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

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 $μ$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 $μ$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$α$, 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.

Figures (21)

• Figure 1: Zoomed-in pseudo-color images of the Cloud 2-N and 2-S clusters obtained with JWST/NIRCam LW and MIRI, produced by combining the NIRCam images F356W (blue) and F444W (green), and the MIRI image F770W (red), representing the MIR band. North is up and east is to the left. The image areas, each with a field of view of approximately $1' \times 1'$, which correspond to the white squares shown in Figures 1 and 2 of Yasui2024, and cover the same sky region as Figure 3 in the reference. The positions of the cluster regions are shown as green ellipses.
• Figure 2: MIRI-NIRCam color--magnitude diagram, $(m_{\rm F356W} - m_{\rm F770W})$ vs. $m_{\rm F770W}$, for the Cloud 2-N and 2-S clusters. All sources detected with more than 10$\sigma$ in F356W and F770W bands are plotted. Class I, II, EV, and III objects are represented by black filled circles enclosed in circles, black filled circles, encircled plus symbols, and black open circles, respectively. The legend shown in the left panel applies to both panels and will be used consistently in subsequent figures. Isochrone tracks for the age of 0.1 Myr are shown with blue curves. Large tick marks on the isochrones denote masses of 3, 1, and 0.1 $M_\odot$ (from top to bottom), and the smaller tick marks below them denote 0.09, 0.08, …, 0.02 $M_\odot$. The sensitivity limits, corresponding to the 10$\sigma$ limiting magnitudes of the F770W and F356W bands, are indicated by dashed lines. Typical photometric uncertainties are indicated by thick gray error bars in the lower-right corners of both panels. The red arrows show the reddening vectors of $A_V = 10$ mag.
• Figure 3: NIRCam LW color--magnitude diagram, $(m_{\rm F356W} - m_{\rm F444W})$ vs. $m_{\rm F356W}$, for the Cloud 2-N and 2-S clusters. All sources detected with more than 10$\sigma$ in both F356W and F444W bands are plotted. Isochrone tracks for the age of 0.1 Myr are shown with blue curves. Tick marks on the isochrones are the same as in Figure \ref{['fig:CM_MIRI_NIRCam']}. The sensitivity limits, corresponding to the 10$\sigma$ limiting magnitudes of the F356W and F444W bands, are indicated by dashed lines. Typical photometric uncertainties are indicated by thick gray error bars in the lower-right corners of both panels. The red arrows show the reddening vectors of $A_V = 10$ mag.
• Figure 4: F405N color--magnitude diagram, $(m_{\rm F356W} - m_{\rm F405N})$ vs. $m_{\rm F405N}$, for the Cloud 2-N and 2-S clusters. All sources detected with more than 10$\sigma$ in both F356W and F405N bands are plotted. The symbols are the same as in Figure \ref{['fig:CM_MIRI_NIRCam']}, based on the classification from the MIR SED slope ($\alpha$). Isochrone tracks for the age of 0.1 Myr are shown with blue curves. Tick marks on isochrones are the same as in Figure \ref{['fig:CM_MIRI_NIRCam']}. The sensitivity limits, corresponding to the 10$\sigma$ limiting magnitudes of the F405N bands, are indicated by dashed lines. Typical photometric uncertainties are indicated by thick gray error bars in the lower-right corners of both panels. The red arrows show the reddening vectors of $A_V = 10$ mag.
• Figure 5: MIRI-NIRCam color--color diagram, $(m_{\rm F444W} - m_{\rm F770W})$ vs. $(m_{\rm F356W} - m_{\rm F444W})$, for the Cloud 2 clusters. The symbols are the same as in Figure \ref{['fig:CM_MIRI_NIRCam']}, based on the classification from the MIR SED slope ($\alpha$). The blue curve represents the dwarf track, defined as the locus of points corresponding to unreddened main-sequence stars. Typical photometric uncertainties are indicated by thick gray error bars in the lower-right corners of both panels.