Jaeyeong Kim

We present two-epoch JWST NIRSpec and MIRI observations of the young protostar EC 53 (V371 Ser), a periodically variable source with well-characterized quiescent and burst phases. The spectra in both epochs show absorption in the CO overtone (∼2.3 μ m) and fundamental (∼4.6 μ m) bands and the H ₂ O stretching (∼2.7 μ m) and bending (∼6.0 μ m) modes. We also obtained high-resolution ( R ≈ 45,000) IGRINS spectra during the burst to constrain the CO overtone line profiles. Local thermodynamic equilibrium (LTE) slab modeling yields gas temperatures of ∼1800 K (CO overtone) and ∼1200 K (CO fundamental), consistent with the overtone tracing hotter gas at smaller radii. The H ₂ O stretching-mode absorption shows no compelling evidence for variability, and the current JWST CO overtone data do not provide a robust constraint on overtone variability. In contrast, the CO fundamental and H ₂ O bending-mode features weaken by a factor of ∼2 during the burst, which is most naturally explained by continuum changes rather than large variations in absorbing gas. To quantify continuum dilution, we introduce a “relative veiling” that treats the quiescent spectrum as an internal reference and measures the change in the continuum excess between the two epochs. This formalism yields burst-to-quiescent hot-continuum ratios of 2.9 ± 0.2 for the CO overtone and 1.71 ± 0.11 for the CO fundamental. Using a viscous-disk prescription, these imply representative accretion-rate ratios of ∼3.6 and ∼2.0, respectively. The differing ratios suggest that inner-disk regions traced at different temperatures, and thus radii, respond differently across the burst cycle, consistent with episodic mass buildup in the inner disk during quiescence followed by more efficient transport through the innermost disk onto the protostar during the burst.

We present the 1.6–28 μ m spectra of the young protostar EC 53, obtained with the James Webb Space Telescope (JWST) Near Infrared Spectograph Integral Field Unit and MIRI Medium Resolution Spectroscopy during the quiescent and burst phases of its periodic brightness variations. To isolate ice absorption features, we modeled and removed the mid-infrared silicate dust absorption using a dedicated continuum-fitting procedure. In the optical depth spectrum, we first fit the broad H ₂ O ice features and then decomposed the major ice components, including NH ₃ , CO ₂ , CH ₃ OH, CO, and CH ₄ , by matching laboratory profiles for both pure and H ₂ O-mixed ices. The 4.62 μ m and 6.85 μ m bands are attributed to OCN ^– and NH ₄ ⁺ ions, respectively. Minor or tentative contributions from complex species (HCOOH, H ₂ CO, CH ₃ COOH, CH ₃ CHO, CH ₃ CH ₂ OH, and NH ₂ CHO) are also considered for our global ice analysis. The silicate-corrected spectra reveal no measurable change in any ice absorption band between the two phases, indicating that moderate and short-period accretion bursts in EC 53 do not significantly alter the physical or chemical state of the ices within its envelope. The derived abundances of these major species relative to H ₂ O significantly exceed the values typically observed toward other embedded protostars. Finally, we place the ice inventory of EC 53 in the context of other protostellar systems observed with JWST, highlighting that its chemically rich, thermally quiescent ice reservoir provides a benchmark for studying ice evolution under episodic accretion.

SPHEREx is a NASA mission designed to perform an all-sky spectroscopic survey in the 0.75–5 μ m wavelength range. Its primary science objectives are to investigate: (1) inflationary cosmology; (2) the history of galaxy formation; and (3) the abundance of molecular ices—critical for prebiotic chemistry—found on the surfaces of interstellar dust grains within planet-forming regions. This paper focuses on the third theme, the SPHEREx Ices Investigation, for which SPHEREx is conducting a spectroscopic survey of nearly 10 million preselected sources throughout the Milky Way and Magellanic Clouds, to characterize their ice absorption features. By selecting targets based on infrared color, spatial isolation, and brightness, the Ices Investigation secures high-signal-to-noise-ratio spectra across a broad range of astrophysical environments that are relatively free of spectral contamination. Rather than attempting to decompose each spectrum into its individual ice components, the Ices Investigation prioritizes accurate measurements of the integrated optical depths of key molecular ice absorption features. This approach enables statistically powerful correlation studies between ice abundances and environmental parameters—including extinction, temperature, gas composition, radiation field strength, cosmic-ray flux, and star formation activity. The data pipeline developed for this purpose incorporates machine learning for continuum estimation, drawing on both SPHEREx and ancillary data sets. Ultimately, the expansive spectral archive produced by SPHEREx, combined with targeted follow-up from facilities like JWST, will transform our understanding of Galactic ice formation, evolution, abundance, and their inheritance into planetary systems and prebiotic inventories.

We present some of the first infrared spectral maps acquired by SPHEREx. These maps, which to our knowledge are the largest of their type ever compiled in the near-infrared, reveal multiple strong lines due to interstellar ices and polycyclic aromatic hydrocarbons (PAHs) throughout the Cygnus X and North American Nebula regions. The maps emphasize the strongest features arising from the 3 μ m H ₂ O, 4.27 μ m CO ₂ , and 4.67 μ m CO lines and the 3.28 μ m PAH feature, all of which are detected over large areas with complex and filamentary spatial distributions. The ice absorption maps of H ₂ O and CO ₂ in particular broadly trace dense, cold, and well-shielded regions across Cygnus X, consistent with the established picture of efficient ice formation in dense molecular clouds. The interstellar ice features are also detected abundantly in diffuse absorption over wide areas. The relative strengths of the H ₂ O and CO ₂ features vary among different lines of sight, indicating possible differences in local physical conditions or chemical variations. The 3.28 μ m PAH emission correlates with the emission from the 7.7 and 11.2 μ m features but shows small differences that may trace the grain-size distribution and variations in the ambient UV field. SPHEREx all-sky spectral imaging—only a small fraction of which is showcased in this work—will support numerous science investigations, including the structure of the Galaxy, the physics of the interstellar medium, and the chemistry of stars.

Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx), a NASA Explorer satellite launched on 2025 March 11, is carrying out the first all-sky near-infrared spectral survey. The satellite observes in 102 spectral bands from 0.75 to 5.0 μ m with a resolving power ranging from λ /Δ λ = 35–130 in 6 . ″ 2 pixels. The observatory obtains a 5 σ depth of 19.5–19.9 AB mag for 0.75 < λ < 3.8 μ m with λ /Δ λ ∼ 40 and 17.8–18.8 AB mag for 3.8 < λ < 5.0 μ m with λ /Δ λ ∼ 120 after mapping the full sky four times over two years. Scientifically, SPHEREx will produce a large galaxy redshift survey over the full sky to constrain the amplitude of inflationary non-Gaussianity. The observations will produce two deep spectral maps near the ecliptic poles that use intensity mapping to probe the evolution of galaxies over cosmic history. By mapping the depth of infrared absorption features over the Galactic plane, SPHEREx will comprehensively survey the abundance and composition of water and other biogenic ice species in the interstellar medium. The project will release initial data rapidly in the form of spectral images, and specialized data products over the life of the mission as the surveys proceed. The science team will also produce spectral catalogs of planet-bearing and low-mass stars, solar system objects, and galaxy clusters three years after launch. We describe the design of the instrument and spacecraft, which flow from the core science requirements. Finally, we present an initial evaluation of the satellite’s in-flight performance and key characteristics.

Sulfur dioxide (SO ₂ ) ice has been tentatively detected in protostellar envelopes, but its reliability as a solid-state sulfur reservoir remains unclear. We present new measurements of SO ₂ ice from 6.8–8.5 μ m toward four Class 0 protostars observed with JWST’s Mid-Infrared (MIRI) Medium Resolution Spectrometer, as part of the COMs ORigin Investigated by the Next-generation Observatory in Space (CORINOS) program. The sample spans a luminosity range from 1 L _⊙ (B335, IRAS 15398–3359) to 10 L _⊙ (L483, Ser-emb 7). To assess continuum placement uncertainty in absorption spectra, we apply randomized polynomial fits over the restricted region. We fit laboratory spectra from the Leiden Ice Database for Astrochemistry using the open-source Python library Omnifit. We detect the 7.7 μ m CH ₄ band in all sources and find its column density robust to baseline choice, providing a reference for evaluating the weaker SO ₂ feature on its blue shoulder and quantifying baseline-related uncertainty. Three SO ₂ laboratory ices were tested: pure SO ₂ ice yields 0.3%–1.2% of volatile sulfur may be locked in SO ₂ ice (lower and upper limits); CH ₃ OH:SO ₂ ice gives 0.02%–0.18%, but with lower quality fitting. The best-fitting H ₂ O:SO ₂ ice yields 0.2%–0.9%, which we consider the most realistic. These ranges define plausible bounds on SO ₂ ice abundances in our sample. We find evidence for SO ₂ in Ser-emb 7, L483, and IRAS 15398–3359, but emphasize the noisy spectrum of B335 prevents a definitive detection. Comparing SO ₂ ice abundances across the different environments, we assess how conditions influence the role of SO ₂ as a potential sulfur reservoir and implications for the longstanding “missing sulfur” problem.

While molecular outflows have been studied in detail with radio interferometry, observations of the hotter gas in protostellar outflows at a comparable physical scale are often challenging. Combined with the Atacama Large Millimeter/submillimeter Array (ALMA), JWST allows us to investigate the cold and hot gas with unprecedented spatial resolution and sensitivity. We present a detailed comparison between the gas distributions probed with ALMA and JWST in the primary outflow of IRAS 15398−3359. At 2000 au scale, the southwestern outflow shows four shell structures in 5–10 μm continuum, whereas the submillimeter H₂CO emission traces two of the four shells closest to the protostar. Submillimeter emission from CS, CCH, c–C₃H₂, and CH₃OH shows the same two shells, and the ¹²CO emission covers most of the outflow region. SO and SiO only trace a condensation at the edge of the shell closest to the protostar. None of these lines observed with ALMA show the outermost shell. At 500 au scale, we find hot H₂ gas inside the outflow cavity with JWST. The derived temperature of H₂ is 1147 ± 198 K within a 0 . ″ 5 aperture at the protostar. The foreground mass column density of dust is (1.4–2.0) × 10⁻³ g · cm⁻² (A_V = 47–66 mag) in the outflow, using the dust model from J. C. Weingartner & B. T. Draine (2001). We also find an 8^∘ difference between the directions toward the [Fe ii] knot and the outermost shell in the MIRI image, which may be interpreted as the precession of the [Fe ii] jet. The dynamical timescale of the [Fe ii] knot is 10 yr, suggesting a current event.

One of the primary objectives of the SPHEREx mission is to understand the origin of molecules such as H2O, CO2, and other volatile compounds at the early stages of planetary system formation. Because the vast majority of these compounds -- typically exceeding 95% -- exist in the solid phase rather than the gaseous phase in the systems of concern here, the observing strategy planned to characterize them is slightly unusual. Specifically, SPHEREx will target highly obscured sources throughout the Milky Way, and observe the species of concern in absorption against background illumination. SPHEREx spectrophotometry will yield ice column density measurements for millions of obscured Milky Way sources of all ages and types. By correlating those column densities with source ages, the SPHEREx mission will shed light on whether those molecules were formed in situ along with their nascent stellar systems, or whether instead they formed elsewhere and were introduced into those systems after their formation. To that end, this work describes version 7$.$1 of the SPHEREx Target List of Ice Sources (SPLICES) for the community. It contains about 8$.$6 million objects brighter than W2~12 Vega mag over much of the sky, principally within a broad strip running the length of the Milky Way midplane, but also within high-latitude molecular clouds and even the Magellanic Clouds.

We present James Webb Space Telescope (JWST) Mid-InfraRed Instrument (MIRI) observations of warm CO and H₂O gas in emission toward the low-mass protostar IRAS 15398-3359, observed as part of the CORINOS program. The CO is detected via the rovibrational fundamental band and hot band near 5 μm, whereas the H₂O is detected in the rovibrational bending mode at 6–8 μm. Rotational analysis indicates that the CO originates in a hot reservoir with an excitation temperature of 1598 ± 118 K, while the water is much cooler at 204 ± 7 K. Neither the CO nor the H₂O line images are significantly spatially extended, constraining the emission to within ∼40 au of the protostar. The compactness and high temperature of the CO are consistent with an origin in the embedded protostellar disk, or in a compact disk wind. In contrast, the water must arise from a cooler region and requires a larger emitting area (compared to the CO) to produce the observed fluxes. The water may arise from a more extended part of the disk, or from the inner portion of the outflow cavity. Thus, the origin of the molecular emission observed with JWST remains ambiguous. Better constraints on the overall extinction, comparison with realistic disk models, and future kinematically resolved observations may all help to pinpoint the true emitting reservoirs.