The attractive properties of gravity enable matter in dense cores to collapse into stars, spanning seven orders of magnitude in space and time, which makes modelling star formation a challenging multi-scale process. To circumvent this scale problem, stars are replaced by a sub-grid sink particle at a much larger scale. Sink particles are created above a threshold density and acquire mass and momentum through accretion. In models where binary star systems form and migrate to separations of a few cells, the accretion flow is unresolved and the relative accretion rate to the sink particles may become inaccurate. We introduce a new recipe for accretion onto binary sink particles that have overlapping accretion regions, and we implement an algorithm to track the angular momentum of sink particles as a proxy for stellar spin. Our preferential binary accretion recipe uses a virtual binary sink particle for the purpose of accretion and redistributes the accreted mass onto the sink particles according to results from models investigating binary accretion in detail. This solves problems common to current algorithms in many codes: (i) accretion is not suppressed due to large velocity differences between gas and stars, when that velocity is only internal to the binary system; (ii) the accretion rates are smoother for the unresolved close binaries in eccentric orbits; and (iii) non-physical suppression of accretion onto the secondary sink particle, when the primary dominates the potential, is eliminated. We test our implementation by comparing simulations at increasing resolution until the binaries are resolved. While not perfect, the algorithm mitigates undesired properties of current algorithms and is particularly useful for global models of star-forming regions. It may also be applied to other unresolved accreting binaries, such as compact objects in evolved star clusters and binary supermassive black holes in cosmological models.

Context . Protoplanetary disks evolve in clustered environments where interactions with nearby stars and interstellar gas are common. Such environmental processes, including stellar flybys and gas infall, can significantly perturb disk structures over a disk’s lifetime and potentially influence the evolution of embedded planets. Aims . We investigate how environmental interactions affect the architecture of class II systems that host both a disk and already-formed planets and assess their impact on disk structure and dynamics, as well as planetary evolution. Methods . We performed 3D simulations using the P HANTOM smoothed particle hydrodynamics code, including multiple dust species treated with a dust-as-particles approach that accounts for dust back-reaction on the gas. We modeled a disk hosting two planets in a 2:1 mean-motion resonance and subjected the system to two types of perturbations: an infalling gaseous cloudlet and a stellar flyby. Results . Infall and flyby perturbations change the disk morphology and dynamical state. Infalling gas increases the disk mass and angular momentum and dynamically excites the dust to produce eccentric and multi-ring dust structures. The stellar flyby truncates the disk, compacting the dust distribution radially and enhancing episodic radial migration of dust grains. These processes excite eccentricity in both gas and dust, leading to distinct accretion pathways for the planets. In particular, the flyby promotes inward dust migration that may enhance solid accretion by the planets, while infall preferentially increases the accretion rate of the inner planet. Conclusions . Environmental interactions during the class II phase can reshape disk-planet systems, imprinting dynamical signatures that may persist into later evolutionary stages. Both late infall and stellar flybys influence the growth and composition of planets; in particular, infall events can lead to the formation of eccentric, narrow debris disks.

Magnetic fields play a central role in the star-formation process, from diffuse gas to the dense, starless, molecular cloud cores that represent the first gravitationally bound structures on the path to star formation. Yet, the evolution of magnetic fields during this critical phase remains poorly understood. Recent studies suggest that cosmic-ray electrons interacting with magnetic fields in prestellar cores can produce detectable synchrotron emission at low radio frequencies, offering a novel probe of their magnetization in tandem with existing observational techniques. However, current instruments lack the angular resolution and sensitivity to exploit this signature. The Square Kilometre Array Observatory (SKAO) will provide the required capabilities enabling detections in nearby star-forming regions within a reasonable number of observation hours in AA* and AA4. Thanks to its large field of view, observations of low- to high-mass star-forming regions within the first kiloparsec from the Sun will enable both targeted studies of individual objects and statistical analyses over several hundreds of prestellar cores per pointing, marking a breakthrough in our understanding of their magnetic field properties. This chapter outlines the scientific context, observational challenges, and prospects for probing magnetic fields in prestellar cores with SKAO, and highlights synergies with complementary facilities such as ALMA, as well as cross-disciplinary collaborations within the SKAO community.

Protoplanetary discs evolve around newly-formed stars through an interplay of infall from surrounding turbulent cloud material, accretion towards the young star, and outflow driven mass-loss. It has been challenging to determine if discs are fed predominantly through infall along the disc midplane, or along the poles, and if accretion occurs in a steady or burst-like fashion. Here, we present a suite of 3D ideal magnetohydrodynamical simulations of protoplanetary disc formation and evolution in a dynamic, large-scale molecular cloud environment using the adaptive mesh refinement framework DISPATCH. We focus on nine stellar systems, where we resolve discs down to a scale of 0.8 au. Across the sample, stellar accretion proceeds at rates of ∼10^-5 M_⊙ yr^-1 over 10^5 yr, with significant variability. Discs grow to 100 au scales and remain gravitationally stable in time, with disc-to-star mass ratios below 10

Context. A significant fraction (>30%) of planet-forming disks and planetary are misaligned with respect to the rotational axis of their host stars, yet the dominant mechanism responsible for these misalignments remains unclear. Aims. We aim to observationally constrain the angular momentum of Class II protoplanetary disks and assess whether late-stage infall of material can bring sufficient angular momentum to tilt them. Methods. We first computed the angular momenta of 15 disks with surface density profiles inferred from dynamical modeling of high angular resolution ALMA observations. Based on this sample, we derived a relation linking disk angular momentum to stellar mass, disk mass, and the radius enclosing 90% of the 13CO flux and used it to estimate angular momenta of 18 more disks. We then compared disk values with theoretical predictions for late-stage accretion from clouds and observed streamers. Results. Angular momentum for most disks is lower than what theoretical models predict for late infall. This is also in qualitative agreement with comparison with streamer observations, however, characterization of mass of reservoirs feeding the streamers is needed to confirm this picture. Conclusions. Interactions with nearby clouds, resulting in late-stage infall of material onto Class II disks, can potentially explain the observed misalignments within disks and planetary systems.

Millimeter continuum spiral arms have so far only been detected in a handful of protoplanetary disks, and thus we have a limited understanding of the circumstances in which they can form. In particular, substructures in small disks ( R ⪅ 50 au) have not been well characterized in comparison with large disks. We present Atacama Large Millimeter/submillimeter Array 1.3 mm continuum observations of the disk around the T Tauri star Haro 6-13 at a resolution of ∼ 0 . ″ 04 (∼5 au). A pair of low-contrast spiral arms are detected at disk radii from ∼10 to 35 au. They can be approximated as Archimedean spirals with pitch angles ranging from ∼10° to 30°. The low value of the disk-averaged spectral index between 1.3 and 3 mm ( α = 2.1) and the high brightness temperatures suggest that the millimeter continuum is likely optically thick and thus may hide sufficient mass for the disk to become gravitationally unstable and form spiral arms. CO observations have shown that Haro 6-13 is surrounded by an envelope, raising the possibility that infall is facilitating spiral arm formation.

The study of the development of structures on multiple scales in the cold interstellar medium has experienced rapid expansion in the past decade, on both the observational and the theoretical front. Spectral line studies at (sub-)millimeter wavelengths over a wide range of physical scales have provided unique probes of the kinematics of dense gas in star-forming regions, and have been complemented by extensive, high dynamic range dust continuum surveys of the column density structure of molecular cloud complexes, while dust polarization maps have highlighted the role of magnetic fields. This has been accompanied by increasingly sophisticated numerical simulations including new physics (e.g., supernova driving, cosmic rays, non-ideal magneto-hydrodynamics, radiation pressure) and new techniques such as zoom-in simulations allowing multi-scale studies. Taken together, these new data have emphasized the anisotropic growth of dense structures on all scales, from giant ISM bubbles driven by stellar feedback on $\sim$50-100 pc scales through parsec-scale molecular filaments down to $<$0.1 pc dense cores and $<$1000 au protostellar disks. Combining observations and theory, we present a coherent picture for the formation and evolution of these structures and synthesize a comprehensive physical scenario for the initial conditions and early stages of star and disk formation.

Stars and their corresponding protoplanetary disks form in diverse environments. To account for these natural variations, we investigate the formation process around nine solar mass stars with a maximum resolution of 2 AU in a Giant Molecular Cloud of (40 pc)$^3$ in volume by using the adaptive mesh refinement code \ramses. The magnetohydrodynamic simulations reveal that the accretion process is heterogeneous in time, in space, and among protostars of otherwise similar mass. During the first roughly 100 kyr of a protostar evolving to about a solar mass, the accretion rates peak around $10^{-5}$ to $10^{-4}$ M$_{\odot}$ yr$^{-1}$ shortly after its birth, declining with time after that. The different environments also affect the spatial accretion, and infall of material to the star-disk system is mostly through filaments and sheets. Furthermore, the formation and evolution of disks varies significantly from star to star. We interpret the variety in disk formation as a consequence of the differences in the combined effects of magnetic fields and turbulence that may cause differences in the efficiency of magnetic braking, as well as differences in the strength and distribution of specific angular momentum.

The short-lived Al-26 and Fe-60 radionuclides are synthesized and expelled into the interstellar medium by core-collapse supernova events. The solar system's first solids, calcium-aluminum refractory inclusions (CAIs), contain evidence for the former presence of the Al-26 nuclide defining the canonical Al-26/(27) Al ratio of similar to 5 x 10(-5). A different class of objects temporally related to canonical CAIs are CAIs with fractionation and unidentified nuclear effects (FUN CAIs), which record a low initial Al-26/Al-27 of 10(-6). The contrasting level of Al-26 between these objects is often interpreted as reflecting the admixing of the Al-26 nuclides during the early formative phase of the Sun. We use giant molecular cloud scale adaptive mesh-refinement numerical simulations to trace the abundance of Al-26 and Fe-60 in star-forming gas during the early stages of accretion of individual low-mass protostars. We find that the Al-26/Al-27 and Fe-60/Fe-56 ratios of accreting gas within a vicinity of 1000 au of the stars follow the predicted decay curves of the initial abundances at the time of star formation without evidence of spatial or temporal heterogeneities for the first 100 kyr of star formation. Therefore, the observed differences in Al-26/Al-27 ratios between FUN and canonical CAIs are likely not caused by admixing of supernova material during the early evolution of the proto-Sun. Selective thermal processing of dust grains is a more viable scenario to account for the heterogeneity in Al-26/Al-27 ratios at the time of solar system formation.