Revealing Potential Initial Mass Function Variations with Metallicity: JWST Observations of Young Open Clusters in a Low-metallicity Environment

• Published in: The Astrophysical journal 2024-11, Vol.975 (1), p.152
• Main Authors: Yasui, Chikako, Izumi, Natsuko, Saito, Masao, Lau, Ryan M., Kobayashi, Naoto, Ressler, Michael E.
• Format: Article
• Language: English
• Subjects: Brown dwarfs; Galaxies; Gas density; Globular clusters; Initial mass function; James Webb Space Telescope; Luminosity; Metallicity; Open clusters; Open star clusters; Spatial resolution; Star & galaxy formation; Star formation; Stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad73a2

We present the substellar mass function of star-forming clusters (≃0.1 Myr old) in a low-metallicity environment (≃−0.7 dex). We performed deep JWST/NIRCam and MIRI imaging of two star-forming clusters in Digel Cloud 2, a star-forming region in the outer Galaxy ( R G ≳ 15 kpc). The very high sensitivity and spatial resolution of JWST enable us to resolve cluster members clearly down to a mass detection limit of 0.02 M ⊙ , enabling the first detection of brown dwarfs in low-metallicity clusters. A total of 52 and 91 sources were extracted in mass- A V -limited samples in the two clusters, from which initial mass functions (IMFs) were derived by model-fitting the F200W band luminosity function, resulting in IMF peak masses (hereafter M C ) log M C / M ⊙ ≃ − 1.5 ± 0.5 for both clusters. Although the uncertainties are rather large, the obtained M C values are lower than those in any previous study ( log M C / M ⊙ ∼ − 0.5 ). Comparison with the local open clusters with similar ages to the target clusters (∼10 6 –10 7 yr) suggests a metallicity dependence of M C , with lower M C at lower metallicities, while the comparison with globular clusters, with similarly low metallicities but considerably older (∼10 10 yr), suggests that the target clusters have not yet experienced significant dynamical evolution and remain in their initial physical condition. The lower M C is also consistent with the theoretical expectation of the lower Jeans mass owing to the higher gas density under such low metallicity. The M C values derived from observations in such an environment would place significant constraints on the understanding of star formation.