Haffner 16 Redux: Revisiting a Young Cluster in the Outer Galaxy

TL;DR

This study revisits the young open cluster Haffner 16 using Gemini data, analyzing its stellar content, age, metallicity, and structure, revealing a diffuse halo and signs of rapid stellar rotation affecting the pre-main sequence stars.

Contribution

It provides new spectroscopic and photometric analysis of Haffner 16, including detailed CMD features, metallicity estimates, and the discovery of a surrounding stellar halo.

Findings

Cluster age ~ 20 Myr

Sub-solar metallicity indicated by spectra

Presence of a diffuse stellar halo

Abstract

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 samples the cluster main sequence (MS) and 5 magnitudes of the pre-MS (PMS). 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 that 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 ~ 20 Myr and a distance modulus of 12.3 +/- 0.2. A light profile of Haffner 16 is constructed in the WISE W1 filter which suggests that the cluster is surrounded by a diffuse stellar halo. Spectra are presented of candidate cluster MS and PMS stars selected according to location on the CMD. The spectra show characteristics that are suggestive of a sub-solar metallicity. Halpha 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.