The spectral lines of the CH molecule are a key carbon (C) abundance diagnostic in FGKM-type stars. These lines are detectable in metal-rich and, in contrast to atomic C lines, also in metal-poor late-type stars. However, only 3D LTE analyses of the CH lines have been performed so far. We test the formation of CH lines in the solar spectrum, using for the first time, 3D Non-LTE (NLTE) models. We also aim to derive the solar photospheric abundance of C, using the diagnostic transitions in the optical (4218-4356 angstrom) and infrared (33025-37944 angstrom). We use the updated NLTE model molecule from S. A. Popa et al. (2023) and different solar 3D radiation-hydrodynamics model atmospheres. The models are contrasted against new spatially resolved optical solar spectra, and the centre-to-limb variation (CLV) of CH lines is studied. We find generally small (similar to 0.01 dex) NLTE effects in the optical and IR diagnostic CH A-X lines in the solar atmosphere. Both 3D NLTE and 3D LTE spectral modelling yield an excellent fit to the solar intensity observations at all viewing angles. The 1D LTE and 1D NLTE models fail to describe the line CLV, and lead to underestimated solar C abundances. The 3D NLTE modelling of diagnostic lines in the optical and IR yields a carbon abundance of A(C) = 8.52 +/- 0.07 dex. The estimate is in agreement with recent results based on neutrino fluxes measured by Borexino. 3D NLTE modelling and tests on spatially resolved solar data are essential to derive robust solar abundances. The analysis presented here focuses on CH, but we expect that similar effects will be present for other molecules of astrophysical interest.

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

Systematic abundance differences that depend on the condensation temperatures of elements have been observed, in particular for stars similar to the Sun; solar twins and solar analogs. Similar differences have also recently been shown to exist between solar abundances and abundances of refractory elements in primitive chondrites. Numerous mechanisms have been proposed to account for these effects, including also differences observed in binary systems. Rather than relying on specific mechanisms, this paper aims to show that the observed effects are a natural and unavoidable outcome of the star formation process itself, in which the associated outflows (winds and jets) carry away material, with efficiency varying with condensation temperature. By using analysis based on modeling results and scaling laws, the trends and magnitudes of the effects are investigated, in three contexts: 1) with respect to differences between the Sun and solar twins, 2) with respect to differences between the Sun and CI-chondrites, and 3) with respect to differences between members of binaries. It is shown that protoplanetary disk outflows indeed are expected to have differential abundance effects, with trends and magnitudes consistent with observed abundance effects. The qualitative as well as semi-quantitative character of the effects are reproduced, in all three contexts. The results demonstrate that the observed systematic differences are likely results of the disk outflows associated with the accretion process. In contrast to mechanisms relying on the tiny mass of planets leaving an observable signature, outflows carry away masses similar to the entire mass of the star, thus much more easily resulting in differential effects with the magnitudes observed, without for example having to assume that the abundance differences are limited to the convection zones of the stars.

We present a GPU-accelerated implementation of a magnetohydrodynamic (MHD) solver using directive-based programming with OpenMP target offloading. The solver is integrated into the DISPATCH framework, which organises the computational domain into a collection of asynchronously updated patches. To reduce GPU kernel launch overhead, patches are grouped into "bunches" that are updated collectively. While porting the particular solver required a complete code refactoring, it yielded performance gains on both GPU and CPU. A stand-alone mini-app achieved a 7.3x speed-up compared to a single NVIDIA A100 GPU to seven AMD 7F72 Rome CPU cores. Within the full DISPATCH framework, the GPU-accelerated MHD Bunch solver showed excellent agreement with the CPU-based reference implementation on standard test problems such as the Sod shock tube and Orszag-Tang vortex. In large-scale 3D tests, the GPU implementation achieved a 9.8x overall speedup, comparing one GPU to 12 CPU cores, with the core MHD update routine being two orders of magnitude faster on the GPU than on a single CPU core. These results demonstrate that OpenMP offloading can provide substantial performance improvements for astrophysical codes while maintaining portability and accuracy. The work also demonstrates how new codes should be structured to allow simple and efficient directive-based GPU offloading.

Context . While class II pre-main-sequence (PMS) stars have already accreted most of their mass, the continued inflow of fresh material via Bondi-Hoyle accretion acts as an additional mass reservoir for their circumstellar disks. This may explain the observed accretion rates of PMS stars, as well as observational inconsistencies in the mass and angular momentum balance of their disks. Aims . Using a new simulation that reproduces the stellar initial mass function (IMF), we want to quantify the role of Bondi-Hoyle accretion in the formation of class II disks, as well as address the prospect of its observational detection with the James Webb Space Telescope (JWST). Methods . We studied the mass and angular momentum of the accreting gas using passively advected tracer particles in the simulation, and we carried out radiative transfer calculations of near-infrared scattering to generate synthetic JWST observations of Bondi-Hoyle trails of PMS stars. Results . Gas accreting on class II PMS stars approximately 1 Myr after their formation has enough mass and angular momentum to strongly affect the evolution of the preexisting disks. The accreted angular momentum is large enough to also explain the observed size of class II disks. The orientation of the angular momentum vector can differ significantly from that of the previously accreted gas, which may result in a significant disk warping or misalignment. We also predict that JWST observations of class II stars will be able to detect Bondi-Hoyle trails with a 80%-100% success rate with only a 2 min exposure time, depending on the filter, if stars with both an accretion rate Ṁ > 5 × 10 −10 M ⊙ /yr and a luminosity of L > 0.5 L ⊙ are selected.

Understanding the origin and evolution of carbon-enhanced metal-poor (CEMP) stars is key to tracing the Galaxy's early chemical enrichment. We investigate how realistic 3D radiation-hydrodynamics (RHD) model atmospheres affect carbon abundances in CEMP stars and implications for their classification and Galactic chemical evolution (GCE). We focus on biases from traditional 1D hydrostatic models. We use the M3DIS code to compute 3D RHD model atmospheres for main-sequence and sub-giant stars over a wide range of metallicities and carbon enhancements. Synthetic spectra of the CH G-band are calculated with 3D radiative transfer and compared to spectra from classical 1D MARCS models. We derive abundance corrections and apply them to a large SAGA database sample to quantify effects on the carbon abundance distribution and CEMP classification. Our new 3D CEMP models predict cooler upper atmospheric layers than in 1D models, resulting in stronger CH absorption and lower inferred carbon abundances by up to -0.9 dex at the lowest metallicities. Carbon enhancement in the atmosphere itself increases molecular opacities and leads to radiative re-heating, partly offsetting adiabatic cooling in 3D models and reducing 3D-1D abundance corrections. Applying these corrections lowers the CEMP fraction by up to 20

Context. Magnetic reconnection is a key mechanism for energy release in the solar atmosphere, but its kinetic-scale microphysics remains difficult to model in large-scale solar geometries.Aims. We investigate whether fully kinetic particle-in-cell (PIC) simulations can be stably and meaningfully embedded within global magnetohydrodynamic (MHD) models of the solar corona using a realistic fan-spine magnetic configuration.Methods. We employed a two-way coupled PIC-MHD scheme implemented in the DISPATCH code framework. The PIC solver is embedded within a reconnecting current sheet in a solar-like topology. A physical adjustment of constants is used to bridge kinetic and fluid scales while maintaining self-consistent plasma ordering.Results. The system evolves stably over more than 45 000 ion plasma periods, exhibiting clear kinetic signatures such as Hall-driven quadrupolar magnetic fields, a reconnection geometry reminiscent of the Petschek configuration, and supra-thermal particle populations. The reconnection rate in the PIC region remains steady and physically consistent, while coupling artefacts are effectively suppressed by fade-in/fade-out boundary weighting.Conclusions. Our results demonstrate that fully kinetic reconnection can be embedded in global solar MHD models with physical fidelity and computational efficiency. This hybrid approach offers a practical pathway to multi-scale kinetic modelling in realistic astrophysical environments.

Aims. Understanding the origin and evolution of carbon-enhanced metal-poor (CEMP) stars is key to tracing the early chemical enrichment of the Galaxy. In this work we investigate how physically realistic 3D radiation-hydrodynamic (RHD) carbon-enhanced model atmospheres affect the inferred carbon abundances in CEMP stars, and assess the implications for their classification and for Galactic chemical evolution (GCE). We pay particular attention to the systematic biases introduced by traditional 1D hydrostatic equilibrium (HE) models.Methods. We used the M3DIS code to compute 3D RHD model atmospheres for main-sequence and sub-giant stars spanning a wide range of metallicities and carbon enhancements. Synthetic spectra of the CH G band were calculated using full 3D radiative transfer and compared to spectra from classical 1D HE MARCS models. We derived abundance corrections and applied them to a large literature sample of metal-poor stars from the SAGA database to quantify systematic effects on the carbon abundance distribution and CEMP classification.Results. Our new 3D CEMP models predict significantly cooler upper atmospheric layers than 1D HE models, resulting in stronger CH absorption and lower inferred carbon abundances by up to -0.9 dex at the lowest metallicities. Carbon enhancement in the atmosphere itself increases molecular opacities and leads to radiative re-heating, which partly offsets the adiabatic cooling in 3D models and reduces the magnitude of 3D - 1D abundance corrections. Applying these corrections lowers the CEMP fraction by up to 20% below [Fe/H] = -3, and furthermore alters the relative contribution of CEMP sub-classes. In particular, the fraction of stars classified as CEMP-no increases and that of CEMP-r/s stars decreases, owing to the downward revision of absolute carbon abundances. These changes bring the Galactic carbon abundance distribution into better agreement with GCE models that assume a contribution from faint supernovae of 20%. Physically realistic model atmospheres are thus essential for a reliable reconstruction of the early chemical enrichment history of the Galaxy.

Protoplanetary disks are traditionally described as finite-mass reservoirs left over by the gravitational collapse of the protostellar core, a view that strongly constrains both disk-evolution and planet-formation models. We propose a different scenario in which protoplanetary disks of pre-main sequence stars are primarily assembled by Bondi-Hoyle accretion from the parent gas cloud. We demonstrate that Bondi-Hoyle accretion can supply not only the mass but also the angular momentum necessary to explain the observed size of protoplanetary disks. Additionally, we predict how the specific angular momentum of protoplanetary disks scales with stellar mass. Our conclusions are based on an analytical derivation of the scaling of the angular momentum in turbulent flows, which we confirmed with a numerical simulation of supersonic turbulence. A key outcome of our analysis is the recognition that density fluctuations in supersonic turbulence-previously overlooked in studies of cloud and core rotation-lead to a significant increase in angular momentum at disk-forming scales. This revised understanding of disk formation and evolution alleviates several long-standing observational discrepancies and compels substantial revisions to current models of disk and planet formation.

Context. Simulating solar flares requires capturing both large-scale magnetohydrodynamic (MHD) evolution and small-scale kinetic processes near reconnection sites. Bridging these scales has been a significant computational challenge. Aims. This study introduces a Particle-In-Cell (PIC) solver integrated within the DISPATCH framework, facilitating seamless embedding within MHD simulations. This development aims to enable self-consistent multi-scale solar flare simulations. Methods. Our PIC solver, inspired by the PhotonPlasma code, addresses the Vlasov-Maxwell equations in collisionless plasma. We validate its accuracy through fundamental plasma tests - including plasma oscillations, two-stream instability, and current sheet reconnection. To make kinetic simulations computationally feasible, we employ physical adjustment of constants (PAC), modifying the speed of light, elementary charge, and electron mass to shift plasma scales. Additionally, we implement and validate a coupling strategy that enables smooth transitions between kinetic and fluid regimes. Results. The PIC solver successfully recovers expected plasma dynamics and electromagnetic field behaviour. Our analysis highlights the effects of PAC on reconnection dynamics, underscoring the importance of transparent and well-documented scaling choices. Test cases involving propagating waves across PIC-MHD interfaces confirm the robustness of our coupling approach. Conclusions. The integration of the PIC solver into the DISPATCH framework makes it possible to run self-consistent, multiscale solar flare simulations. Our approach provides a computationally efficient foundation for investigating reconnection physics in large-scale astrophysical plasmas.

The streaming instability is a mechanism whereby pebble-sized particles in protoplanetary discs spontaneously come together in dense filaments, which collapse gravitationally to form planetesimals upon reaching the Roche density. The extent of the filaments along the orbital direction is nevertheless poorly characterised, due to a focus in the literature on small simulation domains where the behaviour of the streaming instability on large scales cannot be determined. We present here computer simulations of the streaming instability in boxes with side lengths up to 6.4 scale heights in the plane. This is 32 times larger than typically considered simulation domains and nearly a factor 1000 times the volume. We show that the azimuthal extent of filaments in the non-linear state of the streaming instability is limited to approximately one gas scale height. The streaming instability will therefore not transform the pebble density field into axisymmetric rings; rather the non-linear state of the streaming instability appears as a complex structure of loosely connected filaments. Including the self-gravity of the pebbles, our simulations form up to 4000 planetesimals. This allows us to probe the high-mass end of the initial mass function of planetesimals with much higher statistical confidence than previously. We find that this end is well-described by a steep exponential tapering. Since the resolution of our simulations is moderate - a necessary trade-off given the large domains - the mass distribution is incomplete at the low-mass end. When putting comparatively less weight on the numbers at low masses, at intermediate masses we nevertheless reproduce the power-law shape of the distribution established in previous studies.

Context. Simulating solar flares, which involve large-scale dynamics and small-scale magnetic reconnection, poses significant computational challenges. Aims. This study aims to develop an explicit Particle-In-Cell (PIC) solver within the DISPATCH framework to model the small-scale kinetic processes in solar corona setting. This study in the first in a series with the ultimate goal to develop a hybrid PIC-MHD solver, to simulate solar flares. Methods. The PIC solver, inspired by the PhotonPlasma code, solves the Vlasov-Maxwell equations in a collisionless regime using explicit time-staggering and spatial-staggering techniques. Validation included unit tests, plasma frequency recovery, two-stream instability, and current sheet dynamics. Results. Validation tests confirmed the solver's accuracy and robustness in modeling plasma dynamics and electromagnetic fields. Conclusions. The integration of the explicit PIC solver into the DISPATCH framework is the first step towards bridging the gap between large and small scale dynamics, providing a robust platform for future solar physics research.

We describe the Stagger Code for simulations of magneto-hydrodynamic (MHD) systems. This is a modular code with a variety of physics modules that will let the user run simulations of deep stellar atmospheres, sunspot formation, stellar chromospheres and coronae, proto-stellar disks, star formation from giant molecular clouds and even galaxy formation. The Stagger Code is efficiently and highly parallelizable, enabling such simulations with large ranges of both spatial and temporal scales. We, describe the methodology of the code, and present the most important of the physics modules, as well as its input and output variables. We show results of a number of standard MHD tests to enable comparison with other, similar codes. In addition, we provide an overview of tests that have been carried out against solar observations, ranging from spectral line shapes, spectral flux distribution, limb darkening, intensity and velocity distributions of granulation, to seismic power-spectra and the excitation of p modes. The Stagger Code has proven to be a high fidelity code with a large range of uses.

Context. In this paper we present a grid of self-consistent 1D model atmospheres of cool stars, sub-stellar objects, and exoplanets in the effective temperature range 300-3000 K, including cloud formation, chemical non-equilibrium effects, and stellar irradiation.Aims. The new grid extends the classical MARCS model atmosphere grid from 2008 towards lower effective temperatures and a broader range of object types. Methods. The new model atmosphere computations, MSG, are based on a combination of three well-tested codes, the classical MARCS 1D atmospheres, the StaticWeather cloud formation code, and the GGchem chemical equilibrium code. The combined code has been updated with new and more complete molecular and atomic opacities, cloud formation, and advanced chemical equilibrium calculations, and we also added new numerical methods at low temperatures to allow for a more robust convergence. Results. The coupling between the MARCS radiative transfer and GGchem chemical equilibrium computations has effectively made it possible to reach convergence based on the electron pressure for warmer models and gas pressure for cooler models, enabling self-consistent modelling of stellar, sub-stellar, and exoplanetary objects in a very wide range of effective temperatures. We will make new cloud-free and non-irradiated models for solar metallicity and a selected variety of other chemical compositions immediately available from our home page (https://cels.nbi.ku.dk). Illustrative examples of cloudy and irradiated models as well as models based on non-equilibrium chemistry are also presented, and we will describe these in more detail and make them available upon completion at the same place for a larger range of parameter space. Conclusions. For solar metallicity models, the new additional molecular opacities only affect the structure of models cooler than T-eff = 2500 K, and the effect becomes substantial for models below T-eff similar to 1500 K. Atomic line opacities are important for models warmer than similar to 3000 K. The line profile of the molecular opacities may have a larger effect on the model structure than previously anticipated, particularly in the uppermost layers at low gas pressure. The qualitative changes in the relative abundances of TiO, H2O, CH4, NH3, and other molecules in our models follow the observationally defined M, L, T (and Y) sequences, but they also reveal more complex and depth-dependent abundance changes and therefore a spectral classification depending on more parameters. The self-consistent coupling to StaticWeather cloud computations allows for detailed comparison between nucleation and observed relative dimming of different spectral bands, with advanced applications for new identification methods of potential exoplanetary biology.

Context . Large-scale stellar surveys, such as SDSS-V, 4MOST, WEAVE, and PLATO, require accurate atmospheric models and synthetic spectra of stars for accurate analyses of fundamental stellar parameters and chemical abundances. Aims . The primary goal of our work is to develop a new approach to solve radiation-hydrodynamics (RHD) and generate model stellar spectra in a self-consistent and highly efficient framework. Methods . We build upon the Copenhagen legacy RHD code, the MULTI3D non-local thermodynamic equilibrium (NLTE) code, and the DISPATCH high-performance framework. The new approach allows us to calculate 3D RHD models of stellar atmospheres on timescales of a few thousand CPU hours and to perform subsequent spectrum synthesis in local thermodynamic equilibrium (LTE) or NLTE for the desired physical conditions within the parameter space of FGK-type stars. Results . We compare the 3D RHD solar model with other available models and validate its performance against solar observations, including the centre-to-limb variation of intensities and key solar diagnostic lines of H and Fe. We show that the performance of the new code allows to overcome the main bottleneck in 3D NLTE spectroscopy and enables calculations of multi-dimensional grids of synthetic stellar observables for comparison with modern astronomical observations.

Characterizing prestellar cores in star-forming regions is an important step towards the validation of theoretical models of star formation. Thanks to their sub-arcsecond resolution, ALMA observations can potentially provide samples of prestellar cores up to distances of a few kpc, where regions of massive star formation can be targeted. However, the extraction of real cores from dust-continuum observations of turbulent star-forming clouds is affected by complex projection effects. In this work, we study the problem of core extraction both in the idealized case of column-density maps and in the more realistic case of synthetic 1.3\,mm ALMA observations. The analysis is carried out on 12 regions of high column density from our 250 pc simulation. We find that derived core masses are highly unreliable, with only {\em a weak correlation between the masses of cores selected in the synthetic ALMA maps and those of the corresponding three-dimensional cores}. The fraction of real three-dimensional cores detected in the synthetic maps increases monotonically with mass and remains always below 50\%. Above $\sim 1\,M_{\odot}$, the core mass function derived from the column-density maps is steeper than that of the three-dimensional cores, while the core mass function from the synthetic ALMA maps has a slope closer to that of the real three-dimensional cores. Because of the mass uncertainties, proper guidance from realistic simulations is essential if ALMA observations of protoclusters at kpc distances are to be used to test star-formation models.

The dynamical state of massive clumps is key to our understanding of the formation of massive stars. In this work, we study the kinematic properties of massive clumps using synthetic observations. We have previously compiled a very large catalog of synthetic dust-continuum compact sources from our 250 pc, SN-driven, star formation simulation. Here, we compute synthetic $\rm N_{2}H^{+}$ line profiles for a subsample of those sources and compare their properties with the observations and with those of the corresponding three-dimensional (3D) clumps in the simulation. We find that the velocity dispersion of the sources estimated from the $\rm N_{2}H^{+}$ line is a good estimate of that of the 3D clumps, although its correlation with the source size is weaker than the velocity-size correlation of the 3D clumps. The relation between the mass of the 3D clumps, $M_{\rm main}$, and that of the corresponding synthetic sources, $M_{\rm SED}$, has a large scatter and a slope of 0.5, $M_{\rm main} \propto M_{\rm SED}^{0.5}$, due to uncertainties arising from the observational band-merging procedure and from projection effects along the line of sight. As a result, the virial parameters of the 3D clumps are not correlated with the clump masses, even if a negative correlation is found for the compact sources, and the virial parameter of the most massive sources may significantly underestimate that of the associated clumps.

Solar modelling has long been split into ”internal” and ”surface” modelling, because of the lack of tools to connect the very different scales in space and time, as well as the widely different environments and dominating physical effects involved. Significant efforts have recently been put into resolving this disconnect. We address the outstanding bottlenecks in connecting internal convection zone and dynamo simulations to the surface of the Sun, and conduct a proof-of-concept high resolution global simulation of the convection zone of the Sun, using the task-based DISPATCH code framework. We present a new `volleyball' mesh decomposition, which has Cartesian patches tessellated on a sphere with no singularities. We use our new entropy based HLLS approximate Riemann solver to model magneto-hydrodynamics in a global simulation, ranging between 0.655 – 0.995 R_⊙, with an initial ambient magnetic field set to 0.1 Gauss. The simulations develop convective motions with complex, turbulent structures. Small-scale dynamo action twists the ambient magnetic field and locally amplifies magnetic field magnitudes by more than two orders of magnitude within the initial run-time.

Systematic surveys of massive clumps have been carried out to study the conditions leading to the formation of massive stars. These clumps are typically at large distances and unresolved, so their physical properties cannot be reliably derived from the observations alone. Numerical simulations are needed to interpret the observations. To this end, we generate synthetic Herschel observations using our large-scale star-formation simulation, where massive stars explode as supernovae driving the interstellar-medium turbulence. From the synthetic observations, we compile a catalog of compact sources following the exact same procedure as for the Hi-GAL compact source catalog. We show that the sources from the simulation have observational properties with statistical distributions consistent with the observations. By relating the compact sources from the synthetic observations to their three-dimensional counterparts in the simulation, we find that the synthetic observations overestimate the clump masses by about an order of magnitude on average due to line-of-sight projection, and projection effects are likely to be even worse for Hi-GAL Inner Galaxy sources. We also find that a large fraction of sources classified as protostellar are likely to be starless, and propose a new method to partially discriminate between true and false protostellar sources.