Stars and planets form within cold, dark molecular clouds. In these dense regions, where starlight cannot penetrate, cosmic rays (CRs) are the dominant source of ionization—driving interstellar chemistry, setting the gas temperature and enabling coupling to magnetic fields. Together, these effects regulate the collapse of clouds and the onset of star formation. Despite this importance, the CR ionization rate, ζ, has never been measured directly. Instead, this fundamental parameter has been loosely inferred from indirect chemical tracers and uncertain assumptions, limiting our understanding of star formation physics. Here we report the direct detection of CR-excited vibrational H2 emission, using James Webb Space Telescope observations of the starless core Barnard 68 (B68). The observed emission pattern matches theoretical predictions for CR excitation precisely, confirming a decades-old theoretical proposal long considered observationally inaccessible. This result enables direct measurement of ζ, effectively turning molecular clouds into natural, light-year-sized, CR detectors. It opens a transformative observational window into the origin, propagation and role of CRs in star formation and galaxy evolution. The famous nebula Barnard 68 has been used as a giant cosmic-ray detector: cosmic-ray-excited vibrational H2 emission has been observed by JWST, giving a direct measurement of the CR ionization rate.

We present an implementation of radiative transfer with flux-limited diffusion (FLD) for the moving-mesh code AREPO and use the method in a physical model for the formation of protostars with non-ideal radiation-magnetohydrodynamics (RMHD). We follow previous work in splitting the additional terms to the hydrodynamical equations arising from the inclusion of radiation into terms to be integrated explicitly and implicitly, as the diffusion and coupling terms would impose very restrictive time-step criteria. We validate the scheme with standard test problems for radiation diffusion, matter-gas coupling, and radiative shocks from the literature. Our implementation is compatible with local time-stepping, which often presents problems for implicit schemes, and we found very good agreement with results obtained with global time-steps. We present an example application of the new implementation to the collapse of a 1 M- (R) molecular cloud core to a second Larson core modelled with radiation non-ideal magnetohydrodynamics. A high-velocity jet with vrad > 10 km s(-1)is self-consistently launched from the second core, nested within the first core, which produces a lower-velocity magnetorotational outflow. We observe magnetic field amplification up to more than | B |(max) > 10(5) Gin the second core, which is surrounded by a small (< 0 . 5 au) disc. This application demonstrates the robustness of our scheme in multiscale and high-resolution simulations on arbitrary meshes and, as such, the model can be readily used for further simulations of protostar formation at high resolution.

Chemistry in diffuse molecular clouds relies primarily on rapid ion-molecule reactions. Formation of the initial ions, H+ and H 2+ , is dominated by cosmic-ray ionization of H and H2, making the cosmic-ray ionization rate (denoted zeta(X) for species X) an important parameter for chemical modeling. We have made observations targeting absorption lines of H 3+ , one of the most reliable tracers of zeta(H2), toward diffuse molecular cloud sight lines where the H2 column density has been directly measured in the ultraviolet, detecting H 3+ in 12 out of 27 sight lines. The 3D-PDR modeling method introduced by M. Obolentseva et al. was used to infer cosmic-ray ionization rates in the clouds along these sight lines, and our combined sample has a mean ionization rate of 5.3 x 10-17 s-1 with standard deviation 2.5 x 10-17 s-1. By associating H 3+ absorption with gas density peaks derived from the differential extinction maps of G. Edenhofer et al., we have constructed a sparsely sampled 3D map of the cosmic-ray ionization rate in targeted regions within about 1 kpc of the Sun. Specific regions show reasonably uniform ionization rates over length scales of tens of parsecs, with the average ionization rate in each region being different. Large differences (factor of 5) in zeta(H2) are found over length scales of about 100 pc. This supports a picture where the cosmic-ray ionization rate varies smoothly over small size scales, but is not uniform everywhere in the Galactic disk, likely being controlled by proximity to particle acceleration sites.

Stars and planets form within cold, dark molecular clouds. In these dense regions, where starlight cannot penetrate, cosmic rays (CRs) are the dominant source of ionization – driving interstellar chemistry(Dalgarno (2006, PNAS, 103, 12269)), setting the gas temperature(Goldsmith et al. (1969, ApJ, 158, 173)), and enabling coupling to magnetic fields(McKee Ostriker (2007, ARA A, 45, 565; arXiv:0707.3514)). Together, these effects regulate the collapse of clouds and the onset of star formation. Despite this importance, the cosmic-ray ionization rate, ζ, has never been measured directly. Instead, this fundamental parameter has been loosely inferred from indirect chemical tracers and uncertain assumptions, leading to published values that span nearly two orders of magnitude and limiting our understanding of star formation physics. Here, we report the first direct detection of CR-excited vibrational H_2 emission, using James Webb Space Telescope (JWST) observations of the starless core Barnard 68 (B68). The observed emission pattern matches theoretical predictions for CR excitation precisely, confirming a decades-old theoretical proposal long considered observationally inaccessible. This result enables direct measurement of ζ, effectively turning molecular clouds into natural, light-year-sized, cosmic-ray detectors. It opens a transformative observational window into the origin, propagation, and role of cosmic rays in star formation and galaxy evolution.

In dense colloids and simulated glasses, it has been discovered that plastic particle rearrangements correlate with nonphononic low-frequency vibrational modes. Here, we demonstrate that this correlation also holds for a very different material class of complex plasmas under shear, characterized by long-range interactions and nonreciprocal forces. We perform experiments with a deformed amorphous quasi-two-dimensional binary complex plasma under optical pressure, which gives rise to a controlled shear rate, and confirm our findings with extensive particle-resolved computer simulations.

We present simulations of the supernova-driven turbulent interstellar medium (ISM) in a simulation domain of volume (256 pc)(3) within which we resolve the formation of protostellar accretion discs and their stellar cores to spatial scales of similar to 10(4) au, using the moving-mesh code AREPO. We perform simulations with no magnetic fields, ideal magnetohydrodynamics (MHD) and ambipolar diffusion, and compare the resulting first Larson cores and their associated structures, including the accretion discs, their location within the larger-scale structure and the streamers connecting these. We find that discs of sizes 10-100 au form early in the simulations without magnetic fields, while there are no discs larger than 10 au with ideal MHD. Ambipolar diffusion causes large discs to form in a subset of cases (two out of six cores), and generally reduces the strength of outflows, which are seen to play a central role. When they are able to carry away significant angular momentum, they prevent the formation of a rotationally supported disc. Magnetic fields strengths grow from 0.1-1 mG in the protostellar core to more than 10 G in the first Larson core in all simulations with ideal MHD. The rotationally supported discs which form can have rotation speeds > 1 km s(1) even out to further than 100 au from the centre, become gravitationally unstable and form complex spiral substructures with Toomre rho < 1 We conclude that the impact of magnetic fields and non-ideal MHD on the formation of protostellar discs is substantial in realistic formation scenarios from the turbulent ISM.

Magnetic fields have been shown both observationally and through theoretical work to be an important factor in the formation of protostars and their accretion disks. Accurate modelling of the evolution of the magnetic field in low-ionization molecular cloud cores requires the inclusion of non-ideal magnetohydrodynamics (MHD) processes, specifically Ohmic and ambipolar diffusion and the Hall effect. These have a profound influence on the efficiency of magnetic removal of angular momentum from protostellar disks and simulations that include them can avoid the `magnetic-braking catastrophe' in which disks are not able to form. However, the impact of the Hall effect, in particular, is complex and remains poorly studied. In this work, we perform a large suite of simulations of the collapse of cloud cores to protostars with several non-ideal MHD chemistry models and initial core geometries using the moving-mesh code AREPO. We find that the efficiency of angular momentum removal is significantly reduced with respect to ideal MHD, in line with previous results. The Hall effect has a varied influence on the evolution of the disk which depends on the initial orientation of the magnetic field. This extends to the outflows seen in a subset of the models, where this effect can act to enhance or suppress them and open up new outflow channels. We conclude, in agreement with a subset of the previous literature, that the Hall effect is the dominant non-ideal MHD process in some collapse scenarios and thus should be included in simulations of protostellar disk formation.

We employed our recent model of the cosmic-ray (CR) halo to compute the Galactic spectra of stable and unstable secondary nuclei. In this model, confinement of the Galactic CRs is entirely determined by the self-generated Alfvénic turbulence whose spectrum is controlled by nonlinear Landau damping. We analyzed the physical parameters affecting propagation characteristics of CRs and estimated the best set of free parameters providing accurate description of available observational data. We also show that agreement with observations at lower energies may be further improved by taking into account the effect of ion-neutral damping that operates near the Galactic disk.

We study penetration of interstellar cosmic rays (CRs) into molecular clouds surrounded by nonuniform diffuse envelopes. The present work generalizes our earlier model of CR self-modulation [Ivlev ; Dogiel ], in which the value for the envelope’s gas density where CRs excite MHD waves was treated as a free parameter. Now, we investigate the case where the density monotonically increases toward the center. Assuming that CRs are relativistic, we obtain a universal analytical solution which does not depend on the particular shape of gas distribution in the envelope, and self-consistently derive boundaries of the diffusion zone formed within the envelope, where CRs are scattered at the self-excited waves. The values of the gas density at the boundaries are found to be substantially smaller than those assumed in the earlier model, which leads to a significantly stronger modulation of penetrating CRs. We compute the impact of CR self-modulation on the gamma-ray emission and show that the results of our theoretical model are in excellent agreement with recent observations of nearby giant molecular clouds by Yang []. Published by the American Physical Society 2024

Newly computed collisional rate coefficients for the excitation of C-2 in collisions with H-2, presented recently by Najar & Kalugina, are significantly larger than the values adopted previously in models for the excitation of the C-2 molecule, a widely used probe of the interstellar gas density. With these new rate coefficients, we have modeled the C-2 rotational distributions inferred from visible and ultraviolet absorption observations of electronic transitions of C-2 toward a collection of 46 nearby background sources. The inferred gas densities in the foreground interstellar clouds responsible for the observed C-2 absorption are a factor 4-7 smaller than those inferred previously, a direct reflection of the larger collisional rate coefficients computed by Najar & Kalugina. These lower-density estimates are generally in good agreement with the peak densities inferred from 3D extinction maps for the relevant sight lines. In cases where H-3(+) absorption has also been observed and used to estimate the cosmic-ray ionization rate (CRIR), our estimates of the latter will also decrease accordingly because the H-3(+) abundance is a function of the ratio of the CRIR to the gas density.

An amorphous quasi-two-dimensional binary complex plasma has been formed in a capacitively coupled radio-frequency discharge. The particle suspension is elongated in one direction by external confinement and rotates slowly, presumably driven by the inhomogeneity of optical pressure. As such elliptical particle suspension is constantly deformed while rotating, shear is applied. The vibrational modes are measured and decomposed into longitudinal and transverse parts, the latter of which is further decomposed into the phononic and non-phononic components. A positive correlation between the particle rearrangement and the non-phononic component at low-frequency regime is observed experimentally. Langevin dynamics simulation has been performed and the results agree with the experimental observations.

Context. Turbulence is a key component of molecular cloud structure. It is usually described by a cascade of energy down to the dissipation scale. The power spectrum for subsonic incompressible turbulence is proportional to k-5/3, while for supersonic turbulence it is proportional to k-2. Aims. We determine the power spectrum in an actively star-forming molecular cloud, from parsec scales down to the expected magnetohydrodynamic (MHD) wave cutoff (dissipation scale). Methods. We analyzed observations of the nearby NGC 1333 star-forming region in three different tracers to cover the different scales from similar to 10 pc down to 20 mpc. The largest scales are covered with the low-density gas tracer 13CO (1-0) obtained with a single dish, the intermediate scales are covered with single-dish observations of the C18O (3-2) line, while the smallest scales are covered in H13CO+ (1-0) and HNC (1-0) with a combination of NOEMA interferometer and IRAM 30m single-dish observations. The complementarity of these observations enables us to generate a combined power spectrum covering more than two orders of magnitude in spatial scale. Results. We derive the power spectrum in an active star-forming region spanning more than 2 decades of spatial scales. The power spectrum of the intensity maps shows a single power-law behavior, with an exponent of 2.9 +/- 0.1 and no evidence of dissipation. Moreover, there is evidence that the power spectrum of the ions to have more power at smaller scales than the neutrals, which is opposite to the theoretical expectations. Conclusions. We show new possibilities for studying the dissipation of energy at small scales in star-forming regions provided by interferometric observations.

Context. The detections of narrow channels of accretion toward protostellar disks, known as streamers, have increased in number in the last few years. However, it is unclear whether streamers are a common feature around protostars that were previously missed, or if they are a rare phenomenon. Aims. Our goals are to obtain the incidence of streamers toward a region of clustered star formation and to trace the origins of their gas to determine whether they originate within the filamentary structure of molecular clouds or from beyond. Methods. We used combined observations of the nearby NGC 1333 star-forming region, carried out with the NOEMA interferometer and the IRAM 30m single dish. Our observations cover the area between the systems IRAS 4 and SVS 13. We traced the chemically fresh gas within NGC 1333 with HC3N molecular gas emission and the structure of the fibers in this region with N2H+ emission. We fit multiple velocity components in both maps and used clustering algorithms to recover velocity-coherent structures. Results. We find streamer candidates toward 7 out of 16 young stellar objects within our field of view. This represents an incidence of approximately 40% of young stellar objects with streamer candidates in a clustered star-forming region. The incidence increases to about 60% when we only considered embedded protostars. All streamers are found in HC3N emission. Conclusions. Given the different velocities between HC3N and N2H+ emission, and because by construction, N2H+ traces the fiber structure, we suggest that the gas that forms the streamers comes from outside the fibers. This implies that streamers can connect cloud material that falls onto the filaments with protostellar disk scales.

Newly-computed collisional rate coefficients for the excitation of C_2 in collisions with H_2, presented recently by Najar and Kalugina (2020), are significantly larger than the values adopted previously in models for the excitation of the C_2 molecule, a widely used probe of the interstellar gas density. With these new rate coefficients, we have modeled the C_2 rotational distributions inferred from visible and ultraviolet absorption observations of electronic transitions of C_2 towards a collection of 46 nearby background sources. The inferred gas densities in the foreground interstellar clouds responsible for the observed C_2 absorption are a factor 4 to 7 smaller than those inferred previously, a direct reflection of the larger collisional rate coefficients computed by Najar and Kalugina (2020). These lower density estimates are generally in good agreement with the peak densities inferred from 3D extinction maps for the relevant sightlines. In cases where H_3^+ absorption has also been observed and used to estimate the cosmic-ray ionization rate (CRIR), our estimates of the latter will also decrease accordingly because the H_3^+ abundance is a function of the ratio of the CRIR to the gas density.

Physical processes associated with cosmic dust play a critical role at different stages of star formation. Dust grains contribute to gas heating and cooling in various ISM phases, determine the attenuation of the radiation field, represent an essential part of the chemical network, control the gas ionization fraction, etc. In turn, a combination of these processes affects various aspects of the dust evolution in dense gas. This chapter summarizes the main observational constraints on the physical and chemical properties of cosmic dust, and presents a brief review of available models to describe the dust evolution in different ISM phases. The main physical mechanisms resulting in dust heating and charging are highlighted, and the approaches to compute the equilibrium temperature of dust grains, their charge distribution, and their impact on gas ionization are discussed. Furthermore, different mechanisms contributing to grain coagulation and the formation of icy mantles on their surface are investigated, and algorithms and approaches to compute the evolution of dust properties due to these processes are presented. A particular focus is made on highlighting the role of the grain size distribution, and analyzing the dependence on the key ISM parameters.

All current estimates of the cosmic-ray (CR) ionization rate rely on assessments of the gas density along the probed sight lines. Until now, these have been based on observations of different tracers, with C2 being the most widely used in diffuse molecular clouds for this purpose. However, dust extinction maps have recently reached sufficient accuracy to give an independent measurement of the gas density on parsec scales. In addition, they allow us to identify the gas clumps along each sight line, thus localizing the regions where CR ionization is probed. We reevaluate H3+ observations, which are often considered as the most reliable method to measure the H2 ionization rate zeta H2 in diffuse clouds. The peak density values derived from the extinction maps for 12 analyzed sight lines turn out to be, on average, an order of magnitude lower than the previous estimates and agree with the values obtained from revised analysis of C2 data. We use the extinction maps in combination with the 3d-pdr code to self-consistently compute the H3+ and H2 abundances in the identified clumps for different values of zeta H2 . For each sight line, we obtain the optimum value by comparing the simulation results with observations. We show that zeta H2 is systematically reduced with respect to the earlier estimates by a factor of approximate to 9 on average, to approximate to 6 x 10-17 s-1, primarily as a result of the density reduction. We emphasize that these results have profound consequences for all available measurements of the ionization rate.

Context. Gas accretion and sublimation in various astrophysical conditions are crucial aspects of our understanding of the chemical evolution of the interstellar medium. To explain grain growth and destruction in warm media, ice mantle formation and sublimation in cold media, and gas line emission spectroscopy, astrochemical models must mimic the gas--solid abundance ratio. Ice-sublimation mechanisms determine the position of snow lines and the nature of gas emitted by and locked inside planetary bodies in star-forming regions. To interpret observations from the interplanetary and extragalactic interstellar mediums, gas phase abundances must be modelled correctly. Aims. We provide a collection of thermal desorption data for interstellar ice analogues, aiming to put constraints on the trapping efficiency of water ice, as well as data that can be used to evaluate astrochemical models. We conduct experiments on compact, amorphous H2O films, involving pure ices as well as binary and ternary mixtures. By manipulating parameters in a controlled way, we generate a set of benchmarks to evaluate both the kinetics and thermodynamics in astrochemical models. Methods. We conducted temperature-programmed desorption experiments with increasing order of complexity of ice analogues of various chemical compositions and surface coverages using molecular beams in ultrahigh vacuum conditions (1 x 10(-10) hPa) and low temperatures (10 K). We provide TPD curves of pure ices made of Ar, CO, CO2, NH3, CH3OH, H2O, and NH4+HCOO-, their binary ice mixtures with compact amorphous H2O, ternary mixtures of H2O:CH3OH:CO, and a water ice made in situ to investigate its trapping mechanisms. Results. Each experiment includes the experimental parameters, ice desorption kinetics for pure species, and the desorption yield (gas--solid ratio) for ice mixtures. From the desorption yields, we find common trends in the trapping of molecules when their abundance is compared to water: compact amorphous water ices are capable of trapping up to 20% of volatiles (Ar, CO, and CO2), similar to 3% of CH3OH, and similar to 5% NH3 in relation to the water content within the ice matrix; ammonium formate is not trapped in the water ice films, and compact amorphous water ice formed in situ has similar trapping capabilities to a compact amorphous water ice deposited using molecular beams. Conclusions. Deposited or formed in a very compact structure, amorphous water ice of less than 100 layers cannot trap a large fraction of other gases, including CO and CO2. These desorption yields offer insights into the availability of species that can react and form interstellar complex organic molecules during the warm-up phase of ice mantles. Furthermore, in order to be reliable, gas-grain astrochemical models should be able to reproduce the desorption kinetics and desorption yield presented in our benchmark laboratory experiments.

Context . Electron fraction and cosmic-ray ionization rates in star-forming regions are important quantities in astrochemical modeling and are critical to the degree of coupling between neutrals, ions, and electrons, which regulates the dynamics of the magnetic field. However, these are difficult quantities to estimate. Aims . We aim to derive the electron fraction and cosmic-ray ionization rate maps of an active star-forming region. Methods . We combined observations of the nearby NGC 1333 star-forming region carried out with the NOEMA interferometer and IRAM 30 m single dish to generate high spatial dynamic range maps of different molecular transitions. We used the DCO + and H 13 CO + ratio (in addition to complementary data) to estimate the electron fraction and produce cosmic-ray ionization rate maps. Results . We derived the first large-area electron fraction and cosmic-ray ionization rate resolved maps in a star-forming region, with typical values of 10 −65 and 10 −16.5 s −1 , respectively. The maps present clear evidence of enhanced values around embedded young stellar objects (YSOs). This provides strong evidence for locally accelerated cosmic rays. We also found a strong enhancement toward the northwest region in the map that might be related either to an interaction with a bubble or to locally generated cosmic rays by YSOs. We used the typical electron fraction and derived a magnetohydrodynamic (MHD) turbulence dissipation scale of 0.054 pc, which could be tested with future observations. Conclusions . We found a higher cosmic-ray ionization rate compared to the canonical value for N (H 2 ) = 10 21 −10 23 cm −2 of 10 −17 s −1 in the region, and it is likely generated by the accreting YSOs. The high value of the electron fraction suggests that new disks will form from gas in the ideal-MHD limit. This indicates that local enhancements of ζ (H 2 ), due to YSOs, should be taken into account in the analysis of clustered star formation.

In this experimental paper, we demonstrate that turbulence can develop in a fluid system with background damping. For that purpose, we analyze dust acoustic waves, self-excited in a fluid complex plasma where the motion of individual microparticles was recorded with a high-speed video camera. We use the Wiener-Khinchin theorem to calculate the kinetic spectrum during different phases of the highly nonlinear periodic wave motion and show that a turbulent cascade develops at the phases of highest particle compression. We demonstrate that the energy cascade occurs despite the presence of a damping force due to the background neutral gas.

Context. Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30-m Large Program aimed at determining the elemental abundances of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical star-forming filaments. In particular, the elemental abundance of S remains uncertain by several orders of magnitude, and its determination is one of the most challenging goals of this program. Aims. This paper aims to constrain the sulfur elemental abundance in Taurus, Perseus, and Orion A based on the GEMS molecular database. The selected regions are prototypes of low-mass, intermediate-mass, and high-mass star-forming regions, respectively, providing useful templates for the study of interstellar chemistry. Methods. We have carried out an extensive chemical modeling of the fractional abundances of CO, HCO+, HCN, HNC, CS, SO, H2S, OCS, and HCS+ to determine the sulfur depletion toward the 244 positions in the GEMS database. These positions sample visual extinctions from A(V) similar to 3 mag to >50 mag, molecular hydrogen densities ranging from a few x 10(3) cm(-3) to 3 x 10(6) cm(-3), and T-k similar to 10-35 K. We investigate the possible relationship between sulfur depletion and the grain charge distribution in different environments. Results. Most of the positions in Taurus and Perseus are best fitted assuming early-time chemistry, t = 0.1 Myr, zeta(H2) similar to (0.5-1) x 10(-16) s(-1), and [S/H] similar to 1.5 x 10(-6). On the contrary, most of the positions in Orion are fitted with t = 1 Myr and zeta(H2) similar to 10(-17) s(-1). Moreover, similar to 40% of the positions in Orion are best fitted assuming the undepleted sulfur abundance, [S/H] similar to 1.5 x 10(-5). We find a tentative trend of sulfur depletion increasing with density. Conclusions. Our results suggest that sulfur depletion depends on the environment. While the abundances of sulfur-bearing species are consistent with undepleted sulfur in Orion, a depletion factor of similar to 20 is required to explain those observed in Taurus and Perseus. We propose that differences in the grain charge distribution might explain these variations. Grains become negatively charged at a visual extinction of A(V) similar to 3.5 mag in Taurus and Perseus. At this low visual extinction, the S+ abundance is high, X(S+) > 10(-6), and the electrostatic attraction between S+ and negatively charged grains could contribute to enhance sulfur depletion. In Orion, the net charge of grains remains approximately zero until higher visual extinctions (A(V) similar to 5.5 mag), where the abundance of S+ is already low because of the higher densities, thus reducing sulfur accretion. The shocks associated with past and ongoing star formation could also contribute to enhance [S/H].