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High resolution (sub-)millimeter polarization observations have opened a new era in the understanding of how B-fields are organized in star forming regions, unveiling an intricate interplay between the B-fields and the gas in protostellar cores. However, to assess the role of the B-field in the process of solar-type star formation, it is key to be able to understand to what extent these polarized dust emissions are good tracers of the B-field in the youngest protostellar objects. We present a thorough investigation of the fidelity and limitations of using dust polarized emission to map the B-field topologies in low-mass protostars. To assess the importance of these effects, we performed the analysis of B-field properties in 27 realizations of MHD models of star-forming cores. Assuming a uniform population of dust grains whose sizes follow the standard MRN, we analyze the synthetic polarized dust emission maps produced if these grains align with the local B-field thanks to B-RATs. We find that (sub-)millimeter polarized dust emission is a robust tracer of the B-field topologies in inner protostellar envelopes and is successful at capturing the details of the B-field spatial distribution down to radii ~100 au. Measurements of the los averaged B-field orientation using the polarized dust emission are precise to < 15{\deg} in about 75 - 95% of the independent lines of sight peering through protostellar envelopes. Large discrepancies between the integrated B-field mean orientation and the orientation reconstructed from the polarized dust emission are mostly observed in (i) lines of sight where the B-field is highly disorganized and (ii) lines of sight probing large column densities. Our analysis shows that high opacity of the thermal dust emission and low polarization fractions could be used to avoid utilizing the small fraction of measurements affected by large errors., Comment: Accepted for publication in A&A

The Davis-Chandrasekhar-Fermi (DCF) method is widely used to indirectly estimate the magnetic field strength from the plane-of-sky field orientation. In this work, we present a set of 3D MHD simulations and synthetic polarization images using radiative transfer of clustered massive star-forming regions. We apply the DCF method on the synthetic polarization maps to investigate its reliability in high-density molecular clumps and dense cores where self-gravity is significant. We investigate the validity of the assumptions of the DCF method step by step and compare the model and estimated field strength to derive the correction factors for the estimated uniform and total (rms) magnetic field strength at clump and core scales. The correction factors in different situations are catalogued. We find the DCF method works well in strong field cases. However, the magnetic field strength in weak field cases could be significantly overestimated by the DCF method when the turbulent magnetic energy is smaller than the turbulent kinetic energy. We investigate the accuracy of the angular dispersion function (ADF, a modified DCF method) method on the effects that may affect the measured angular dispersion and find that the ADF method correctly accounts for the ordered field structure, the beam-smoothing, and the interferometric filtering, but may not be applicable to account for the signal integration along the line of sight in most cases. Our results suggest that the DCF methods should be avoided to be applied below $\sim$0.1 pc scales if the effect of line-of-sight signal integration is not properly addressed., 40 pages, 24 figures, accepted by ApJ

Aims. The main goal of the following analysis is to assess the potential role of magnetic fields in regulating the envelope rotation, the formation of disks and the fragmentation of Class 0 protostars in multiple systems. Methods. We use the Submillimeter Array to carry out observations of the dust polarized emission at 0.87 mm, in the envelopes of a large sample of 20 Class 0 protostars. We estimate the mean magnetic field orientation over the central 1000 au envelope scales to characterize the orientation of the main component of the organized magnetic field at the envelope scales in these embedded protostars. This direction is compared to that of the protostellar outflow in order to study the relation between their misalignment and the kinematics of the circumstellar gas. The latter is traced via velocity gradient observed in the molecular line emission (mainly N2H+) of the gas at intermediate envelope scales. Results. We discover a strong relationship between the misalignment of the magnetic field orientation with the outflow and the amount of angular momentum observed at similar scales in the protostellar envelope, revealing a potential link between the kinetic and the magnetic energy at envelope scales. The relation could be driven by favored B-misalignments in more dynamical envelopes or a dependence of the envelope dynamics with the large-scale B initial configuration. Comparing the trend with the presence of fragmentation, we observe that single sources are mostly associated with conditions of low angular momentum in the inner envelope and good alignment of the magnetic field with protostellar outflows, at intermediate scales. Our results suggest that the properties of the magnetic field in protostellar envelopes bear a tight relationship with the rotating-infalling gas directly involved in the star and disk formation: we find that it may not only influence the fragmentation of protostellar cores into multiple stellar systems, but also set the conditions establishing the pristine properties of planet-forming disks., This project has received funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation programme (MagneticYSOs project, grant agreement No. 679937, PI: Maury). J.M.G. is supported by the grant AYA2017-84390- C2-R (AEI/FEDER, UE).

Context. Recent observational progress has challenged the dust grain-alignment theories used to explain the polarized dust emission routinely observed in star-forming cores. Aims. In an effort to improve our understanding of the dust grain alignment mechanism(s), we have gathered a dozen ALMA maps of (sub)millimeter-wavelength polarized dust emission from Class 0 protostars and carried out a comprehensive statistical analysis of dust polarization quantities. Methods. We analyze the statistical properties of the polarization fraction Pfrac and the dispersion of polarization position angles S. More specifically, we investigate the relationship between S and Pfrac as well as the evolution of the product S × Pfrac as a function of the column density of the gas in the protostellar envelopes. We compare the observed trends with those found in polarization observations of dust in the interstellar medium and in synthetic observations of non-ideal magneto-hydrodynamic (MHD) simulations of protostellar cores. Results. We find a significant S Pfrac-0.79 correlation in the polarized dust emission from protostellar envelopes seen with ALMA; the power-law index significantly differs from the one observed by Planck in star-forming clouds. The product S × Pfrac, which is sensitive to the dust grain alignment efficiency, is approximately constant across three orders of magnitude in envelope column density (from NH2 = 1022 cm-2 to NH2 = 1025 cm-2), with a mean value of 0.36-0.17+0.10. This suggests that the grain alignment mechanism producing the bulk of the polarized dust emission in star-forming cores may not systematically depend on the local conditions such as the local gas density. However, in the lowest-luminosity sources in our sample, we find a hint of less efficient dust grain alignment with increasing column density. Our observations and their comparison with synthetic observations of MHD models suggest that the total intensity versus the polarized dust are distributed at different intrinsic spatial scales, which can affect the statistics from the ALMA observations, for example, by producing artificially high Pfrac. Finally, synthetic observations of MHD models implementing radiative alignment torques (RATs) show that the statistical estimator S × Pfrac is sensitive to the strength of the radiation field in the core. Moreover, we find that the simulations with a uniform perfect alignment (PA) of dust grains yield, on average, much higher S × Pfrac values than those implementing RATs; the ALMA values lie among those predicted by PA, and they are significantly higher than the ones obtained with RATs, especially at large column densities. Conclusions. Ultimately, our results suggest that dust alignment mechanism(s) are efficient at producing dust polarized emission in the various local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. Further studies will be needed to understand how more efficient grain alignment via, for example, different irradiation conditions, dust grain characteristics, or additional grain alignment mechanisms can reproduce the observations., V.J.M.L.G. acknowledges the support of the ESO Studentship Program. A.J.M. acknowledges support from the Joint ALMA Observatory Visitor Program, and ESO Visitor program. J.M.G. is supported by the Spanish grant AYA2017-84390-C2-R (AEI/FEDER, UE). C.L.H.H. acknowledges the support of both the NAOJ Fellowship as well as JSPS KAKENHI grants 18K13586 and 20K14527. This work has benefited from the support of the European Research Council under the Horizon 2020 Framework Programme (Starting Grant MagneticYSOs with grant agreement no. 679937)

The interstellar medium (ISM) is typically a hostile environment: cold, dilute and irradiated. Nevertheless, it appears very fertile for molecules. The localized heating resulting from turbulence dissipation is a possible channel to produce and excite molecules. However, large-scale simulations cannot resolve the dissipative scales of the ISM. Here, we present two-dimensional small-scale simulations of decaying hydrodynamic turbulence using the chemses code, with fully resolved viscous dissipation, time-dependent heating, cooling, chemistry and excitation of a few rotational levels of H2. We show that molecules are produced and excited in the wake of strong dissipation ridges. We carefully identify shocks and we assess their statistics and contribution to the molecular yields and excitation. We find that the formation of molecules is strongly linked to increased density as a result of shock compression and to the opening of endothermic chemical routes because of higher temperatures. We identify a new channel for molecule production via H2 excitation, illustrated by CH+ yields in our simulations. Despite low temperatures and the absence of magnetic fields (favouring CH+ production through ion-neutral velocity drifts), the excitation of the first few rotational levels of H2 shrinks the energy gap to form CH+. The present study demonstrates how dissipative chemistry can be modelled by statistical collections of one-dimensional steady-state shocks. Thus, the excitation of higher J levels of H2 is likely to be a direct signature of turbulence dissipation, and an indirect probe for molecule formation. We hope these results will help to bring new tools and ideas for the interpretation of current observations of H2 rotational lines carried out using the Stratospheric Observatory for Infrared Astronomy (SOFIA), and pave the way for a better understanding of the high-resolution mapping of H2 emission by future instruments, such as theJames Webb Space Telescope and the Space Infrared Telescope for Cosmology and Astrophysics.

Molecular clouds are essentially made up of atomic and molecular hydrogen, which in spite of being the simplest molecule in the ISM plays a key role in the chemical evolution of molecular clouds. Since its formation time is very long, the H2 molecules can be transported by the turbulent motions within the cloud toward low density and warm regions, where its enhanced abundance can boost the abundances of molecules with high endothermicities. We present high resolution simulations where we include the evolution of the molecular gas under the effect of the dynamics, and we analyze its impact on the abundance of CH+., Comment: To appear in: Proceedings IAU Symposium No. 332, 2017

Context. The amount of data collected by spectrometers from radio to ultraviolet (UV) wavelengths opens a new era where the statistical and chemical information contained in the observations can be used concomitantly to investigate the thermodynamical state and the evolution of the interstellar medium (ISM). Aims. In this paper, we study the statistical properties of the HI-to-H2 transition observed in absorption in the local diffuse and multiphase ISM. Our goal is to identify the physical processes that control the probability of occurrence of any line of sight and the origins of the variations of the integrated molecular fraction from one line of sight to another. Methods. The turbulent diffuse ISM is modeled using the RAMSES code, which includes detailed treatments of the magnetohydrodynamics, the thermal evolution of the gas, and the chemistry of H2. The impacts of the UV radiation field, the mean density, the turbulent forcing, the integral scale, the magnetic field, and the gravity on the molecular content of the gas are explored through a parametric study that covers a wide range of physical conditions. The statistics of the HI-to-H2 transition are interpreted through analytical prescriptions and compared with the observations using a modified and robust version of the Kolmogorov-Smirnov test. Results. The analysis of the observed background sources shows that the lengths of the lines of sight follow a flat distribution in logarithmic scale from ~100 pc to ~3 kpc. Without taking into account any variation of the parameters along a line of sight or from one line of sight to another, the results of one simulation, convolved with the distribution of distances of the observational sample, are able to simultaneously explain the position, the width, the dispersion, and most of the statistical properties of the HI-to-H2 transition observed in the local ISM. The tightest agreement is obtained for a neutral diffuse gas modeled over ~200 pc, with a mean density n̅H̅ = 1−2 cm−3, illuminated by the standard interstellar UV radiation field, and stirred up by a large-scale compressive turbulent forcing. Within this configuration, the 2D probability histogram of the column densities of H and H2, poetically called the kingfisher diagram, is remarkably stable and is almost unaltered by gravity, the strength of the turbulent forcing, the resolution of the simulation, or the strength of the magnetic field Bx, as long as Bx < 4 μG. The weak effect of the resolution and our analytical prescription suggest that the column densities of HI are likely built up in large-scale warm neutral medium and cold neutral medium (CNM) structures correlated in density over ~20 pc and ~10 pc, respectively, while those of H2 are built up in CNM structures between ~3 and ~10 pc. Conclusions. Combining the chemical and statistical information contained in the observations of HI and H2 sheds new light on the study of the diffuse matter. Applying this new tool to several atomic and molecular species is a promising perspective to understanding the effects of turbulence, magnetic field, thermal instability, and gravity on the formation and evolution of molecular clouds.

How and when in the star formation sequence do dust grains start to grow into pebbles is a cornerstone question to both star and planet formation. We compute the polarized radiative transfer from a model solar-type protostellar core, using the POLARIS code, aligning the dust grains with the local magnetic field, following the radiative torques (RATs) theory. We test the dependency of the resulting dust polarized emission with the maximum grain size of the dust size distribution at the envelope scale, from amax = 1 micron to 50 micron. Our work shows that, in the framework of RAT alignment, large dust grains are required to produce polarized dust emission at levels similar to those currently observed in solar-type protostellar envelopes at millimeter wavelengths. Considering the current theoretical dificulties to align a large fraction of small ISM-like grains in the conditions typical of protostellar envelopes, our results suggest that grain growth (typically > 10 micron) might have already significantly progressed at scales 100-1000 au in the youngest objects, observed less than 10^5 years after the onset of collapse. Observations of dust polarized emission might open a new avenue to explore dust pristine properties and describe, for example, the initial conditions for the formation of planetesimals., Accepted for publication in MNRAS. Accepted 2019 July 23. Received 2019 July 23; in original form 2019 April 05

Analyzing the properties of dust and its evolution in the early phases of star formation is crucial to put constraints on the collapse and accretion processes as well as on the pristine properties of planet-forming seeds. We analyze PdBI observations at 1.3 and 3.2 mm for 12 Class 0 protostars obtained as part of the CALYPSO survey and derive dust emissivity index (beta) profiles as a function of the envelope radius at 200-2000 au scales. Most of the protostellar envelopes show low dust emissivity indices decreasing toward the central regions. The decreasing trend remains after correction of the (potentially optically-thick) central region emission, with surprisingly low beta (lower than 1) values across most of the envelope radii of NGC1333-IRAS4A, NGC1333-IRAS4B, SVS13B, and Serpens-SMM4. We discuss the various processes that could explain such low and varying dust emissivity indices at envelope radii 200-2000 au. Our observations of extremely low dust emissivity indices could trace the presence of large (mm-size) grains in Class 0 envelopes and a radial increase of the dust grain size toward the inner envelope regions. While it is expected that large grains in young protostellar envelopes could be built via grain growth and coagulation, we stress that the typical timescales required to build mm grains in current coagulation models are at odds with the youth of our Class 0 protostars. Additional variations in the dust composition could also partly contribute to the low beta we observe. The steepness of the beta radial gradient depends strongly on the envelope mass, which might favor a scenario where large grains are built in high density protostellar disks and transported to the intermediate envelope radii, for example with the help of outflows and winds., Comment: 17 pages, 11 figures, accepted for publication in A&A

Although, from a theoretical point of view, magnetic fields are believed to have a significant role during the early stages of star formation, especially during the main accretion phase, the magnetic field strength and topology is poorly constrained in the youngest accreting Class 0 protostars that lead to the formation of solar-type stars. We carried out observations of the polarized dust continuum emission with the SMA interferometer at 0.87mm, in order to probe the structure of the magnetic field in a sample of 12 low-mass Class 0 envelopes, including both single protostars and multiple systems, in nearby clouds. Our SMA observations probe the envelope emission at scales of 600-5000 au with a spatial resolution ranging from 600 to 1500 au depending on the source distance. We report the detection of linearly polarized dust continuum emission in all of our targets, with average polarization fractions ranging from 2% to 10% in these protostellar envelopes. The polarization fraction decreases with the continuum flux density, which translates into a decrease with the H2 column density within an individual envelope. Our analysis show that the envelope-scale magnetic field is preferentially observed either aligned or perpendicular to the outflow direction. Interestingly, our results suggest for the first time a relation between the orientation of the magnetic field and the rotational energy of envelopes, with a larger occurrence of misalignment in sources where strong rotational motions are detected at hundreds to thousands of au scales. We also show that the best agreement between the magnetic field and outflow orientation is found in sources showing no small-scale multiplicity and no large disks at about 100 au scales., Comment: 22 pages, 9 figures, accepted for publication in A&A

Molecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H formation in a variety of interstellar environments., VW’s research is funded by an ERC Starting Grant (3DICE, grant agreement 336474). GV acknowledges financial support from the National Science Foundation’s Astronomy & Astrophysics Division (Grants No. 1311958 and 1615897). LH acknowledges support from ERC Consolidator Grant GRANN (grant agreement no. 648551). GN acknowledges support from the Swedish Research Council. VW, FD and SM acknowledge the CNRS program ”Physique et Chimie du Milieu Interstellaire” (PCMI) co-funded bythe Centre National d’Etudes Spatiales (CNES). SDP acknowledges funding from STFC, UK. V.V acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (MagneticYSOS project, grant agreement No 679937).

Magnetic fields are believed to redistribute part of the angular momentum during the collapse and could explain the order-of-magnitude difference between the angular momentum observed in protostellar envelopes and that of a typical main sequence star. The Class 0 phase is the main accretion phase during which most of the final stellar material is collected on the central embryo. To study the structure of the magnetic fields on 50-2000 au scales during that key stage, we acquired SMA polarization observations (870μm) of 12 low-mass Class 0 protostars. In spite of their low luminosity, we detect dust polarized emission in all of them. We observe depolarization effects toward high-density regions potentially due to variations in alignment efficiency or in the dust itself or geometrical effects. By comparing the misalignment between the magnetic field and the outflow orientation, we show that the B is either aligned or perpendicular to the outflow direction. We observe a coincidence between the misalignment and the presence of large perpendicular velocity gradients and fragmentation in the protostar (Galametz et al. 2018). Our team is using MHD simulations combined with the radiative transfer code POLARIS to produce synthetic maps of the polarized emission. This work is helping us understand how the magnetic field varies from the large-scale to the small-scales, quantify beam-averaging biases and study the variations of the polarization angles as a function of wavelength or the assumption made on the grain alignment (see poster by Valdivia).

This paper assesses the roles of the presence of warm H2, and the increased formation rate due to the ion-neutral drift. We performed ideal MHD simulations that include the heating and cooling of the multiphase ISM, and where we treat dynamically the formation of H2. In a post-processing step we compute the abundances of species at chemical equilibrium. We show that CH+ is efficiently formed at the edge of clumps, in regions where the H2 fraction is low, but nevertheless higher than its equilibrium value, and where the gas temperature is high. We show that warm and out of equilibrium H2 increases the integrated column densities of CH+ by one order of magnitude, up to values still 3-10 times lower than those observed in the diffuse ISM. We balance the Lorentz force with the ion-neutral drag to estimate the ion-drift velocities (vd). We find that the vd distribution peaks around 0.04 km s-1, and that high vd are too rare to have a significant statistical impact on the abundances of CH+. Compared to previous works, our multiphase simulations reduce the spread in vd, and our self-consistent treatment of the ionisation leads to much reduced vd. Nevertheless, our resolution study shows that this velocity distribution is not converged: the ion-neutral drift has a higher impact on CH+ at higher resolution. On the other hand, our ideal MHD simulations do not include ambipolar diffusion, which would yield lower drift velocities. Within these limitations, we conclude that warm H2 is a key ingredient in the efficient formation of CH+ and that the ambipolar diffusion has very little influence on the abundance of CH+, mainly due to the small drift velocities obtained. However, we point out that small-scale processes and other non-thermal processes not included in our MHD simulation may be of crucial importance, and higher resolution studies with better controlled dissipation processes are needed., Comment: Accepted for publication in section 6. Interstellar and circumstellar matter of Astronomy and Astrophysics. Date of acceptance: 08/12/2016. (13 pages, 12 figures.)

Magnetic fields play a key role during the gravitational collapse of dense protostellar cores. In recent years mm and sub-mm observations of dust polarized emission have been used to unveil the morphology of the magnetic field, but this method relies on the assumption that non-spherical dust grains are well aligned with the magnetic field.Using non-ideal MHD numerical simulations, we study the evolution of the magnetic field during the gravitational collapse. We use the state-of-the-art radiative transfer code POLARIS to compute the Stokes parameters and produce synthetic observations of mm/submm polarized dust emission. We compare the results obtained using the radiative torques (RAT) mechanism to the results obtained by assuming that grains are perfectly aligned to constrain how well polarized dust emission traces the magnetic field orientation.The complexity of the magnetic field produces a mild depolarization. The depolarization observed in the inner regions is rather caused by a decrease of the dust alignment efficiency and it cannot be reproduced by just scaling down the polarisation degree obtained for a uniform efficiency. We find that the magnetic field orientation is well constrained by the polarized dust emission as long as its 3D topology remains organized.

We performed high-resolution, 3D MHD simulations and we compared to observations of translucent molecular clouds. We show that the observed populations of rotational levels of H2 can arise as a consequence of the multi-phase structure of the ISM., 2 pages, 1 figure. Due to appear in the proceedings of the 6th Zermatt ISM Symposium: "Conditions and Impact of Star Formation: From Lab to Space"

Context. Ultraviolet radiation plays a crucial role in molecular clouds. Radiation and matter are tightly coupled and their interplay influences the physical and chemical properties of gas. In particular, modeling the radiation propagation requires calculating column densities, which can be numerically expensive in high-resolution multidimensional simulations. Aims. Developing fast methods for estimating column densities is mandatory if we are interested in the dynamical influence of the radiative transfer. In particular, we focus on the effect of the UV screening on the dynamics and on the statistical properties of molecular clouds. Methods. We have developed a tree-based method for a fast estimate of column densities, implemented in the adaptive mesh refinement code RAMSES. We performed numerical simulations using this method in order to analyze the influence of the screening on the clump formation. Results. We find that the accuracy for the extinction of the tree-based method is better than 10%, while the relative error for the column density can be much more. We describe the implementation of a method based on precalculating the geometrical terms that noticeably reduces the calculation time. To study the influence of the screening on the statistical properties of molecular clouds we present the probability distribution function (PDF) of gas and the associated temperature per density bin and the mass spectra for different density thresholds. Conclusions. The tree-based method is fast and accurate enough to be used during numerical simulations since no communication is needed between CPUs when using a fully threaded tree. It is then suitable to parallel computing. We show that the screening for far UV radiation mainly affects the dense gas, thereby favoring low temperatures and affecting the fragmentation., 14 pages, 20 figures