supervirial conditions, and followed the so-called warm collapse model in its evolution, whereas Berkeley 96 would have formed with a subvirial structure, and would have evolved following a cold collapse path. Both processes would be able to reproduce the suggested degree of mass segregation and their spatial distribution by mass range. Finally, the mass distributions of the clusters, from the most massive stars down to PMS stars around 1.3 M⊙, are calculated. An acceptable general agreement with the Salpeter initial mass function slope is found.