Mass function of a young cluster in a low-metallicity environment. Sh 2-209

TL;DR

This study presents deep near-infrared imaging of the low-metallicity HII region S209, revealing two star-forming clusters, their initial mass functions, and demonstrating the ability to accurately derive IMFs in such environments.

Contribution

First detailed derivation of the initial mass function in a low-metallicity environment using high-resolution NIR imaging and Gaia data.

Findings

IMFs show slightly flatter high-mass slopes than Salpeter.

Break mass of IMFs is around 0.1 solar masses, lower than in the solar neighborhood.

Main cluster contains approximately 1500 members, enabling precise IMF analysis.

Abstract

We present deep near-infrared (NIR) imaging of Sh 2-209 (S209), a low-metallicity (${\rm [O/H]} = - 0.5$ dex) HII region in the Galaxy. From the NIR images, combined with astrometric data from Gaia EDR3, we estimate the distance to S209 to be 2.5 kpc. This is close enough to enable us to resolve cluster members clearly ($\simeq$1000 AU separation) down to a mass-detection limit of $\simeq$0.1 $M_\odot$, and we have identified two star-forming clusters in S209, with individual cluster scales $\sim$1 pc. We employ a set of model luminosity functions to derive the underlying initial mass functions (IMFs) and ages for both clusters. The IMFs we obtained for both clusters exhibit slightly flat high-mass slopes ($\Gamma \simeq -1.0$) compared to the Salpeter IMF ($\Gamma = -1.35$), and their break mass of $\simeq$0.1 $M_\odot$ is lower than those generally seen in the solar neighborhood ($\sim$0.3 $M_\odot$). In particular, because the S209 main cluster is a star-forming cluster with a larger number of members ($\sim$1500) than the number ($\sim$100) in regions previously studied in such environments, it is possible for the first time to derive the IMF in a low-metallicity environment with high accuracy over the wide mass range 0.1--20 $M_\odot$.