LZ-STAR Survey: Low-metallicity star formation survey of Sh2-284

II. The initial mass function

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
² Indian Institute of Science Education and Research (IISER), Mohali, India
³ Department of Space, Earth & Environment, Chalmers University of Technology, 45293 Gothenburg, Sweden
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ Space Telescope Science Institute, Baltimore, MD, USA
⁷ Department of Astronomy, Univ. of Virginia, Charlottesville, VA 22904, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 May 2025
Accepted: 19 May 2026

Abstract

Context. To fully understand the star formation process, we are compelled to study it in a variety of environments. Of particular interest are how star formation and the resulting initial mass function (IMF) vary as a function of metallicity.

Aims. Using JWST/NIRCam, we observed an embedded young cluster in the vicinity of Sh2-284 (hereafter S284), the HII region associated with the open cluster Dolidze 25, with the aim of studying star formation in a metal-poor (i.e., about 1/3 of solar) environment. In particular, we aim to measure the peak of the IMF.

Methods. Using JWST/NIRCam photometry, we identified the embedded cluster S284-EC1 and resolved its low-mass content. Via a comparison with pre-main-sequence evolutionary tracks, we determined the mass and extinction for the individual cluster members. Extinction-limited samples were created based on the distribution of extinction and the completeness of the data. For the region with a completeness of 60% or higher, we fit a log-normal distribution to the IMF.

Results. Adopting an age of 1 Myr for the cluster members, supported by the high extinction and outflow activity in the region, the peak of the IMF is at m_c = 0.17 ± 0.02M_⊙. This value lies toward the low-mass end of the range reported for local young clusters in the literature However, the result is sensitive to the age of the cluster. If an older age of 2 Myr is adopted, the peak mass increases to 0.28 M_⊙.

Conclusions. We have found evidence of IMF variation as a function of metallicity, i.e., the peak of the IMF shifts to lower masses as one goes from solar to 1/3 solar metallicity. However, we caution that the result is sensitive to the assumed age of the stellar population, i.e., with peak mass rising if an age older than 1 Myr is adopted. This study further motivates the need for expanded samples of low-metallicity regions and their content to enable more comprehensive measures of the IMF in such environments.