High-mass stars form in protoclusters, where gravo-magnetic processes elongate collapsing clouds and clumps preferentially perpendicular to magnetic (B) fields. Yet it remains unclear whether gravo-magnetic processes still govern the formation of 0.01-pc-scale condensations in massive star-forming protoclusters, which are crucial for understanding the stellar initial mass function and multiplicity. Here we report on statistical evidence that the condensation elongations are preferentially aligned with local B fields, based on dust polarization observations towards 30 massive star-forming regions with the Atacama Large Millimeter/submillimeter Array. Our clustered massive star formation simulations reveal that this more parallel alignment is exclusively observed in models where the initial turbulence dominates the B fields. By contrast, models in which the initial B fields dominate the turbulence distinctly exhibit a more perpendicular alignment. The comparison between observations and simulations indicates that turbulence could play a more important role than B fields in the formation of condensations in the context of clustered massive star formation. Moreover, we find a possibly turbulence-induced preferential misalignment between the B field and rotation axis of condensations, which may potentially reduce the magnetic braking efficiency and facilitate massive disk formation. Our findings indicate that turbulence could be more critical than previously thought in determining the initial stellar properties.

Cosmic rays (CRs) are an integral part of the nonthermal pressure budget in the interstellar medium (ISM) and are the leading-order ionization mechanism in cold molecular clouds. We study the impacts that different microphysical CR diffusion coefficients and streaming speeds have on the evolution of isothermal, magnetized, turbulent plasmas, relevant to the cold ISM. We utilized a two-moment CR magnetohydrodynamic model, allowing us to dynamically evolve both CR energy and flux densities with contributions from Alfv & eacute;nic streaming and anisotropic diffusion. We identify coupled and decoupled regimes, and define dimensionless Prandtl numbers Pm-c and Pm-s, which quantify whether the plasma falls within these two regimes. In the coupled regime-characteristic of slow streaming (Pm-s < 1) and low diffusion (Pm-c < 1)-the CR fluid imprints upon the plasma a mixed equation of state between P-c proportional to rho(4/3) (relativistic fluid) and P-c proportional to rho(2/3) (streaming), where P-c is the CR pressure and rho is the plasma density. By modifying the sound speed, the coupling reduces the turbulent Mach number, and hence the amplitude of the density fluctuations, while supporting secular heating of the CR fluid. In contrast, in the decoupled regime (Pm-s > 1 or Pm-c > 1) the CR fluid and the plasma have negligible interactions. We further show that CR heating is enabled by coherent structures within the compressible velocity field, with no impact on the turbulence spectrum of incompressible modes.

Cosmic rays (CRs) are known to play a key role in many astrophysical environments: they can modify shock dynamics, influence the thermochemistry and the ionization of the interstellar medium, regulate galaxy mass content by driving galactic winds, and be released by jets from active galactic nuclei. They also serve as important observational tracers through γ-ray emission, radio synchrotron, and secondary particle production. Since CR particles follow power-law distributions in momentum space spanning many decades in energy, and because diffusion and radiative losses further shape these spectra, it is crucial to model spectrally resolved CRs in numerical simulations and to assess the impact of this modeling on gas dynamics and observational signatures. We present a consistent multi-group spectral method in momentum space for CR protons called RAMSES-MCR in the adaptive mesh refinement code RAMSES, based on the two-moment formalism that evolves both CR energy and number density in momentum space, together with their associated flux. The modeled CR processes include advection, anisotropic/isotropic diffusion, streaming instability, Coulomb and hadronic losses, adiabatic changes, and feedback onto the gas. We also show that the method can be naturally extended to CR electrons (e.g. including synchrotron losses) and generalized to multiple CR species. The implementation is validated against a suite of standard multi-dimensional tests. We finally apply RAMSES-MCR to the three-dimensional expansion of a supernova remnant including CRs with anisotropic diffusion and energy losses, and demonstrate how CR energy redistributes in a momentum-dependent manner and modifies the gas momentum during the snowplough phase.

Context. The formation and early evolution of protoplanetary disks during a gravitational collapse are governed by a wide variety of physical processes. Observations have begun probing disks in their earliest stages, and have favored the magnetically regulated disk formation scenario. Disks are also expected to exhibit ellipsoidal morphologies in the early phases, an aspect that has been widely overlooked. Aims. We aim to describe the birth and evolution of the disk while accounting for the eccentric motions of fluid parcels. Methods. Using 3D radiative magnetohydrodynamic simulations with ambipolar diffusion, we self-consistently modeled the collapse of isolated 1 M-circle dot and 3 M-circle dot cores and the subsequent formation of a central protostar surrounded by a disk. We accounted for dust dynamics and employed gas tracer particles to follow the thermodynamical history of fluid parcels. Results. We find that magnetic fields and turbulence drive highly anisotropic accretion onto the disk via dense streamers. This streamer-fed accretion, occurring from the vertical and radial directions, drives vigorous internal turbulence that facilitates efficient angular momentum transport and rapid radial spreading. Crucially, the anisotropic inflow delivers material with an angular momentum deficit that continuously generates and sustains a significant disk eccentricity (e similar to 0.1). Conclusions. Our results reveal ubiquitous eccentric kinematics in Class 0 disks, with direct implications for disk evolution, planetesimal formation, and the interpretation of cosmochemical signatures in Solar System meteorites.

Although protostars and disks are often studied separately owing to numerical and observational challenges, breakthroughs in recent years have highlighted the need to study both objects in concert. The role of magnetic fields in this regard must be investigated. We aim to describe the birth of the protostar and that of its disk, as well as their early joint evolution following the second collapse. We wish to study the structure of the nascent star-disk system, while focusing on the innermost sub-AU region. We carry out high resolution 3D RMHD simulations, describing the collapse of dense cloud cores to stellar densities. The calculations reach $\approx 2.3$ yr after protostellar birth. Our simulations are also compared to their hydro counterpart to better isolate the role of magnetic fields. When accounting for ambipolar diffusion, the efficiency of magnetic braking is drastically reduced and the nascent protostar reaches breakup velocity, thus forming a rotationally supported disk. The diffusion of the magnetic field also allows for the implantation of a $\sim \mathrm{kG}$ field in the protostar, which is thereafter maintained. The magnetic field is mainly toroidal in the star-disk system, although a notable vertical component threads it. We also show that the nascent disk is prone to the MRI, although our resolution is inadequate to capture the mechanism. We note a sensitivity of the disk's properties with regards to the angular momentum inherited prior to the second collapse, as well as the magnetic field strength. These calculations carry multiple implications on several issues in stellar formation theory, and offer perspectives for future modeling of the system. Should the fossil field hypothesis to explain the origins of magnetic fields in young stellar objects hold, we show that a $\sim \mathrm{kG}$ field strength may be implanted and maintained in the protostar at birth.

Context. Dust grains play a crucial role in the modeling of protostellar formation, particularly through their opacity and interaction with the magnetic field. The destruction of dust grains in numerical simulations is currently modeled primarily by temperaturedependent functions. However, a dynamical approach could be necessary to accurately model the vaporization of dust grains. Aims. We focused on modeling the evolution of dust grains during star formation, specifically on the vaporization of the grains by chemisputtering. We also investigated the evolution of non-ideal magnetohydrodynamic resistivities and the Planck and Rosseland mean opacities influenced by the grain evolution. Methods. We modeled the evolution of the dust by considering spherical grains at thermal equilibrium with the gas phase, composed only of one kind of material for each grain. We then took into account the exchange processes that can occur between the grains and the gas phase and that make the grain size evolve. We considered three materials for the grains: carbon, silicate, and aluminum oxide. Given a temporal evolution in temperature and density of the gas phase, we computed the evolution of a dust grain distribution. This evolution was then used to compute the non-ideal magnetohydrodynamic resistivities and the Planck and Rosseland mean opacities. Results. We observed a significant dependence of the sublimation temperature of the carbon grains on the dynamical evolution of the gas phase. The application of our method to trajectories where the temperature and density of the gas decrease after the sublimation of a portion of the grain distribution highlights the limitations of current vaporization prescriptions in simulations. Conclusions. The dynamical approach leads to more accurate results for the carbon grain quantity when the temperature and density of the gas evolve quickly. The dynamical approach application to collapse and disk evolution is then foreseen with its integration into hydrodynamic simulations.

The early stages of stellar birth comprise a two-step process involving the formation of two hydrostatic cores. The second step of gravitational collapse sets the radiative efficiency and accretion rate of the young protostar. These two parameters, which are of prime importance for protostellar evolution, dictate the luminosities and thus play a key role in deciphering the current discrepancy between observational surveys and theoretical models. In this Letter, we provide quantitative estimates on the evolution of the radiative efficiency and accretion rate obtained from self-consistent, high-resolution, radiative hydrodynamic simulations performed using the codes PLUTO and RAMSES. The main highlight of our result is that the radiative efficiency reaches unity, that is, supercriticality, relatively quickly after protostellar birth. Supercriticality at the accretion shock is a necessary condition for cold accretion. Our results thus support a rapid transition to the cold accretion scenario, which is one of the assumptions used in pre-main sequence (PMS) models when working towards solutions to explain observational data. We briefly discuss the implications of the time evolution of the radiative efficiency factor in the context of the luminosity problem, the protostellar luminosity function, PMS evolution, accurate sink properties, and the stellar initial mass function.

The formation mechanism of brown dwarfs (BDs), whether akin to stars or ejected planetary-mass objects, remains debated. We present the first 3D radiation-MHD simulations of magnetized, turbulent, gravitationally unstable low-mass cores (0.05–0.1 M _⊙ ) collapsing into proto-BDs. Using the RAMSES code with adaptive mesh refinement, we model the full dynamical range (10 ^5 –10 ^22 cm ^−3 ), including radiative transfer (flux limited diffusion) and nonideal MHD (ambipolar diffusion). Our simulations self-consistently follow the isothermal collapse, first hydrostatic core formation, H _2 dissociation, and BD birth. The resulting BDs have initial radii ≈0.75 R _⊙ and masses ≈0.8 M _Jup , growing via accretion as we follow the early evolution of the object. Crucially, we find that BDs may form similarly to low-mass stars but with a prolonged first-core phase, supporting a starlike formation scenario.

Cosmic rays (CRs) are an important source of feedback in a variety of astrophysical contexts. Magneto-hydrodynamical (MHD) simulations treating CRs as a fluid have shown that how their feedback operates is strongly dependent on their transport properties such as diffusion and streaming. In this paper we introduce the numerical implementation, in the adaptive-mesh-refinement MHD code RAMSES, of the grey two-moment formulation of CR fluid dynamics, which follows the energy density and its associated three-dimensional flux. This method is tested for CR diffusion, streaming, and advection in a series of multi-dimensional tests including shocks to check the robustness and stability of this numerical two-moment CRMHD solver. We finally use the new two-moment CR implementation in a complex simulation of an isolated galactic disc producing galaxy-wide outflows launched by small-scale supernova explosions, and compare it with a previously existing one-moment formulation in the same code.

We investigate the previously unexplored role of magnetic fields in the formation of second-generation (SG) stars in proto-globular clusters (GCs) using 3D radiation-magnetohydrodynamical simulations. This study is based on the asymptotic giant branch (AGB) scenario and incorporates photoionization feedback and stellar winds from AGB stars. We model SG formation within a young (34 Myr) massive (10^6 Msun) proto-GC moving through a magnetized, homogeneous interstellar medium. Our results indicate that variations in magnetic field strength and orientation significantly influence the gas geometry and SG star-forming regions around the cluster. Overall, magnetic fields limit SG formation to the very center of the cluster, with stronger magnetic fields tending to form more compact SG clusters. For magnetic field strengths of 0.5 and 5 microG, we observe no substantial changes in the mass of formed SG stars. However, with a strong 50 microG field, we see a 25 percent increase or a 70 percent decrease in total SG mass, for a field aligned parallel or perpendicular to the cluster's motion, respectively. This variation reflects how magnetic fields influence gas accretion, as our results suggest that gas accreted from the interstellar medium (ISM) slightly dominates over AGB ejecta in the cluster, except in cases of strong perpendicular fields, where gas accretion is efficiently suppressed. Additionally, stronger magnetic fields limit the cluster's ability to retain its ejecta, leading to the formation of stars with lower helium abundances. On the other hand, a strong perpendicular magnetic field produces SG stars that originate from AGB ejecta and exhibit the highest helium abundances.

A complete understanding of the initial conditions of high-mass star formation and what processes determine multiplicity requires the study of the magnetic field in young massive cores. Using Atacama Large Millimeter/submillimeter Array (ALMA) 250 GHz polarization observations (0 .'' 3 = 1000 au) and ALMA 220 GHz high-angular-resolution observations (0 .'' 05 = 160 au), we have performed a full energy analysis including the magnetic field at core scales and have assessed what influences the multiplicity inside a massive core previously believed to be in the prestellar phase. With a mass of 31 M circle dot, the G11.92 MM2 core has a young CS molecular outflow with a dynamical timescale of a few thousand years. At high resolution, the MM2 core fragments into a binary system, with a projected separation of 505 au and a binary mass ratio of 1.14. Using the Davis-Chandrasekhar-Fermi method with an angle dispersion function analysis, we estimate in this core a magnetic field strength of 6.2 mG and a mass-to-magnetic-flux ratio of 18. The MM2 core is strongly subvirialized, with a virial parameter of 0.064, including the magnetic field. The high mass-to-magnetic-flux ratio and low virial parameter indicate that this massive core is very likely undergoing runaway collapse, which is in direct contradiction with the core accretion model. The MM2 core is embedded in a filament that has a velocity gradient consistent with infall. In line with clump-fed scenarios, the core can grow in mass at a rate of 1.9-5.6 x 10-4 M circle dot yr-1. In spite of the magnetic field having only a minor contribution to the total energy budget at core scales (a few thousands of astronomical units), it likely plays a more important role at smaller scales (a few hundreds of astronomical units) by setting the binary properties. Considering energy ratios and a fragmentation criterion at the core scale, the binary system could have been formed by core fragmentation. The binary system properties (projected separation and mass ratio), however, are also consistent with radiation-magnetohydrodynamic simulations with super-Alfvenic or supersonic (or sonic) turbulence that form binaries by disk fragmentation.

High Performance Computing (HPC) based simulations are crucial in Astrophysics & Cosmology (A&C), helping scientists investigate and understand complex astrophysical phenomena. Taking advantage of exascale computing capabilities is essential for these efforts. However, the unprecedented architectural complexity of exascale systems impacts legacy codes. The SPACE Centre of Excellence (CoE) aims to re-engineer key astrophysical codes to tackle new computational challenges by adopting innovative programming paradigms and software (SW) solutions. SPACE brings together scientists, code developers, HPC experts, hardware (HW) manufacturers, and SW developers. This collaboration enhances exascale A&C applications, promoting the use of exascale and post-exascale computing capabilities. Additionally, SPACE addresses high-performance data analysis for the massive data outputs from exascale simulations and modern observations, using machine learning (ML) and visualisation tools. The project facilitates application deployment across platforms by focusing on code repositories and data sharing, integrating European astrophysical communities around exascale computing with standardised SW and data protocols.

Context The presence of kiloparsec-sized bubble structures on both sides of the Galactic plane suggests active phases of Sgr A ⋆ , the central supermassive black hole of the Milky Way in the last 1–6 Myr. We investigated the contribution of such events to the cosmic-ray (CR) flux measured in the solar neighborhood with numerical simulations. Aims. We evaluate whether the population of high-energy charged particles emitted by the Galactic center could be sufficient to significantly impact the CR flux measured in the solar neighborhood. Methods. We present a set of 3D magnetohydrodynamical simulations following the anisotropic propagation of CRs in a Milky Way-like Galaxy. We followed independent populations of CRs through time. We followed CRs originating from two different source types, namely supernovae and the Galactic center. To assess the evolution of the CR flux spectrum properties, we split these populations into two independent energy groups of 100 GeV and 10 TeV. Results. We find that the anisotropic nature of CR diffusion dramatically affects the amount of CR energy received in the solar neighborhood. The typical timescale required to observe measurable changes in the CR spectrum slope is of the order 10 Myr, largely surpassing estimated ages of the Fermi bubbles in the active galactic nuclei (AGN) jet-driven scenario. Conclusions. We conclude that a CR outburst from the Galactic center in the last few million years is unlikely have produced any observable feature in the local CR spectrum in the TeV regime within times consistent with current estimates of the age of the Fermi bubbles.

Context. Protostellar disks are the product of angular momentum conservation during protostellar collapse. Understanding their formation is crucial because they are the birthplace of planets and their formation is also tightly related to star formation. Unfortunately, the initial properties of Class 0 disks and their evolution are still poorly constrained both theoretically and observationally. Aims. We aim to better understand the mechanisms that set the statistics of disk properties as well as to study their formation in massive protostellar clumps. We also want to provide the community with synthetic disk populations to better interpret young disk observations. Methods. We used the ramses code to model star and disk formation in massive protostellar clumps with magnetohydrodynamics, including the effect of ambipolar diffusion and radiative transfer as well as stellar radiative feedback. Those simulations, resolved up to the astronomical unit scale, have allowed us to investigate the formation of disk populations. Results. Magnetic fields play a crucial role in disk formation. A weaker initial field leads to larger and massive disks and weakens the stellar radiative feedback by increasing fragmentation. We find that ambipolar diffusion impacts disk and star formation and leads to very different disk magnetic properties. The stellar radiative feedback also have a strong influence, increasing the temperature and reducing fragmentation. Comparing our disk populations with observations reveals that our models with a mass-to-flux ratio of 10 seems to better reproduce observed disk sizes. This also sheds light on a tension between models and observations for the disk masses. Conclusions. The clump properties and physical modeling significantly impact disk populations. It is critical to for the tension, with respect to disk mass estimates, between observations and models to be solved with synthetic observations. This is particularly important in the context of understanding planet formation.

Context. Population III (pop III) stars were born in halos characterised by a pristine gas composition. In such a halo, once the gas density reaches n(H) similar to 1 cm(-3), molecular cooling leads to the collapse of the gas and the birth of pop III stars. Halo properties, such as the chemical abundances, mass, and angular momentum can affect the collapse of the gas, thereby leading to the pop III initial mass function (IMF) of star formation. Aims. We want to study the properties of primordial halos and how halos that host early star formation differ from other types of halos. The aim of this study is to obtain a representative population of halos at a given redshift hosting a cold and massive gas cloud that enables the birth of the first stars. Methods. We investigated the growth of primordial halos in a Lambda CDM Universe in a large cosmological simulation. We used the hydrodynamic code Ramses and the chemical solver Krome to study halo formation with non-equilibrium thermochemistry. We then identified structures in the dark and baryonic matter fields, thereby linking the presence or absence of dense gas clouds to the mass and the physical properties of the hosting halos. Results. In our simulations, the mass threshold for a halo for hosting a cold dense gas cloud is similar or equal to 7 x 10(5) M-circle dot and the threshold in the H-2 mass fraction is found to be similar or equal to 2 x 10(-4). This is in agreement with previous works. We find that the halo history and accretion rate play a minor role. Here, we present halos with higher HD abundances, which are shown to be colder, as the temperature in the range between 10(2)-10(4) cm(-3) depends on the HD abundance to a large extent. The higher fraction of HD is linked to the higher spin parameter that is seen for the dense gas.

High Performance Computing based simulations are crucial in Astrophysics and Cosmology, helping scientists investigate and understand complex astrophysical phenomena. Taking advantage of Exascale computing capabilities is essential for these efforts. However, the unprecedented architectural complexity of exascale systems impacts simulation codes. The SPACE Center of Excellence aims to re-engineer key astrophysical codes to adapt to these new computational challenges by adopting innovative programming paradigms and software solutions. Through co-design activities, SPACE brings together scientists, code developers, HPC experts, hardware manufacturers, and software developers. This collaboration enhances exascale astrophysics and cosmology applications, promoting the use of exascale and post-exascale computing capabilities. Additionally, SPACE addresses high-performance data analysis for the massive data outputs from exascale simulations, using machine learning and visualization tools. The project facilitates application deployment across platforms by focusing on code repositories and data sharing, integrating European astrophysical communities around exascale computing with standardized software and data protocols. In this paper, we present the SPACE Center of Excellence and the preliminary results achieved by the project.

Recent observations have begun probing the early phases of disc formation, but little data yet exists on disc structure and morphology of Class 0 objects. Using simulations, we are able to lay out predictions of disc morphologies expected in future surveys of young discs. Based on detailed simulations of ab initio star formation by core collapse, we predict that early discs must be eccentric. In this letter, we study the morphology and, in particular, the eccentricity of discs formed in non-ideal magnetohydrodynamic (MHD) collapse simulations. We attempt to show that discs formed by cloud collapse are likely to be eccentric. We ran non-ideal MHD collapse simulations in the adaptive mesh refinement code RAMSES with radiative transfer. We used state-of-the-art analysis methods to measure the disc eccentricity. We find that despite no asymmetry in the initial conditions, the discs formed are eccentric, with eccentricities on the order of 0.1. These results may have important implications for protoplanetary disc dynamics and planet formation. The presence of eccentricity in young discs that is not seen at later stages of disc evolution is in tension with current viscous eccentricity damping models. This implies that there may be an as-yet undiscovered circularisation mechanism in circumstellar discs.

We present a plausible and coherent view of the evolution of the protosolar disk that is consistent with the cosmochemical constraints and compatible with observations of other protoplanetary disks and sophisticated numerical simulations. The evidence that high-temperature condensates, CAIs and AOAs, formed near the protosun before being transported to the outer disk can be explained by either an early phase of vigorous radial spreading of the disk, or fast transport of these condensates from the vicinity of the protosun towards large disk radii via the protostellar outflow. The assumption that the material accreted towards the end of the infall phase was isotopically distinct allows us to explain the observed dichotomy in nucleosynthetic isotopic anomalies of meteorites and leads to intriguing predictions on the isotopic composition of refractory elements in comets. When the infall of material waned, the disk started to evolve as an accretion disk. Initially, dust drifted inwards, shrinking the radius of the dust component to 45 au, probably about 1/2 of the width of the gas component. Then structures must have emerged, producing a series of pressure maxima in the disk which trapped the dust on My timescales. This allowed planetesimals to form at radically distinct times without changing significantly of isotopic properties. There was no late accretion of material onto the disk via streamers. The disk disappeared in 5 Myr, as indicated by paleomagnetic data in meteorites. In conclusion, the evolution of the protosolar disk seems to have been quite typical in terms of size, lifetime, and dust behavior, suggesting that the peculiarities of the Solar system with respect to extrasolar planetary system probably originate from the chaotic nature of planet formation and not at the level of the parental disk.

Signposts of early planet formation are ubiquitous in substructured young discs. Dense, hot and high-pressure regions formed during gravitational collapse process, integral to star formation, facilitate dynamical mixing of dust within the protostellar disc. This provides an incentive to constrain the role of gas-dust interaction and resolve zones of dust concentration during star-disc formation. We explore if thermal and dynamical conditions developed during disc formation can generate gas flows that efficiently mix and transport well-coupled gas and dust components. We simulated the collapse of dusty molecular cloud cores with the hydrodynamics code PLUTO augmented with radiation transport and self-gravity. We used a 2D axisymmetric geometry and follow the azimuthal component of velocity. Dust was treated as Lagrangian particles that are subject to drag from the gas, whose motion is computed on a Eulerian grid. We considered 1, 10 and 100 micron-sized neutral spherical dust. Importantly, the equation of state accurately includes molecular hydrogen dissociation. We focus on molecular cloud core masses of 1 and 3 Msun and explore effects of initial rotation rates and cloud core sizes. Our study underlines mechanisms for early transport of dust from inner hot disc regions via the occurrence of meridional flows and outflow. The vortical flow fosters dynamical mixing and retention of dust while thermal pressure driven outflow replenishes dust in the outer disc. Young dynamical precursors to planet-forming discs exhibit regions with complex hydrodynamical gas features and high-temperature structures. These can play a crucial role in concentrating dust for subsequent growth into protoplanets. Dust transport, especially, from sub-au scales surrounding the protostar to outer relatively cooler parts, offers an efficient pathway for thermal reprocessing during pre-stellar core collapse. [Abridged]

Context. The first hydrostatic core (FHSC) phase is a brief stage in the protostellar evolution that is difficult to detect. Its chemical composition determine that of later evolutionary stages. Numerical simulations are the tool of choice to study these objects. Aims. Our goal is to characterize the chemical evolution of gas and dust during the formation of the FHSC. Moreover, we are interested in analyzing, for the first time with 3D magnetohydrodynamic (MHD) simulations, the role of grain growth in its chemistry. Methods. We postprocessed 2 × 10 5 tracer particles from a RAMSES non-ideal MHD simulation using the codes NAUTILUS and SHARK to follow the chemistry and grain growth throughout the simulation. Results. Gas-phase abundances of most of the C, O, N, and S reservoirs in the hot corino at the end of the simulation match the ice-phase abundances from the prestellar phase. Interstellar complex organic molecules such as methyl formate, acetaldehyde, and formamide are formed during the warm-up process. Grain size in the hot corino ( n H > 10 11 cm −3 ) increases forty-fold during the last 30 kyr, with negligible effects on its chemical composition. At moderate densities (10 10 < n H < 10 11 cm −3 ) and cool temperatures 15 < T < 50 K, increasing grain sizes delay molecular depletion. At low densities ( n H ~ 10 7 cm −3 ), grains do not grow significantly. To assess the need to perform chemo-MHD calculations, we compared our results with a two-step model that reproduces well the abundances of C and O reservoirs, but not the N and S reservoirs. Conclusions. The chemical composition of the FHSC is heavily determined by that of the parent prestellar core. Chemo-MHD computations are needed for an accurate prediction of the abundances of the main N and S elemental reservoirs. The impact of grain growth in moderately dense areas delaying depletion permits the use of abundance ratios as grain growth proxies.