The embedded cluster DBSB 48 in the nebula Hoffleit 18: Comparison with Trumpler 14

Abstract

We derive fundamental parameters of the embedded cluster DBSB 48 in the southern nebula Hoffleit 18 and the very young open cluster Trumpler 14, by means of deep JHK s infrared photometry. We build colour–magnitude and colour–colour diagrams to derive reddening and age, based on main sequence and pre-main sequence distributions. Radial stellar density profiles are used to study cluster structure and guide photometric diagram extractions. Field-star decontamination is applied to uncover the intrinsic cluster sequences in the diagrams. Ages are inferred from K-excess fractions. A prominent pre-main sequence population is present in DBSB 48, and the K-excess fraction f K = 55 ± 6% gives an age of 1.1 ± 0.5 Myr. A mean reddening of A K s = 0.9 ± 0.03 was found, corresponding to A V = 8.2 ± 0.3. The cluster CMD is consistent with the far kinematic distance of 5 kpc for Hoffleit 18. For Trumpler 14 we derived similar parameters as in previous studies in the optical, in particular an age of 1.7 ± 0.7 Myr. The fraction of stars with infrared excess in Trumpler 14 is f K = 28 ± 4%. Despite the young ages, both clusters are described by a King profile with core radii R core = 0.46 ± 0.05 pc and R core = 0.35 ± 0.04 pc, respectively, for DBSB 48 and Trumpler 14. Such cores are smaller than those of typical open clusters. Small cores are probably related to the cluster formation and/or parent molecular cloud fragmentation. In DBSB 48, the magnitude extent of the upper main sequence is Δ K s ≈ 2 mag, while in Trumpler 14 it is Δ K s ≈ 5 mag, consistent with the estimated ages.