A Mid-Infrared Spectroscopic Study of Young Stellar Objects in the SMC Region NGC 346: JWST Detects Dust, Accretion, Ices and Outflows

TL;DR

This study uses JWST/MIRI MRS to obtain mid-infrared spectra of intermediate- to high-mass YSOs in NGC 346, a low-metallicity region in the SMC (~$1/5\,Z_{\odot}$). By combining MRS data with archival photometry and applying SED fitting, the authors derive stellar masses up to ~$18\,M_{\odot}$ and identify a suite of dust, PAH, and ice features, along with hydrogen recombination and molecular lines. They report detections of protostellar outflows, including a resolved jet-like structure around Y535 extending to ~30,000 AU, and find accretion rates on the order of ~$10^{-2}$ to ~$10^{-1}\,M_{\odot}$ yr$^{-1}$, with indications that younger, cooler YSOs accrete more rapidly. The results demonstrate JWST’s power to characterize the physical conditions and evolutionary states of YSOs in low-metallicity environments, informing star-formation processes during cosmic noon and the role of metallicity in dust, ice, and outflow phenomena.

Abstract

We present mid-infrared spectroscopic observations of intermediate- to high-mass young stellar objects (YSOs) in the low-metallicity star-forming region NGC 346 located within the Small Magellanic Cloud (SMC). We conduct these integral-field-unit observations with the Mid-Infrared Instrument Medium Resolution Spectroscopy instrument on board JWST. The brightest and most active star-forming region in the SMC, NGC 346 has a metallicity of $\sim$1/5 $Z_{\odot}$, analogous to the era when star formation in the early Universe ($z$$\simeq$2) peaked. We discuss the emission and absorption features present in the spectral energy distributions (SEDs) of five YSOs with coverage from 4.9-27.9$μ$m and three other sources with partial spectral coverage. Via SED model-fitting, we estimate their parameters, finding masses ranging from 2.9-18.0 M$_{\odot}$. These targets show dusty silicates, polycyclic aromatic hydrocarbons and ices of CO$_2$, CO, H$_2$O and CH$_3$OH in their protostellar envelopes. We measure emission from H$_2$ and atomic fine-structure lines, suggesting the presence of protostellar jets and outflows. We detect H I lines indicating ongoing accretion and estimate accretion rates for each source which range from 2.50x10$^{-6}$-2.23x10$^{-4}$ M$_{\odot}$yr$^{-1}$ based on H I (7-6) line emission. We present evidence for a $\sim$30,000AU protostellar jet traced by fine-structure, H I and H$_2$ emission about the YSO Y535, the first detection of a resolved protostellar outflow in the SMC, and the most distant yet detected.

Figures (20)

• Figure 1: JWST multicolor MIRI image of NGC 346 combining the F770W (blue), F1000W (cyan), F1130W (green), F1500W (yellow), and F2100W (red) filters of NGC 346. The locations of the four MIRI/MRS pointings are marked with red circles. NASA, ESA, CSA, STScI, N. Habel (JPL, Caltech). Image processing: Alyssa Pagan (STScI) and Patrick Kavanagh (Maynooth University).
• Figure 2: JWST NIRCam F444W imaging of the field surrounding our four MRS observations. The FOV of the four MIRI MRS channels (1, 2, 3, and 4) is shown in blue, green, yellow, and red, respectively.
• Figure 3: JWST MIRI F770W images of Y544 and Y533 showing their multiple components. Y544 is composed of two sources, Y544A & B. Source Y533 is composed of Y533A, Y533B, Y533C, Y533D & Y533E. Constituent sources are roughly ordered by decreasing mid-IR luminosity.
• Figure 4: Six slices of the IFU cube targeting Y535: H$_2$ 0-0 S(7) emission at 5.514 $\mu$m (top left), 6.2 $\mu$m PAH continuum emission (top middle), 11.2 $\mu$m PAH continuum emission (top right), H1 7-6 emission at at 12.379 $\mu$m (bottom left), [Ne2] emission at 18.821 $\mu$m (bottom middle) and H$_2$ 0-0 S(1) emission at 17.044 $\mu$m (bottom right). Contours levels for all panels are at 2500, 5000, 10000, 25000, and 50000 MJy sr$^{-1}$. PAH slices are a width of 0.2 $\mu$m centered at the given wavelength.
• Figure 5: Six slices of the IFU cube targeting Y544: H$_2$ 0-0 S(7) emission at 5.514 $\mu$m (top left), 6.2 $\mu$m PAH continuum emission (top middle), 11.2 $\mu$m PAH continuum emission (top right), H1 7-6 emission at at 12.379 $\mu$m (bottom left), [Ne2] emission at 18.821 $\mu$m (bottom middle) and H$_2$ 0-0 S(1) emission at 17.044 $\mu$m (bottom right). Contours levels for all panels are at 300, 400, 500, 1000, 250 and 4000 MJy sr$^{-1}$. PAH slices are a width of 0.2$\mu$m centered at the given wavelength.