Berkeley 94 and Berkeley 96: Two Young Clusters with Different Dynamical Evolution

TL;DR

This study compares two young star clusters, Berkeley 94 and Berkeley 96, revealing differences in their dynamical evolution, initial conditions, and mass segregation, supported by photometric data and numerical simulations.

Contribution

It provides the first detailed comparison of the dynamical evolution and initial conditions of these two young clusters using multi-band photometry and simulations.

Findings

Berkeley 94 formed under supervirial conditions, following warm collapse.

Berkeley 96 formed with subvirial conditions, following cold collapse.

Both clusters show mass segregation consistent with their dynamical histories.

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 LogAge(yr)=7.5+-0.07, and 7.0+-0.07, respectively. A sequence of optical 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. According to the results from numerical simulations, the spatial distributions of members in different mass ranges are interpreted as suggesting different initial conditions and evolutionary dynamical paths 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 Msun, are calculated. An acceptable general agreement with the Salpeter IMF slope is found.