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.

Comets are frozen remnants from our solar system’s birth, 4.6 billion years ago. Comparing their composition with that found in planet-forming disks surrounding young solar analogues (104 — 106 years old) provides a diagnostic of the evolutionary processes that can shape planetary systems. Moreover, it allows the tracing of chemical signatures from parental molecular clouds to planets, bridging the gap between interstellar chemistry and planetary formation (Ceccarelli C., et al., 2023, Mumma M. J., & Charnley S. B.). Nevertheless, systematic comparisons between protostellar environments, disks, and comets remain scarce, often limited to a few target selections (e.g., Drozdovskaya M.et al., 2019; Bianchi E., et al., 2019).In this work, we present the first statistical analysis of [CH3CN]/[CH3OH] abundance ratios across a diverse sample, including 13 comets, 24 low-mass hot corinos, and 6 planet-forming disks. This statistical approach allows us to identify whether inconsistencies are present in existing datasets and provides a more comprehensive view of the various stages of planet formation (Lippi M., et al., 2024).While we observe significant variations of the [CH3CN]/[CH3OH] abundance ratios within the planet-forming disk sample – most likely driven by evolutionary processes – in comets and hot corinos this ratio shows a remarkable similarity (see Figure 1). This suggests that the transition from protostellar envelopes to cometary bodies is driven by a consistent chemistry that converges over time, even when material is continuously reprocessed.Figure 1: Comparison of the [CH3CN]/[CH3OH] abundance ratio in hot corinos, Class 0 to Class II disks, and comets. References: Ceccarelli, C., Codella, C., Balucani, N., et al. 2023, in Astronomical Society of the Pacific Conference Series, Vol. 534, Protostars and Planets VII; Mumma M. J., Charnley S. B., Annual Review of Astronomy and Astrophysics, 2011, 49, 471-524; Drozdovskaya, M. N., van Dishoeck, E. F., Rubin, M., Jørgensen, J. K., & Al- 470; Bianchi, E., Codella, C., Ceccarelli, C., et al. 2019, MNRAS, 483, 1850; Lippi M., Podio L., Codella C., Faggi S., De Simone M., Villanueva G. L., Mumma M. J., Ceccarelli C., The Astrophysical Journal, 2024, 970.

Disks of gas and dust orbiting young stars are the arenas and material reservoirs for planet formation. Over the past decade, multiwavelength observations, from infrared to radio, have resolved the spatial distribution of hundreds of protoplanetary disks in nearby star-forming regions, revealing a diverse zoo of substructures. These substructures are morphological features such as rings, gaps, spirals, vortices, asymmetries, warps, or clumps that trace variations in density, temperature, or composition relative to an otherwise smooth distribution of gas and dust. Many unknowns persist as to the origin of these substructures, their role in planet assembly, and their true properties. SKA-Mid Band 5b continuum observations, offering angular resolutions of ∼ 0.05” (∼ 0.15”) with AA4 (AA*) at 12.5 GHz / 2.4 cm, will enable new progress at this frontier. In this chapter, we outline the open questions in the field of disk substructure that SKA-Mid is uniquely poised to address, with a lens on dust thermal emission.

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.

A few years ago, silicon oxysulfide, OSiS, was suggested as a possible interstellar molecule containing both sulfur and silicon. Its detection using remote spectroscopy has so far been unsuccessful, and only upper limits have been reported in the literature. It remains to be seen whether efficient formation pathways exist that warrant further search. An initial suggestion for the OSiS formation indicated the O + SiS reaction, but this cannot form OSiS under the rarefied conditions of the interstellar medium, since stabilizing secondary or three-body collisions are highly unlikely. For this reason, following the example of reactions such as OH + CO, OH + CS, and OH + SiO we report a computational investigation of the OH + SiS reaction. According to our electronic structure calculations, which allowed us to construct the potential energy surface of the reactive system, the OH + SiS reaction can lead to the formation of OSiS + H, but a much more competitive pathway leads to SiO + HS. Our preliminary kinetic analysis showed that the yield of OSiS + H is ca. 17

About 350 molecules have been identified in the interstellar medium (ISM), including complex molecules relevant to prebiotic chemistry. A remarkable level of molecular diversity has been observed from the earliest stages of star formation, providing the initial chemical inventory inherited by planetary systems. Radio observations have played a pivotal role in these discoveries, starting with the identification of the first polyatomic molecule, NH_3 (Cheung et al. 1968). (Sub-)millimeter observations have revealed complex organic molecules of prebiotic relevance, including formamide (NH_2CHO), glycolaldehyde (CH_2OHCHO), and even urea ((NH_2)_2CO), and hydroxylamine (NH_2OH), which are possible precursors of RNA nucleotides (Ceccarelli et al. 2023; Jiménez-Serra et al. 2020). However, in dense protostellar regions, dust opacity hampers the detection of molecular emission. Additionally, large molecules and those containing heavy atoms, which have rotational transitions at lower frequencies, often remain inaccessible to current instruments. The Square Kilometre Array Observatory (SKAO) will provide an unprecedented combination of sensitivity and angular resolution at radio wavelengths. This will allow for the detection of prebiotic species and offer new insights into the chemical pathways that shape emerging planetary systems (Jiménez-Serra et al. 2022). This chapter details the scientific questions and advancements that the SKAO, and more specifically, SKA-Mid equipped with the Band 5 receivers, will pursue in the field of astrochemistry, focusing on the chemical complexity in both high-mass and solar-type star-forming regions.

We report the first detection of TiO in star-forming regions based on Atacama Large Millimeter/submillimeter Array observations of Orion Source I, a well-characterized massive protostar. Multiple rotational transitions are identified, with emission spatially resolved within ∼ 50 au, showing a compact distribution with a velocity structure consistent with the base of a rotating outflow. The spatial and velocity distributions of TiO are consistent with those of AlO, with both species being key dust seeding refractory molecules. The column density of TiO is derived to be (3.0 ± 0.4)×10^15 cm^-2, corresponding to X_ TiO/SiO∼ 12.8 ± 1.7 ×10^-3, higher than CI chondrites and indicative of efficient dust-to-gas conversion near the protostar. We also identify a tentative detection of AlOH, which exhibits a more extended distribution along the disk surface, possibly indicating different conditions from those traced by TiO and AlO. The detection of TiO, a key dust seeding species, offers important constraints on refractory chemistry and the formation environments of primitive minerals, linking astrochemical processes in protostellar systems to the earliest stages of Solar System material formation.

The chemical composition of planets is inherited from that of the natal protoplanetary disk at the time of planet formation. In recent years, we have made huge progress in characterizing disk chemistry. (Sub-)millimeter interferometers, such as ALMA, allowed us to detect emission lines from simple to complex organic molecules and to probe their radial and vertical distribution in disks. On the other hand, JWST has started to unveil the composition of disk ices, and line emission from the innermost disk regions. The advent of SKA will open new domains in the field, by observing emission lines from heavier molecules including heavy carbon chains and rings, and prebiotic molecules with peak emission in the cm range. Moreover, SKA will probe molecular emission from regions which are obscured by dust opacity at mm wavelengths, hence from the disk midplane, and often from the inner 30 au region. These observations will constrain the initial conditions for disk evolution and planet formation, allowing us to predict the chemical composition of the forming planets and their atmospheres. Comparison with forthcoming results on exoplanet atmospheres and on the chemistry of pristine bodies in the Solar System will provide new hints on the origin and evolution of planetary systems including our own.

Understanding how solid material in planet-forming disks evolves from micron-sized dust to planetary cores is a central challenge in modern astrophysics. This study has advanced dramatically in the past decade, largely driven by ALMA and high-contrast imaging facilities. However, major uncertainties remain regarding the presence, evolution, and role of centimeter-sized grains (the pebbles) in planet formation. The SKAO will fill this gap by enabling the first large-scale, high-resolution survey of disk emission at centimeter wavelengths. This chapter presents the scientific rationale and observational strategies to detect and characterize pebbles in the planet-forming disks of nearby star-forming regions. By resolving their spatial distribution, spectral properties, and evolutionary trends, SKA will offer essential constraints on dust growth and disk dynamics. This work provides observational strategies, target selection, and predictions on the detectability of hundreds of nearby disks. The chapter also explores SKA's potential to uncover the actual dust mass in disks, protoplanets and their circumplanetary disks, and other aspects of the planet formation. Together, these capabilities will establish SKAO as a cornerstone facility for planet formation science in the coming decade.

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.

Jets and outflows are ubiquitous phenomena associated with the formation of young stellar objects (YSOs). They play a crucial role in removing angular momentum from the accreting system and in regulating star-formation efficiency. Theoretical studies and observations with ALMA and VLA have shown that jets and winds may have a crucial role in promoting dust growth in the envelope-disc system and in shaping the physical and chemical properties of the surrounding environment. Despite these significant advances, many fundamental questions remain unanswered regarding the acceleration, collimation, and chemical impact of jets and outflows from YSOs. The SKA-project will overcome the limitations of current mm/cm-facilities by enabling high-angular resolution and high-sensitivity cm-observations, crucial for probing jets/outflows near YSOs. Radio recombination lines, combined with proper motions, offer a unique opportunity to study the 3D-kinematics of jets. Non-thermal linearly polarised synchrotron emission will allow measuring magnetic field strength and morphology at unprecedented scales of a few au. Observations of dust emission in outflow cavities will allow studying how dust grows and is eventually transported from the disc to the envelope and back. Finally, the SKA-project will allow exploring the dust composition and chemical enrichment in shocks, where sputtering/shattering of grains cause the release of their mantles and refractory cores in the gas-phase. Complementary to ALMA's detection of simple and complex organic molecules, the SKAO will probe, for the first time, long carbon chains/rings, several Cl-, Al-, Mg-, and other metal-bearing species (missed by current sub-mm facilities).

We present an infrared, millimetre, and radio survey of 20 Class 0-III young stellar objects in the Ophiuchus A L1688 star-forming cluster, combining high-resolution (7-25 au) VLA and JWST observations with archival ALMA data. We implement physically motivated models to derive dust and ionised gas properties, spectral behaviour and their relative contributions through the millimetre-centimetre radio spectral energy distribution. Our models reveal circumstellar dust disks that are, on average, tens to hundreds of times more massive than millimetre-only estimates (subject to uncertainties arising from the choice of dust opacity) and contain millimetre-sized grains even at the Class 0 stage. Owing to the VLA's high resolution we are able to connect outflows to their origins, detecting protostellar jet emission on scales of 10s-1000s au. Our results represent a homogeneous characterisation of the dust and ionised gas properties in Ophiuchus and present a potential solution to the long-standing 'missing disk mass' problem. However, our understanding is still limited by resolution and sensitivity at frequencies <40 GHz. Future facilities like the SKA and ngVLA are needed to provide the necessary capabilities to fully spatially resolve this emission (<0.18") even in one of the closest star-forming regions.

Young binary stars with discs provide unique laboratories for studying the earliest stages of planet formation in star-forming environments. The detection of substructure in discs around Class I protostars challenges current models of disc evolution, and suggests that planets may form earlier than previously expected (< 1 Myr). In the context of the FAUST Large Program, we present observations of the circumbinary disc (CBD) around the young binary system L1551 IRS 5. The CBD exhibits two prominent over-densities in the continuum emission at the edge of the cavity, with the northern over-density being about 20% brighter than the southern one. By analysing the disc morphology and kinematics of L1551 IRS 5, we delineate dynamical constraints on the binary's orbital parameters. Additionally, we present 3D hydrodynamical models of the CBD to predict both the dust and the gas surface densities. Then, we compare the resulting synthetic observations with ALMA observations of the continuum emission at 1.3 mm and the (CO)-O-18 line emission. Our analysis suggests that the density enhancements observed with ALMA in L1551 IRS 5 can be caused by interactions between the binary stars and the CBD, leading to dust concentration within the disc. We conclude that the observed over-density corresponds to a location where solids could potentially grow in size under favourable conditions.

The luminosity of embedded protostars is commonly measured via observations of the dust continuum spectral energy distribution from millimetre to infrared wavelengths. However, this method cannot be applied to embedded protostars in binary or multiple systems, where their components are usually unresolved over this extended wavelength range. We propose a new method, based on the idea that a molecule formed (mainly) on the grain surfaces only emits lines in the region where it thermally sublimates from the grain mantles, heated by the photons emitted by the embedded source. In this respect, carbonyl sulfide (OCS) is an optimal molecule, because of its low binding energy and rotational lines in the millimetre. We apply the method to the protobinary system NGC1333 IRAS4A, using ALMA high-spatial resolution (similar to 50 au) observations of the OCS(19-18) line as part of the ALMA Large Programme FAUST. We also present new quantum mechanics calculations of the OCS binding energy distribution, essential for the application of the method. We found that the two binary components, A1 and A2, have a comparable luminosity within the error bars, 7.5 +/- 2.5 and 7 +/- 1 L-circle dot, respectively. We discuss the reliability of the estimated luminosities and the potential of this new method for measuring the luminosity of embedded protostars in binary and multiple systems.

Protoplanetary disks represent a crucial stage in the evolution of Young Stellar Objects towards the formation of fully formed planetary systems. While substantial progress has been made in the last decades in the characterization of the dust and molecular gas in these systems, the ionized component remains poorly understood. Ionized gas traces important processes such as photoevaporation, accretion, disk winds, and jets, and therefore is key to studying disk dynamics, evolution, and ultimately planet formation. In this paper, we investigate the capabilities of the forthcoming SKA telescope to probe this component in protoplanetary disks within nearby star forming regions. We present state-of-the-art simulations of photoevaporative, magneto-thermal, and magnetohydrodynamic winds, and generate theoretical predictions and synthetic SKAO observations to assess its potential in detecting and characterizing free-free emission and Hydrogen recombination lines. Finally, we discuss synergies with complementary facilities and how they will provide a comprehensive, multi-scale view of disk winds and offer critical insights on the mechanisms driving disk evolution and the onset of planet formation.

Two main formation routes have been proposed for interstellar complex organic molecules (iCOMs): on dust grain surfaces and in the gas phase. Observing such molecules in protostellar outflow shock regions - provided that their ages are well-constrained - can help distinguish between these pathways by probing chemical evolution over time. This study focuses on the potential daughter-mother relationship of glycolaldehyde (CH_2OHCHO) and ethanol (C_2H_5OH), previously proposed in the literature. We test whether gas-phase reactions converting ethanol into glycolaldehyde derived in these works can explain the observed abundance of the latter in star-forming regions. We target the southern outflow of L1157, which hosts three shock regions, B0, B1 and B2, of increasing ages: about 900, 1500 and 2300 yr. We obtained high-resolution IRAM NOEMA maps of three lines of glycolaldehyde and one line of ethanol. We derived their abundances in the three shocks and used a pseudo time-dependent astrochemical model to simulate gas-phase and grain-surface formation scenarios for glycolaldehyde. Ethanol is assumed to form on grains and be released in the gas by shocks, where it is gradually converted into glycolaldehyde via the ethanol-tree reaction network. We present the first spatially resolved maps of glycolaldehyde and ethanol in the L1157 southern outflow, and more generally toward solar-like star forming regions. The abundance ratio [CH_2OHCHO]/[C_2H_5OH] increases from B1 to B2, consistent with model predictions. However, the model cannot reproduce all three shocked regions simultaneously, suggesting that one of the assumptions of our model, such as the same excitation temperature and grain composition in B0, B1 and B2, or gas temperature evolution, is wrong. Nonetheless, our modeling rules out the possibility that all the observed gaseous glycolaldehyde is a grain-surface product.

Astrochemical observations have revealed a surprisingly high level of chemical complexity, including long carbon chains, in the earliest stages of Sun-like star formation. The origin of these species and whether they undergo further growth, possibly contributing to the molecular complexity of planetary systems, remain open questions. We present recent observations performed using the 100-m Green Bank Telescope of the prestellar core L1544, and the protostellar system IRAS 16293-2422. In L1544, we detected several complex carbon-bearing species, including C2S, C3S, C3N, c-C3H, C4H and C6H, complementing previously reported emission of cyanopolyynes. In IRAS 16293-2422, we detected c-C3H and, for the first time, HC7N. Thanks to the high spectral resolution, we refine the rest frequencies of several c-C3H and C6H transitions. We perform radiative transfer analysis, highlighting a chemical difference between the two sources: IRAS 16293-2422 shows column densities 10 to 100 times lower than L1544. We perform astrochemical modeling, employing an up-to-date chemical network with revised reaction rates. Models reproduce the general trends, with cyanopolyyne and polyynyl radical abundances decreasing as molecular size increases, but underestimate the abundances of cyanopolyynes longer than HC5N by up to two orders of magnitude. Current models, which include the dominant neutral-neutral formation routes, cannot account for this discrepancy, suggesting that the chemical network is incomplete. We propose that additional ion-molecule reactions are crucial for the formation of these species. Developing a more comprehensive chemical network for long carbon chains is essential for accurately interpreting present and future observations.

Utilizing the James Webb Space Telescope (JWST), the Atacama Large Millimeter/submillimeter Array (ALMA), and the Very Large Array (VLA), we present high angular resolution (0 . ″ 06–0 . ″ 42), multiwavelength (4 μ m–3 cm) observations of the VLA 1623-2417 protostellar system to characterize the origin, morphology and, properties of the continuum emission. JWST observations at 4.4 μ m reveal outflow cavities for VLA 1623 A and, for the first time, VLA 1623 B, as well as scattered light from the upper layers of the VLA 1623 W disk. We model the millimeter-centimeter spectral energy distributions to quantify the relative contributions of dust and ionized gas emission, calculate dust masses, and use spectral index maps to determine where optical depth hinders this analysis. In general, all objects appear to be optically thick down to ∼90 GHz, show evidence for significant amounts (tens to hundreds of M ⊕ ) of large (>1 mm) dust grains, and are dominated by ionized gas emission for frequencies ≲​​​​​​15 GHz. In addition, we find evidence of unsettled millimeter dust in the inclined disk of VLA 1623 B possibly attributed to instabilities within the circumstellar disk, adding to the growing catalog of unsettled Class 0/I disks. Our results represent some of the highest-resolution observations possible with current instrumentation, particularly in the case of the VLA. However, our interpretation is still limited at low frequencies (≲22 GHz) and thus motivates the need for next-generation interferometers operating at centimeter wavelengths.

Utilising JWST, ALMA and the VLA we present high angular resolution (0.06''- 0.42''), multi-wavelength (4 micron - 3cm) observations of the VLA 1623-2417 protostellar system to characterise the origin, morphology and, properties of the continuum emission. JWST observations at 4.4 micron reveal outflow cavities for VLA 1623 A and, for the first time, VLA 1623 B, as well as scattered light from the upper layers of the VLA 1623 W disk. We model the millimetre-centimetre spectral energy distributions to quantify the relative contributions of dust and ionised gas emission, calculate dust masses, and use spectral index maps to determine where optical depth hinders this analysis. In general, all objects appear to be optically thick down to ~90 GHz, show evidence for significant amounts (10's - 100's M_Earth) of large (>1 mm) dust grains, and are dominated by ionised gas emission for frequencies ~<15 GHz. In addition, we find evidence of unsettled millimetre dust in the inclined disk of VLA 1623 B possibly attributed to instabilities within the circumstellar disk, adding to the growing catalogue of unsettled Class 0/I disks. Our results represent some of the highest resolution observations possible with current instrumentation, particularly in the case of the VLA. However, our interpretation is still limited at low frequencies (~<22 GHz) and thus motivates the need for next-generation interferometers operating at centimetre wavelengths.