Protoplanetary Disk Evolution in a Low-Metallicity Environment: JWST's First Mid-Infrared Census of Low-Mass Stars

TL;DR

This paper reports the first high-resolution mid-infrared study of protoplanetary disks in a low-metallicity environment using JWST, revealing that disk properties are surprisingly similar to those in solar-metallicity regions despite reduced dust content.

Contribution

It provides novel JWST observations of low-metallicity star-forming regions, demonstrating that young disks retain high fractions of optically thick disks and active accretion rates comparable to solar-metallicity environments.

Findings

Approximately 75% of sources retain optically thick disks.

Lack of 2 μm excess suggests diminished inner disk emission.

Brown dwarf candidates show a high disk fraction (~75%).

Abstract

This study presents the first high-resolution, high-sensitivity mid-infrared (MIR) investigation of protoplanetary disks in a low-metallicity environment, using JWST/NIRCam and MIRI observations of Digel Cloud 2, a star-forming region in the outer Galaxy ($D \simeq 8$ kpc, ${\rm [M/H]} \simeq -0.7$ dex). It hosts two very young ($\sim$0.1 Myr) embedded clusters, Cloud 2-N and Cloud 2-S, offering a window into disk evolution under conditions analogous to the early universe, where low metallicity implies reduced dust content. Imaging across 1-20 $\mu$m, including F770W and complementary bands (F356W, F444W, F405N), enables probing disk properties with unprecedented spatial resolution and stellar mass sensitivity down to $\sim$0.1 $M_\odot$. Among 89 and 95 sources detected in F770W in Cloud 2-N and 2-S, respectively, we identify candidate stellar-mass cluster members using infrared photometry, from which stellar mass and extinction are estimated. Among these, $\simeq$75 % retain optically thick disks in both clusters based on MIR SED slopes, consistent with similarly aged solar-metallicity regions. In contrast, a lack of 2 $\mu$m excess suggests diminished inner disk emission, possibly due to enhanced silicate grains with low sublimation temperatures. Using the F405N narrow-band filter covering Br$\alpha$, we detect accretion signatures in $\simeq$35 % of sources selected by extinction criteria, with rates $\gtrsim$10$^{-6}$ $M_\odot$ yr$^{-1}$, comparable to or exceeding those in nearby low-mass stars. Brown dwarf candidates, identified across multiple bands including F770W and shorter wavelengths, exhibit a high disk fraction of $\sim$75 %, indicating robust disk retention across mass ranges even under low-metallicity conditions.