Haffner 16 redux: revisiting a young cluster in the outer galaxy.
Abstract (from CDS):
Images and spectra recorded with the Gemini Multi-Object Spectrograph on Gemini South are used to investigate the stellar content of the open cluster Haffner 16. The (i′ ,g′ -i′ ) color-magnitude diagram (CMD) constructed from these data extends over 10 mag in i′ , sampling the cluster main sequence (MS) and 5 mag of the pre-MS (PMS). The fraction of unresolved equal mass binaries among PMS stars is estimated to be 0.6 ± 0.1. The isochrones do not track the PMS on the CMD, in the sense that the PMS has a shallower slope on the CMD than predicted by the models. Still, a dip in star counts, which is associated with the relaxation of PMS stars onto the MS, is identified near i′ =17. The depth and brightness of this feature-as well as the morphology of the cluster MS on the CMD-are matched by models with a slightly sub-solar metallicity that have an age of ∼20 Myr and a distance modulus of 12.3 ± 0.2. A light profile of Haffner 16 is constructed in the W1 filter (λcen=3.4 µm), which suggests that the cluster is surrounded by a diffuse stellar halo. Spectra of candidate cluster MS and PMS stars selected according to location on the CMD are presented. The spectra show characteristics that are suggestive of a sub-solar metallicity. Hα emission is common among objects on the PMS locus on the CMD near i′ =18. It is suggested that the location of the Haffner 16 PMS on the CMD is affected by large-scale cool spot activity, likely induced by rapid stellar rotation.