Investigating the borderline between a young star cluster and a small stellar association: a test case with Bochum 1

Abstract

Context. Usually, a loose stellar distribution can be classified as an OB stellar group, an association, or a young open cluster. We compare data with the typical OB association Vul OB1. Aims. Here, we discuss the nature of Bochum 1, a typical example of an object affected by the above classification problem. Methods. Field-decontaminated 2MASS photometry is used to analyse colour–magnitude diagrams (CMDs) and stellar radial density profiles (RDPs) of the structures present in the region of Bochum 1. Results. The field-decontaminated CMD of Bochum 1 shows main sequence (MS) and pre-main sequence (PMS) stars. We report two new small angular-size, compact young clusters and one embedded cluster in the area of Bochum 1. Vul OB1 harbours the young open cluster NGC 6823 and the very compact embedded cluster Cr 404. The Vul OB1 association includes the H II region Sh2-86, and its stellar content is younger (≈3 Myr) than that of Bochum 1 (≈9 Myr), which shows no gas emission. Bochum 1 harbours one of the newly found compact clusters as its core. The RDP of Bochum 1 is irregular and cannot be fitted by a King-like profile, which suggests significant erosion or dispersion of stars from a primordial cluster. Similarly to Bochum 1, the decontaminated CMD of NGC 6823 presents conspicuous MS and PMS sequences. Taken separately, RDPs of MS and PMS stars follow a King-like profile. The core shows an excess density of MS stars that mimics the profile of a post-core collapse cluster. At such a young age, it can be explained by an excess of stars formed in the prominent core. Conclusions. The present study suggests that Bochum 1 is a star cluster fossil remain that might be dynamically evolving into an OB association. Bochum 1 may be a missing link connecting early star cluster dissolution with the formation of low-mass OB associations.