Alessio Caratti o Garatti

Context . Very low-mass stars ( M _⋆ < 0.2 M _⊙ ) are known to host a larger fraction of terrestrial planets compared to higher mass stars. Studying the structure of their protoplanetary disk is therefore crucial to understanding the formation of Earth-like planets. Aims . In this work, we aim to characterize the innermost regions of the disk around a known very low-mass star, 2MASS J16053215-1933159 (J1605 hereafter), by analyzing new JWST/NIRSpec observations taken in 2024, two years after the first observation with JWST/MIRI in 2022. We intend to investigate the atomic and molecular emission in both spectra to establish a global view of the system and study its variability. Methods . After identifying both the fundamental band of CO and HI recombination emission lines from both spectra corrected from the contribution of the stellar photosphere, we used the HI lines to derive the stellar accretion rate at each epoch. The CO emission was fitted with a slab model to derive the gas characteristics. Results . We find that between the two epochs (2022 and 2024), the CO flux is reduced by a factor of ∼3, the flux of the only HI line seen by both NIRSpec and MIRI (the HI 10-6 line) is reduced by a factor of ∼13, and the total continuum-density flux is reduced by ∼15%. The accretion rate derived from individual HI lines is consistent with a decrease of a factor of ∼13 in two years. After correcting the spectrum from the CO absorption present in the J1605 stellar photosphere, we find that the decrease in the disk’s CO emission is consistent with a CO gas of the same column density and temperature at each epoch, but originating from a smaller emitting area when the accretion rate is low (NIRSpec’s epoch). While warm hydrocarbons (≃500 K) are detected with MIRI-MRS (2022) with extremely high column densities of C ₂ H ₂ , no hydrocarbon features are seen with NIRSpec (2024). We propose that non-Local Thermal Equilibrium (LTE) effects quench the near-IR emission of hydrocarbons in the warm reservoir. Taking the newly detected hot CO (∼800 K) as a reference and assuming LTE, we constrained the column density of C ₂ H ₂ , HCN, and CH ₄ to be less than 30% of CO in the hot reservoir. This value suggests that the C/O in the inner disk is unlikely to be much larger than unity. Conclusions . The variation of the accretion luminosity of the very low-mass star J1605 correlates with the variation of its fundamental CO luminosity similarly to T-Tauri stars, supporting the idea that very low-mass stars could be seen as scaled-down versions of T-Tauri stars. The detection of the emission of the fundamental band of CO in disks around cold objects (very low-mass stars, brown dwarfs, and giant planets) is challenging because of the presence of CO absorption in their photosphere. Future observations (e.g., with ELT/METIS) looking for such emission will require robust ways to constrain the photosphere emission and follow the time variability of the sources.

Context . The JWST gives unique access to the chemical and physical conditions in inner disks ( 0.4 M _⊙ ). In V1094 Sco, even cold C ₄ H ₂ is detected. In contrast, the IM Lup spectrum is dominated by O-bearing species. No one-to-one correspondence is found between the gas in the outer disk, as traced by the C ₂ H (3–2)/C ¹⁸ O (2–1) flux ratios, and that of the inner disk, as traced by the C ₂ H ₂ /H ₂ O flux ratios. To account for these observational results, we propose a scenario based on a toy model of halted pebble drift relevant to the pebble-rich disks. We show that a volatile C/O ratio close to unity and low elemental C and O abundance can be achieved in inner disks only if (1) about 95% of the icy grains are blocked in the outer disk, (2) the outer disk is chemically evolved to reach elevated gas-phase C/O and low (C+O)/H abundance ratio, and (3) the gas in the outer disk had time to be advected to the inner disk. In this scenario, DL Tau and perhaps V1094 Sco would be the rare examples for which all these conditions are met. Therefore, a high C ₂ H ₂ /H ₂ O flux ratio in pebble-rich disks would have a different origin than that proposed for very low-mass stars, for which fast drift of oxygen-rich pebbles would eventually leave a carbon-rich inner disk. Destruction of carbonaceous grains appears to be a less compelling scenario to account for the high C ₂ H ₂ /H ₂ O flux ratio in DL Tau and V1094 Sco because existing models require strong pebble drift and low accretion rates to enrich the inner disk in volatile carbon. Alternatively, the mid-IR molecular features might be related to the low optical depth of the dust, which would reveal a normally hidden reservoir of C ₂ H ₂ deeper down to the midplane. Finally, we show for the first time that the disks with a high C ₂ H ₂ /H ₂ O flux ratio exhibit a prominent silica dust component, a result found in four disks published so far (V1094 Sco, DL Tau, CY Tau, and DoAr 33). We propose that the re-formation of dust at the sublimation front of silicates in a gas with super-solar (but below unity) C/O ratio leads to a silica stoichiometry (SiO ₂ ). In turn, silica constitutes a promising diagnostic of the C/O ratio in the inner disks. Conclusions . The diversity of inner disks as revealed by mid-IR spectroscopy might be driven by the joint evolution of the outer versus inner disk via chemical evolution of the cold outer disk and radial redistribution of gas and ice-coated dust. Studies gathering large samples and forward modeling are required to confirm this proposal and obtain a global picture of disk evolution.

Context . The evolution of protoplanetary disks is regulated by the interplay between accretion onto the central star and mass loss through jets and winds. While atomic and ionised outflows are commonly observed, direct detections of molecular winds in evolved class II disks remain rare, limiting our understanding of their role in disk evolution and dispersal. Aims . We aim to characterise the spatial, thermal, kinematic, and dynamical properties of molecular hydrogen (H ₂ ) emission associated with the nearly edge-on class II disk HV Tau C, and to assess the dynamical impact of its molecular wind. We also simultaneously constrained the accretion properties using H I recombination lines detected in the same mid-infrared (mid-IR) spectrum, enabling us to investigate the accretion–ejection connection in this system. Methods . We analyse mid-IR JWST /MIRI-MRS data from the MINDS Cycle 1 GTO programme, extracting spatially resolved pure-rotational H ₂ emission. Rotational diagrams were used to determine excitation temperatures and column densities, while position–velocity diagrams provide the kinematics of extended emission. Combining excitation and kinematic information, we estimated the wind mass-loss rate, momentum rate, and mechanical luminosity. Accretion luminosities and mass-accretion rates are derived from H I lines, accounting for uncertainties in visual extinction due to the edge-on geometry. Results . We detect spatially extended pure-rotational H ₂ emission from HV Tau C, revealing a wide-angled, bi-conical molecular wind extending well beyond the near-IR scattered-light disk, the ALMA 887 µm dust continuum, and the compact ¹² CO ( J =3–2) gas disk. The H ₂ rotational diagram requires at least two temperature components: a warm component at ∼600 K and a hot component at ∼2000 K, similar to those found in much younger protostars. The kinematics indicate outward motions of a few tens of km s ⁻¹ and dynamical timescales of tens to hundreds of years. The inferred mass-loss rate is about 10 ⁻⁸ M _⊙ yr ⁻¹ , while accretion rates lie between 10 ⁻¹⁰ and 10 ⁻⁸ M _⊙ yr ⁻¹ . The measured accretion rate may be underestimated due to geometrical effects, and the true accretion rate could be higher than that inferred from the mid-IR H I lines. Conclusions . These results suggest that wide-angled molecular H ₂ winds may persist into the class II phase, with inferred outflow rates that are comparable to those observed in some protostellar systems. Such outflows may play a significant role in angular momentum removal, disk evolution, and disk dispersal even in relatively evolved systems.

Molecular winds may play a key role in governing angular momentum transport and accretion during the early evolution of protostars. We present the morphology and kinematic properties of the H ₂ emission in five young, envelope-dominated protostars across a broad bolometric luminosity range, from 0.2 to 10 ⁴ L _⊙ , observed with the NIRSpec/integral field unit and MIRI/Medium Resolution Spectrograph on board JWST as part of the Investigating Protostellar Accretion (IPA) program. A rich set of pure rotational lines of H ₂ , up to v = 0–0 S (18), and a few rovibrational lines are detected in the winds, revealing bipolar structures. The H ₂ lines show a stratified/onion-like structure morphologically and kinematically, where the lines with higher E _u / k show a higher degree of collimation and higher velocities. Additionally, the wind velocity scales with the L _bol of the host protostellar system. In four out of five protostars, H ₂ emission fills the outflow cavity without showing pronounced limb brightening. We also report a tentative detection of H ₂ wind rotation in IRAS 16253, which suggests a launch radius of ∼4 au and a magnetic lever arm parameter of ∼5–10. Taken together, these properties of the H ₂ winds can be explained by the magnetohydrodynamic disk wind models. We detect a collimated, high-velocity H ₂ jet toward HOPS 370, which is more evolved than the extremely young source HH 211 but is accreting at a high accretion rate. This suggests that the presence of collimated molecular jets in protostars is more closely connected to accretion rate than system age.

Accretion-driven outbursts are a fundamental process in star formation, determining the final masses of stars and the chemistry of protoplanetary disks, yet their detection in the youngest, deeply embedded protostars remains challenging. Radiatively pumped methanol masers, to date associated exclusively with high-mass young stellar objects, exhibit flares that are interpreted as signatures of accretion bursts and serve as beacons to identify massive protostellar outbursts. Indeed, for all such methanol maser flares studied thus far, the driving source has been identified as the associated high mass protostar. By combining James Webb Space Telescope (JWST) and Atacama Large Millimeter/submillimeter Array (ALMA) observations, we prove that the recent methanol maser flare in the massive star-forming region IRAS 18134-194216 was driven by a nearby lower-mass protostar undergoing a strong accretion outburst, rather than the central massive object. Our infrared spectroscopy captures the physical and chemical response of the protostellar core to the accretion burst. Our results demonstrate that methanol masers flares can uncover the hidden population of embedded low-mass bursting protostars in massive clusters, and challenge the standard paradigm where such maser activity is exclusively attributed to massive protostars, demonstrating that cavities opened by low-mass companions can provide a radiative path for triggering flares of pre-existing masers excited by massive protostars.

Complex organic molecules (COMs) are the key to understanding the chemical evolution from simple interstellar molecules to potential prebiotic material. Although COMs have been extensively studied in the gas phase toward protostars, their counterparts in ices, where they are thought to form at earlier stages, remain far less constrained. A number of diagnostic features of complex ices lie between 6.8 and 8.8 μ m, a region known as the “COM ice fingerprint range,” but previous infrared facilities lacked the sensitivity and spectral resolution required to quantify the weak bands therein. With the unprecedented sensitivity and resolving power of the James Webb Space Telescope, these limitations can now be overcome. Here, we present the first large-sample quantitative study of the absorption features at 7.02, 7.24, 7.40, and 7.67 μ m, using mid-infrared spectra of 21 protostars observed by the Medium Resolution Spectroscopy mode of the Mid-Infrared Instrument. The CH ₄ band at 7.67 μ m is the strongest band and shows remarkably uniform peak positions (7.67–7.68 μ m) and full widths at half-maximum (FWHMs; 0.06–0.08 μ m), suggesting CH ₄ ice as its dominant carrier. The 7.24 and 7.40 μ m bands exhibit larger source-to-source variations in peak positions and FWHMs, but their occurrence and intensities are strongly correlated with each other. Comparisons with existing and new laboratory spectra suggest HCOO ⁻ as the most likely carrier of these two bands, yet HCOO ⁻ cannot fully reproduce their intensity ratios, implying additional contributions from other species such as C ₂ H ₅ OH, CH ₃ CHO, and CH ₃ COCH ₃ . Our results reveal, for the first time, the potential ubiquity of weak features of complex ices in protostars, which have remained largely undetected due to observational limitations.

HK Tau is a roughly equal-mass pre-main-sequence binary system consisting of a low-inclination primary (57̧irc) and an edge-on ( ) secondary. We present JWST Mid-Infrared Instrument (MIRI) observations targeting both sources, taken as part of the JWST GTO program MIND. The mid-infrared spectra reveal a line-rich -dominated primary and a line-poor secondary. This evidence, albeit in line with the evolution-motivated trend uncovered by recent observations of binaries at MIRI wavelengths, is likely due to the different configuration of the two sources. Thermochemical disc models coupled with radiative transfer indeed show that at inclinations comparable to that of HK Tau B, only ionised atomic lines are expected to remain visible in the spectra. While it blocks molecular emission lines, however, the edge-on configuration allows ice absorption bands to be visible against the continuum. In this framework, the HK Tau system provides an unprecedented opportunity for a simultaneous view of the solid and gaseous component of a pair of coeval protoplanetary discs thanks to the complementary inclination of the two sources. We detect water ice at 6.2 and 13.6 ice at 15.2 ice at 6.85 , 83 CO2 CO2 and NH4+ in the spectrum of HK Tau B. An additional absorption band between 8.3 and 9 emission is present around both sources, but is much more elongated in HK Tau B. The distinctive X shape centred at the location of B, combined with the intensity, morphology, and spectral characteristics of the ionised atomic lines is compatible with both silicate stretching and C-H bending. Neither the primary nor the secondary shows signs of polycyclic aromatic hydrocarbons. Extended H2 Ar II Ne II , and Ne III , suggests a low-velocity wind origin with a wide (∼ ) semi-opening angle. The lower forbidden line fluxes and smaller spatial extent of the emission around A imply that if a wind is launched from the primary as well, it is too cold or dense to be ionised and emit brightly. 70 H2

Nitrogen-bearing molecules are generally more difficult to observe than oxygen-bearing ones in both gas and ice, mainly due to the lower abundance of nitrogen in the interstellar medium. Therefore, the formation pathways of many of these species is still under debate. Studies prior to the launch of the Space Telescope (JWST) did not have the sensitivity to observe ices toward the youngest and most deeply embedded Class 0 objects. The time is now ripe to study nitrogen-bearing molecules toward ice-rich Class 0 and Class I objects with the JWST. Here we focus on OCN-$, CH_3CN, C_2H_5CN, NO, and N$_2O in ices to better understand their formation. James Webb We used the data from JWST Observations of Young protoStars (JOYS+) program. Particularly, we studied the objects that have the JWST NIRSpec-IFU observations (8 Class 0 and 11 Class I) to measure the ice column densities of the targeted molecules. We firmly detect OCN-$ in ices for all these objects, tentatively detect CH_3CN, C_2H_5CN, and N_2O toward three sources, and find upper limits on the NO abundance in ices. The OCN^-/CO_2 ratios are found to be higher by a factor of ∼2-3 for the objects that have a visible CO_2 (15.2,μm) double peak (a sign of ice thermal processing), which points to the moderate effect of temperature on OCN^- production. Considering H_2O, CO_2, and OCN^- relations with A_̊m V, we tentatively find that OCN^- forms at a later stage compared to H_2O and CO_2. We find that the ratios of CH_3CN, C_2H_5CN, and N_2O with respect to OCN^- are relatively constant within one order of magnitude across our objects, likely suggesting that they have similar ice environments. The upper limit abundances of NO are around one order of magnitude lower than what was previously predicted in ices of a mature protoplanetary disk to explain the gas-phase detection of this molecule in the disk. This indicates that gas-phase NO may be a product of another molecule such as N$_2O in the ices. We conclude that after OCN-$ forms, it can increase at higher temperatures by only a factor of ∼2-3, and thus OCN^- detection alone does not imply ice heating. Large-sample studies of OCN$^- toward pre-stellar cores will be useful to further confirm the formation timeline of this molecule.