Abstract Water and methanol are key components of interstellar ices and gas in star- and planet-forming regions, but direct observations of water in low-mass protostars are challenging due to atmospheric absorption. We present high-resolution ( R = 70,500) mid-infrared spectroscopy of the class I protostar SVS13-A with EXES on board SOFIA at 26 μ m, targeting both H 2 O and CH 3 OH absorption lines. Several lines of each species are detected, tracing warm gas with rotational temperatures of ∼140–170 K. Remarkably, the methanol column density is a factor of ∼4 higher than that of water, well above typical interstellar ice ratios (<10%). Comparison with previous millimeter observations indicates that absorption and emission probe distinct regions, with the mid-IR lines likely tracing cooler gas along the line of sight. The surprising observed CH 3 OH/H 2 O ratio may reflect selective sublimation due to the distribution of binding energies or ice stratification in the inner envelope. These observations probe the inner regions of the protostar, where planets are expected to form and inherit the chemical composition of their natal environment, providing a direct link between ice sublimation and gas-phase chemistry. Our results represent the first high-spectral-resolution mid-infrared view of both water and methanol toward a low-mass protostar, offering a unique window into the chemical composition of the innermost envelope and planet-forming region and highlighting the diagnostic power of high-resolution mid-infrared spectroscopy to uncover hidden chemical layers and the ice-to-gas transition in embedded protostars.

Water and methanol are key components of interstellar ices and gas in star- and planet-forming regions, but direct observations of water in low-mass protostars are challenging due to atmospheric absorption. We present high-resolution (R = 70,500) mid-infrared spectroscopy of the Class I protostar SVS13-A with EXES on board SOFIA at 26 μm, targeting both H_2O and CH_3OH absorption lines. Several lines of each species are detected, tracing warm gas with rotational temperatures of ∼140–170 K. Remarkably, the methanol column density is a factor of ∼4 higher than that of water, well above typical interstellar ice ratios (<10%). Comparison with previous millimeter observations indicates that absorption and emission probe distinct regions, with the mid-IR lines likely tracing cooler gas along the line of sight. The surprising observed CH_3OH/H_2O ratio may reflect selective sublimation due to the distribution of binding energies or ice stratification in the inner envelope. These observations probe the inner regions of the protostar, where planets are expected to form and inherit the chemical composition of their natal environment, providing a direct link between ice sublimation and gas-phase chemistry. Our results represent the first high-spectral-resolution mid-infrared view of both water and methanol toward a low-mass protostar, offering a unique window into the chemical composition of the innermost envelope and planet-forming region, and highlighting the diagnostic power of high-resolution mid-infrared spectroscopy to uncover hidden chemical layers and the ice-to-gas transition in embedded protostars.

We present the first Hubble Space Telescope - Space Telescope Imaging Spectrograph (HST-STIS) far-ultraviolet (FUV) and near-ultraviolet (NUV) high-resolution spectra of the red supergiant (RSG) Antares (M1.5 Iab), isolated from its long-period B-star binary companion, and compare them to spectra of Betelgeuse (M2 Iab) to assess the similarities and differences in chromospheric heating and atmospheric dynamics. While Betelgeuse has two well-known circumstellar CO outflows, previous radio and infrared observations of Antares did not detect the presence of circumstellar CO. The Antares FUV STIS spectra reveal CO electronic Fourth-Positive absorption bands formed exterior to the chromosphere. The derived column density is 4 orders of magnitude smaller than the CO MOLsphere located interior to 1.7R*. The numerous NUV Fe ii emission lines show classical wind-scattering profiles, with velocities close to the star of vwind similar or equal to 19 km s-1, consistent with that previously inferred from absorption spectra observed against the B star at a projected radius of similar to 150R*. Surprisingly, the NUV semi-forbidden emission lines show evidence of wind scattering, indicative of the massive column densities present in the winds of low-surface-gravity RSGs. Differential comparison with Betelgeuse of chromospheric fluxes from emission lines excited by electron collisions, electron collisions combined with photospheric photoexcitation, and coincidental line pumping reveals similar levels of chromospheric heating. C ii] indicates comparable chromospheric electron densities and column densities. With its well-constrained distance and stellar parameters, Antares provides a good source for understanding the processes driving RSG outflows. Chromospherically, Antares is effectively a twin of Betelgeuse.

Water and methanol are key components of interstellar ices and gas in star- and planet-forming regions, but direct observations of water in low-mass protostars are challenging due to atmospheric absorption. We present high-resolution (R = 70,500) mid-infrared spectroscopy of the class I protostar SVS13-A with EXES on board SOFIA at 26 mu m, targeting both H2O and CH3OH absorption lines. Several lines of each species are detected, tracing warm gas with rotational temperatures of similar to 140-170 K. Remarkably, the methanol column density is a factor of similar to 4 higher than that of water, well above typical interstellar ice ratios (<10%). Comparison with previous millimeter observations indicates that absorption and emission probe distinct regions, with the mid-IR lines likely tracing cooler gas along the line of sight. The surprising observed CH3OH/H2O ratio may reflect selective sublimation due to the distribution of binding energies or ice stratification in the inner envelope. These observations probe the inner regions of the protostar, where planets are expected to form and inherit the chemical composition of their natal environment, providing a direct link between ice sublimation and gas-phase chemistry. Our results represent the first high-spectral-resolution mid-infrared view of both water and methanol toward a low-mass protostar, offering a unique window into the chemical composition of the innermost envelope and planet-forming region and highlighting the diagnostic power of high-resolution mid-infrared spectroscopy to uncover hidden chemical layers and the ice-to-gas transition in embedded protostars.

In Titan’s atmosphere, the chemistry of simple hydrocarbons (e.g., CH 4 and C 2 H 2 ) and nitrogen bearing species (e.g., N 2 and CN) represents an important link between molecular species and the ubiquitous organic haze that gives Titan its characteristic orange hue. Here we present a new search for two previously undetected molecules, triacetylene (C 6 H 2 ) and the gas phase dicyanoacetylene (C 4 N 2 ), using the Echelon-Cross-Echelle Spectrograph instrument on board the Stratospheric Observatory for Infrared Astronomy aircraft. We do not detect these two molecules but determine upper limits for their mixing ratios and column abundances. We find the 3 σ upper limits on the uniform volume mixing ratio (VMR) above 100 km for C 6 H 2 to be 4.3 × 10 −11 , which is lower than the photochemical model predictions. This new upper limit suggests that the growth of linear molecules is inhibited. We also put a strict upper limit on the uniform VMR for gas phase C 4 N 2 above 125 km to be 1.0 × 10 −10 . This upper limit is well below the saturation mixing ratio at this altitude for C 4 N 2 and greatly limits the feasibility of C 4 N 2 forming ice from condensation.

We present the first astrophysical detection of methanol (CH _3 OH) in the torsional band near 25 μ m. Using high-resolution mid-infrared (MIR) spectroscopy, we identified over 70 gas-phase CH _3 OH absorption lines between 20 and 28 μ m toward the massive protostar NGC 7538 IRS 1 with the Stratospheric Observatory for Infrared Astronomy/Echelon-Cross-Echelle Spectrograph. We derive a temperature of 180 K and a total column density of 2 × 10 ^17 cm ^−2 , comparable to submillimeter measurements. Complementary analysis of acetylene (C _2 H _2 ) absorption lines is also included. Both CH _3 OH and C _2 H _2 reveal an unresolved second velocity component. These MIR absorption lines likely probe the molecular material in two edge-on disks, supporting the scenario that NGC 7538 IRS 1 consists of multiple protostars. We provide an updated line list for the torsional band of CH _3 OH, which was generated from lab work and model calculations. This discovery and the updated line list will enable the search for CH _3 OH in JWST/MIRI spectra.

Infrared spectra of hydrocarbon dust absorption bands toward the bright hypergiant Cygnus OB2-12 are compared to published spectra of the Quintuplet Cluster, a sightline to the Galactic center. The Cyg OB2-12 data include a new ground-based 2.86−3.70 μ m spectrum and a previously published, but here further analyzed, spectrum of the 5.50–7.34 μ m region. Higher-spectral-resolution data for the Cyg OB2-12 sightline in the 3 μ m region allows a detailed comparison of the 3.4 μ m aliphatic bands to those observed toward the Quintuplet. Despite differences in interstellar environments along each sightline, strong similarities are observed in the central wavelengths and relative strengths for bands at ∼3.3, 3.4, 5.85, 6.2, and 6.85 μ m. Analysis of these bands, produced by aromatic, aliphatic, olefinic, hydrogenated, and oxygenated components, shows that carbonaceous dust is a significant component of the diffuse interstellar medium (ISM), second in abundance only to silicates, and is primarily aromatic in nature. The grains producing these bands likely consist of large aromatic carbon cores with thin aliphatic mantles composed of hydrogenated amorphous carbon. Laboratory analog spectra reproduce the observed aliphatic absorption bands well, supporting the presence of such mantles. We present evidence that the carriers of both the 3.4 μ m aliphatic and the 3.3 μ m aromatic bands reside exclusively in the diffuse ISM, and that the 3.3 μ m bands observed in the diffuse ISM differ from the 3.25 μ m band seen in dense clouds, implying chemically distinct carriers.

The near-infrared (NIR) emission of the youngest protostars still needs to be characterized to better understand the evolution of their accretion and ejection activity. We analyze James Webb Space Telescope NIRSpec 1.7–5.3 μ m observations of two deeply embedded sources in the S68N protostellar core in Serpens. The North Central source exhibits a highly obscured spectrum ( A K ∼ 4.8 mag) that is modeled with a pre-main-sequence photosphere and a hot disk component. The photospheric parameters are consistent with a young, low-mass photosphere, as suggested by the low surface gravity, log g of 1.95 ±0.15 cm s −2 . The hot disk suggests that accretion onto the central protostellar embryo is ongoing, although prototypical accretion-tracing emission lines H i are not detected. The South Central source, which is even more embedded ( A K ∼ 8 mag; no continuum is detected shortward of 3.6 μ m) appears to be driving the large-scale S68N protostellar outflow, and launches a collimated hot molecular jet detected in H 2 and CO rovibrational lines. Shock modeling of the H 2 (ro)vibrational lines establishes that fast C -type shocks (≥30 km s −1 ), with high pre-shock density (≥10 7 cm −3 ), and strong magnetic field ( b ∼ 3–10, where B = b × n H ( cm − 3 ) μ G ) best match the data. The bright CO fundamental line forest suggests energetic excitation, with the contribution of non-LTE effects, i.e., irradiation pumping. Detected OH and CH + rovibrational lines support this hypothesis. These two Class 0 protostars seem to be in very young evolutionary stages and still have to acquire the bulk of their final stellar masses. These results demonstrate that JWST enables unprecedented diagnostics of these first stages of the protostellar evolutionary phase.

Supernova shocks into dense molecular cores in IC 443 (clumps B, C, and G) and 3C 391 were observed using the Stratospheric Observatory for Infrared Astronomy and complemented by archival data from the Herschel Space Observatory. The pure rotational transitions 0-0 S(1) and S(5) of H2, and the ground-state 110-101 transition of H2O, are all broadened, arising from molecules that survive the passage of the shock front. Theoretical models from the Paris-Durham shock code were analyzed to generate synthetic profiles that approximately match the observations. The observations can be fit with two shock conditions, which approximate the range of densities in the pre-shock molecular cloud. The width and brightness of the S(5) lines require shocks into gas with a density of order 2,000 cm-3, into which the IC 443 blast wave drives shocks with speed 60 km/s. The brightness and narrower width of the S(1) lines requires different shocks, into gas with density of order 10^5 cm-3, with shock speeds of 10 km/s. The H2O velocity distribution is also consistent with these shocks. The existence of shocks into dense gas shows that the bright shocked clumps in IC~443 were prestellar cores. It is unlikely that they will form stars soon after the passage of the shock front, given the input of kinetic and thermal energy from the shocks.

Using the EXES instrument on SOFIA, we have obtained velocity-resolved spectra of several pure rotational lines of H _2 toward shocked molecular gas within three Galactic sources: the supernova remnant (SNR) IC 443 (clump C), a protostellar outflow in the intermediate-mass star-forming region NGC 2071, and the SNR 3C 391. These observations had the goal of searching for expected velocity shifts between ortho- and para-H _2 transitions emitted by C-type shocks. In contrast in our previous similar study of HH7, the result of our search was negative: no velocity shifts were reliably detected. Several possible explanations for the absence of such shifts are discussed: these include a preshock ortho-to-para ratio that is already close to the high-temperature equilibrium value of 3 (in the case of IC 443C), the more complex shock structures evident in all these sources, and the larger projected aperture sizes relative to those in the observations of HH7.

Using the EXES instrument on SOFIA, we have obtained velocity-resolved spectra of several pure rotational lines of H2 toward shocked molecular gas within three Galactic sources: the supernova remnant (SNR) IC443 (Clump C), a protostellar outflow in the intermediate-mass star-forming region NGC 2071, and the SNR 3C391. These observations had the goal of searching for expected velocity shifts between ortho- and para-H2 transitions emitted by C-type shocks. In contrast in our previous similar study of HH7, the result of our search was negative: no velocity shifts were reliably detected. Several possible explanations for the absence of such shifts are discussed: these include a preshock ortho-to-para ratio that is already close to the high-temperature equilibrium value of 3 (in the case of IC443C), the more complex shock structures evident in all these sources, and the larger projected aperture sizes relative to those in the observations of HH7.

We present in this paper mid-infrared (5–8 μ m) spectroscopy toward the massive young binary W3 IRS 5, using the Echelon Cross Echelle Spectrograph (EXES) spectrometer in high-resolution mode ( R ∼ 50,000) from the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA). Many (∼180) ν 2 = 1–0 and (∼90) ν 2 = 2–1 absorption rovibrational transitions are identified. Two hot components over 500 K and one warm component of 190 K are identified through Gaussian fittings and rotation diagram analysis. Each component is linked to a CO component identified in the IRTF/iSHELL observations ( R = 88,100) through their kinematic and temperature characteristics. Revealed by the large scatter in the rotation diagram, opacity effects are important, and we adopt two curve-of-growth analyses, resulting in column densities of ∼10 19 cm −2 . In one analysis, the model assumes a foreground slab. The other assumes a circumstellar disk with an outward-decreasing temperature in the vertical direction. The disk model is favored because fewer geometry constraints are needed, although this model faces challenges as the internal heating source is unknown. We discuss the chemical abundances along the line of sight based on the CO-to-H 2 O connection. In the hot gas, all oxygen not locked in CO resides in water. In the cold gas, we observe a substantial shortfall of oxygen and suggest that the potential carrier could be organics in solid ice.

We present the first high spectral resolution mid-infrared survey in the Orion BN/KL region, covering 7.2–28.3 μ m. With SOFIA/EXES, we target the enigmatic source Orion IRc2. While this is in the most prolifically studied massive star-forming region, longer wavelengths and molecular emission lines dominated previous spectral surveys. The mid-infrared observations in this work access different components and molecular species in unprecedented detail. We unambiguously identify two new kinematic components, both chemically rich with multiple molecular absorption lines. The “blue clump” has v LSR = −7.1 ± 0.7 km s −1 , and the “red clump” has 1.4 ± 0.5 km s −1 . While the blue and red clumps have similar temperatures and line widths, molecular species in the blue clump have higher column densities. They are both likely linked to pure rotational H 2 emission also covered by this survey. This work provides evidence for the scenario that the blue and red clumps are distinct components unrelated to the classic components in the Orion BN/KL region. Comparison to spectroscopic surveys toward other infrared targets in the region show that the blue clump is clearly extended. We analyze, compare, and present in-depth findings on the physical conditions of C 2 H 2 , 13 CCH 2 , CH 4 , CS, H 2 O, HCN, H 13 CN , HNC, NH 3 , and SO 2 absorption lines and an H 2 emission line associated with the blue and red clumps. We also provide limited analysis of H 2 O and SiO molecular emission lines toward Orion IRc2 and the atomic forbidden transitions [Fe ii ], [S i ], [S iii ], and [Ne ii ].

We present results from a high spectral resolution (6 km s-1) survey of five massive protostars in the wavelength range of 2.95 and 3.25 µm, conducted with iSHELL at the InfraRed Telescope Facility (IRTF). Our targets are Mon R2 IRS 2, Mon R2 IRS 3, AFGL 2136, Orion BN and S140 IRS 1. Two of our five targets (Mon R2 IRS 3 and AFGL 2136) show transitions from organic species, with MonR2 IRS 3 showing HCN lines in emission, and AFGL 2136 showing HCN and C2H2 lines in absorption. The velocity of the emission lines of HCN of MonR2 IRS 3A are consistent with CO emission features in lines up to J = 26, as both are red-shifted with respect to the systemic velocity. Carbon monoxide lines also show blue-shifted absorption. This P-Cygni line profile, commonly observed towards massive young stellar objects, is likely due to an expanding shell, which is supported by sub-millimetre velocity maps of HCN. Alternatively HCN emission may arise from the upper layers of a disk photosphere, as has been suggested for the massive protostar AFGL 2591. Absorption lines in AFGL 2136 may either originate in foreground cloud or in the disk photosphere. For a foreground cloud, the data require that the foreground gas only covers the source partially (0.3) at 13 µm. In contrast, absorption lines at 3 and 7 µm require a covering factor of >0.9. Analysing the 13 µm HCN absorption lines in terms of absorption by gas in the photosphere of a disk, results in physical conditions that are consistent over all three vibrational modes. C2H2 absorption lines reveal an increasing temperature and abundance with decreasing wavelength, indicative of a radial abundance gradient. We conclude that the disk model is the best interpretation of the absorption lines of AFGL 2136.

We have performed a high-resolution 4–13 μ m spectral survey of the hot molecular gas associated with the massive protostars AFGL 2591 and AFGL 2136. Here we present the results of the analysis of the ν 2 band of H 2 O, detected with the Echelon Cross Echelle Spectrograph on board the Stratospheric Observatory for Infrared Astronomy between wavelengths of 5 and 8 μ m. All lines are seen in absorption. Rotation diagrams indicate that the gas is optically thick and lines are observed to saturate at 40% and 15% relative to the continuum for AFGL 2136 and AFGL 2591, respectively. We applied two curve of growth analyses to derive the physical conditions, one assuming a foreground origin and one a circumstellar disk origin. We find temperatures of 400–600 K. A foreground origin would require the presence of externally heated clumps that are smaller than the continuum source. The disk analysis is based on stellar atmosphere theory, which takes into consideration the temperature gradient in the disk. We discuss the challenges with each model, taking into consideration the properties of other species detected in the spectral survey, and conclude that further modeling efforts are required to establish whether the absorption has a disk or foreground origin. The main challenge to the foreground model is that molecules are expected to be observed in emission. The main challenges to the disk model are the midplane heating mechanism and the presence of narrow absorption lines shifted from the systemic velocity.

We have measured the gas temperature in the IC 63 photodissociation region (PDR) using the S(1) and S(5) pure rotation lines of molecular hydrogen with SOFIA/EXES. We divide the PDR into three regions for analysis based on the illumination from $\gamma$ Cas: "sunny," "ridge" and "shady." Constructing rotation diagrams for the different regions, we obtain temperatures of T$_{ex}$=$562^{+52}_{-43}$ K towards the "ridge" and T$_{ex}$=$495^{+28}_{-25}$ K in the "shady" side. The H$_2$ emission was not detected on the "sunny" side of the ridge, likely due to the photo-dissociation of H$_2$ in this gas. Our temperature values are lower than the value of T$_{ex}$=685$\pm$68 K using the S(1), S(3), and S(5) pure rotation lines, derived by Thi et al. (2009) using lower spatial-resolution ISO-SWS data at a different location of the IC 63 PDR. This difference indicates that the PDR is inhomogeneous and illustrates the need for high-resolution mapping of such regions to fully understand their physics. The detection of a temperature gradient correlated with the extinction into the cloud, points to the ability of using H$_2$ pure rotational line spectroscopy to map the gas temperature on small scales. We used a PDR model to estimate the FUV radiation and corresponding gas densities in IC 63. Our results shows the capability of SOFIA/EXES to resolve and provide detailed information on the temperature in such regions.

We report NASA-DLR SOFIA upGREAT circumstellar [O i ] 63.2 μ m and [C ii ] 157.7 μ m emission profiles and FIFI-LS [O i ] 63.2 μ m, [O i ] 145.5 μ m, and [C ii ] 157.7 μ m fluxes obtained shortly after Betelgeuse’s 2019/2020 Great Dimming event. Haas et al. noted a potential correlation between the [O i ] 63.2 μ m flux and V magnitude based on three Kuiper Airborne Observatory observations made with the CGS and FIFI instruments. The FIFI observation was obtained when V ≃ 0.88 and revealed a 3 σ non-detection at a quarter of the previous CGS flux measurement made when V ≃ 0.35. A potential explanation could be a change in dust-gas drag heating by circumstellar silicates caused by variations in the photospheric radiation field. SOFIA observations provide a unique test of this correlation because the V -band brightness went to its lowest value on record, V ≃ 1.61, with the SOFIA observations being made when V FIFI−LS ≃ 1.51 and V upGREAT ≃ 1.36. The upGREAT spectra show a [O i ] 63.2 μ m flux larger than previous space observatory measurements obtained when V ≃ 0.58. The profile is consistent with formation in the slower, more turbulent inner S1 outflow, while the [C ii ] 157.7 μ m profile is consistent with formation farther out in the faster S2 outflow. Modeling of dust-gas drag heating, combined with 25 yr of Wing three-filter and V photometry, reveals that it is unlikely that the S1 circumstellar envelope and [O i ] 63.2 μ m fluxes are dominated by the dust-gas drag heating and that another heating source is also active. The [O i ] 63.2 μ m profile is hard to reconcile with existing outflow velocity models.

We present the first mid-infrared (MIR) detections of HNC and (HCN)-C-13 in the interstellar medium, and numerous resolved HCN rovibrational transitions. Our observations span 12.8-22.9 mu m toward the hot core Orion IRc2, obtained with the Echelon-Cross-Echelle Spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). In particular, similar to 5 km.s(-1) resolution distinguishes individual rovibrational transitions of the HNC and HCN P, Q, and R branches; and the (HCN)-C-13 R branch. This allows direct measurement of the species' excitation temperatures, column densities, and relative abundances. HNC and (HCN)-C-13 exhibit a local standard of rest velocity of -7 km.s(-1) that may be associated with an outflow from nearby radio source I and an excitation temperature of about 100 K. We resolve two velocity components for HCN, the primary component also being at -7 km.s(-1) with a temperature of 165 K. The hottest component, which had never before been observed, is at 1 km.s(-1) with a temperature of 309 K. This is the closest component to the hot core's center measured to date. The derived C-12/C-13 = 13 +/- 2 is below expectation for Orion's Galactocentric distance, but the derived HCN/ HNC = 72 +/- 7 is expected for this extreme environment. Compared to previous sub-millimeter and millimeter observations, our SOFIA line survey of this region shows that the resolved MIR molecular transitions are probing a distinct physical component and isolating the chemistry closest to the hot core.