Margaret Meixner

NGC 346 is a massive star-forming region located at a distance of ∼62 kpc, in the Small Magellanic Cloud. Due to its low metallicity ( Z ∼ 1/5 Z _⊙ ), it is an ideal environment to study star formation and stellar population analogs to those at Cosmic Noon. In this work, we produce a combined JWST NIRCam and MIRI photometric catalog of NGC 346. We characterise different stellar populations in the region: the upper main sequence (UMS), red giant branch (RGB), and red clump (RC), as well as pre-main-sequence (pre-MS) stars and young stellar objects (YSOs). We performed point-spread function-weighted photometry in 11 wavelength bands across NIRCam and MIRI and utilized multiple color–magnitude diagrams to identify the various stellar populations in the field. Our final photometric catalog of NGC 346 comprises 249,519 unique sources, including 2024 UMS stars, 2755 RGB stars, and 742 RC stars. In addition, we identified 6274 candidate pre-MS stars, 7350 candidate YSOs, and 23,819 IR-excess sources. Combining these three categories, we characterised 1583 strong and 3761 likely pre-MS/YSO candidates. By utilizing the F115W − F200W versus F115W − F187N color–color diagram, we found 239 nonspurious sources with Pa α excess, indicating accretion and star formation in NGC 346. Using JWST NIRCam and MIRI observations, we produced the deepest catalog to date of the star formation region NGC 346 in the near- and mid-IR range (1–21 μ m). Our catalog and characterisations of the young and old populations will provide the basis for detailed follow-up studies.

Context . The outer rings (ORs) of Supernova SN 1987A were ejected some 20 000 years before the explosion. Their characterisation is crucial for constraining the properties of the progenitor of this famous SN. Aims . While numerous studies have investigated the SN ejecta, equatorial ring (ER), and reverse shocks in detail, very few were dedicated to the ORs. We aim to fill this gap and investigate the ORs’ physical properties. We analysed data obtained over a long temporal period, from multiple instruments, and over a wide wavelength range from the optical to mid-infrared of the northern and southern ORs (NOR and SOR) of SN 1987A. We combined observations taken with the Hubble Space Telescope (HST) between 1994 and 2022, with VLT/MUSE in 2023, and with the James Webb Space Telescope (JWST) in 2022 and 2024. Methods . We measured emission flux in different regions of the ORs in narrow-band HST and broad- and narrow-band JWST/NIRCam filter images. We extracted optical and mid-infrared spectra for the ORs in the MUSE and JWST/MIRI/MRS data and measured emission-line fluxes. We analysed the evolution of the ORs clumps’ morphology over time with HST. Results . The optical light curves of the ORs have shown a steady decline with time over the last 30 years, both in the H α and [O III ] λ 5007 filters. This behaviour is expected as the ORs were ionised by the initial SN UV flash and have been fading since then. The observations do not show any sign of interaction of the SN ejecta with the ORs, which would appear as a brightening in the ORs’ optical light curve. We estimated the decay times for [O III ] to be 900 and 630–730 days for the NOR and SOR, respectively, and for H α +[N II ] to be 15 870 and 7160 days for the NOR and SOR, respectively. We provide the first mid-resolution spectra of the ORs in the optical and mid-infrared. We constrained the temperature from the optical [N II ] lines to 13 400–16 900 K and 11 800–14 500 K for the NOR and SOR, respectively. We constrained the electron density from the optical [S II ] lines to 610–670 cm ⁻³ and 720–790 cm ⁻³ for the NOR and SOR, respectively. Conclusions . The spectra of the ORs differ significantly from the spectrum of the ER, not only in the detected lines, but also in the line ratios of given lines. The ORs will likely keep on fading in the coming years until the SN ejecta sweep up the ORs. Continued monitoring of SN 1987A and its ring system in the X-ray to mid-infrared is essential to capture this instant.

Supernova (SN) 1987A provides a unique laboratory for investigating many aspects of SN physics and evolution. An observation at Day 12927 (35.4 yr) since the explosion with the Mid-Infrared Instrument (MIRI) Medium Resolution Spectrometer (MRS) on the James Webb Space Telescope provided the first spatially resolved spectroscopic study of SN 1987A in the mid-infrared (mid-IR), yielding insights into the evolution of dust, the ejecta, the equatorial ring (ER), and shocks in the system. Here we present a second epoch with MIRI/MRS at Day 13311 (36.4 yr), allowing the mid-IR spatially resolved spectroscopic temporal evolution of SN 1987A to be probed for the first time. Analysis of the ER-dominated dust continuum showed little evolution between Days 12927 and 13311. However, spatial analysis reveals that the inner ER is fading while the outermost regions are brightening. Broad ejecta emission lines detected at Day 12927 are evolving rapidly, driven by the recent onset of ejecta–ER interaction in the northeast and southwest portions of the ER. Most lines from the ER show no change during the 384 days between epochs, although some, such as [Ne ii ] and [Ar ii ], have faded. We identify mid-IR H ₂ emission associated with the ejecta for the first time. Using near- and mid-IR [Fe ii ] lines as density and temperature diagnostics of the ejecta in the interaction region, we find it likely that the dense, inner Fe-rich ejecta has now reached the reverse shock. Continued monitoring of SN 1987A is essential to observe the evolving ejecta–ER interaction and dust components.

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 Medium Resolution Spectroscopy mode of the Mid-Infrared Instrument on board JWST. The brightest and most active star-forming region in the SMC, NGC 346, has a metallicity of ∼1/5 Z _⊙ , analogous to the era when star formation in the early Universe ( z ≃ 2) peaked. We discuss the emission and absorption features present in the spectral energy distributions (SEDs) of five YSOs with coverage from 4.9 to 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 to 18.0 M _⊙ . 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 that range from 2.50 × 10 ⁻⁶ to 2.23 × 10 ⁻⁴ M _⊙ yr ⁻¹ , based on the H i (7–6) line emission. We present evidence for a ∼30,000 au protostellar jet traced by fine-structure, H i , and H ₂ emission about YSO Y535—the first mid-infrared detection of a resolved protostellar outflow in the SMC and the most distant yet detected by JWST.

We present JWST/MIRI spectra from the Medium-resolution Spectrometer of I Zw 18, a nearby dwarf galaxy with a metallicity of ∼3% Solar. Here, we investigate warm molecular hydrogen, H ₂ , observed in spectra extracted in ∼120 pc apertures centered on eleven regions of interest. We detect seven H ₂ rotational lines, some of which are among the weakest ever measured. The H ₂ population diagrams are fit with local-thermodynamic-equilibrium models and models of photodissociation regions. We also fit the ortho-/para-H ₂ ratios (OPRs); in three of the six regions for which it was possible to fit the OPR, we find values significantly greater than 3, the maximum value for local thermodynamic equilibrium. To our knowledge, although predicted theoretically, this is the first time that OPR significantly >3 has been measured in interstellar gas. We find that an OPR tends to increase with decreasing H ₂ column density, consistent with the expected effects of self-shielding in advancing photodissociation fronts. The population diagrams are consistent with H nucleon densities of ∼10 ⁵ cm ⁻³ , and an interstellar radiation field scaling factor, G ₀ , of ∼10 ³ . This warm, dense H ₂ gas coexists with the same highly ionized gas that emits [O IV] and [Ne V]. Emission from T ≳ 50 K dust is detected, including an as-yet-unidentified dust emission feature near 14 μ m; possible identification of Al ₂ O ₃ is discussed. The continuum emission from several regions requires that a considerable fraction of the refractory elements be incorporated in dust. Despite stacking spectra in the SE where H ₂ is found, no significant emission from polycyclic aromatic hydrocarbons is detected.

Using NIRSpec on JWST, we studied a sample of 15 intermediate-mass (1.8–4.1 M _⊙) young stellar objects (YSOs) previously identified with MIRI photometry in the low-metallicity NGC 346 star-forming cluster in the Small Magellanic Cloud. All objects, observed in the 1.7–5.3 μm range, show strong hydrogen recombination lines in the Paschen, Brackett, Pfund, and Humphreys series, confirming their very young ages. The spectra of 11 YSOs show prominent absorption bands from the three most important ice species (H₂O, CO₂, CO), marking the first detection of these ices in intermediate-mass YSOs beyond our Galaxy. In three YSOs, water ice appears to be in crystalline form. In some objects, we also detect ¹³CO₂ and OCS ices—never before observed beyond the Milky Way (MW)—and methanol ice in at least one star. We compared the column densities of H₂O, CO₂, and CO ices with those measured in more and less massive protostars in the MW and Large Magellanic Cloud, finding that, in NGC 346, ice column densities reach values nearly an order of magnitude lower than in more massive objects (∼1 × 10¹⁷ cm⁻² for water and ∼1 × 10¹⁶ cm⁻² for CO₂ and CO). However, the relative proportions of the ice species abundances do not differ from those in massive MW YSOs. This suggests that metallicity may not significantly affect ice chemistry in protoplanetary disks and that, shielded by the protostellar envelope or deep in the midplane, circumstellar material is likely impervious to the radiation environment.

We present JWST/MIRI spectra from the Medium-Resolution Spectrometer of I Zw 18, a nearby dwarf galaxy with a metallicity of ∼3% solar. Its proximity enables a detailed study of highly ionized gas that can be interpreted in the context of newly discovered high-redshift dwarf galaxies. We derive aperture spectra centered on 11 regions of interest; the spectra show very low extinction, A _V ≲ 0.1, consistent with optical determinations. The gas is highly ionized; we have detected 10 fine-structure lines, including [O iv] 25.9 μm with an ionization potential (IP) of ∼55 eV, and [Ne v] 14.3 μm with an IP of ∼97 eV. The ionization state of I Zw 18 falls at the extreme upper end of all of the line ratios we analyzed, but not coincident with galaxies containing an accreting massive black hole (active galactic nucleus). Comparison of the line ratios with state-of-the-art photoionization and shock models suggests that the high-ionization state in I Zw 18 is not due to shocks. Rather, it can be attributed to metal-poor stellar populations with a self-consistent contribution of X-ray binaries or ultra-luminous X-ray sources. It could also be partially due to a small number of hot low-metallicity Wolf−Rayet stars ionizing the gas; a small fraction (a few percent) of the ionization could come from an intermediate-mass black hole. Our spectroscopy also revealed four 14 μm continuum sources, ≳30–100 pc in diameter, three of which were not previously identified. Their properties are consistent with H ii regions ionized by young star clusters.

The first JWST observations of SN 1987A provided clear evidence that a compact object is ionizing the innermost ejecta. Here, we analyze a second epoch of JWST NIRSpec and MIRI/Medium-Resolution Spectrometer observations to better characterize the properties of this region, aided by a higher spectral resolving power for the new NIRSpec data. We confirm the presence of the previously identified narrow lines from the central region, i.e., ([Ar vi] 4.5292 μm, [Ar ii] 6.9853 μm, [S iv] 10.5105 μm, and [S iii] 18.7130 μm), and also identify similar components in [Ca v] 4.1585 μm, [Cl ii] 14.3678 μm, and possibly [Fe ii] 1.6440 μm. These lines are blueshifted by ∼−250 km s⁻¹, while the emission region is spatially unresolved and located southeast of the center. The offset and blueshift could imply a kick velocity of 510 ± 55 km s⁻¹ for the neutron star. We also identify [Ca iv] 3.2068 μm near the center, but it is displaced to the north and has a redshift of ∼700 km s⁻¹. We find that scattering by dust in the ejecta with a typical grain size ∼0.3 μm can explain the [Ca iv] properties and the absence of other narrow lines at shorter wavelengths, while dust absorption is important at λ ≳ 8 μm. Photoionization models for a pulsar wind nebula and a cooling neutron star are both compatible with the observations, with the exception of the [Fe ii] feature. The two models primarily differ at short wavelengths, where new lines are expected to emerge over time as the optical depth of dust in the expanding ejecta decreases.