arXiv subjects

Vincent Guillet

Publications and source records attributed to Vincent Guillet.

15 recordsLinked to original sources

arXiv2026

Large Interstellar Polarisation Survey. III. Observational constraints on the structure of grains

Dust in the diffuse interstellar medium remains incompletely understood with regard to the structure, composition, size distribution, and alignment properties of the grains. Joint observations of reddening, starlight polarisation spectra, and polarised dust emission for individual sightlines provide essential constraints on such properties. We study a far-UV selected sample of 96 reddening curves, for which optical linear polarisation spectra were obtained with FORS at the VLT as part of the Large Interstellar Polarisation Survey (LIPS). Starlight polarisation spectra for 60 stars are presented in this work. These data are combined with Gaia distance estimates and Planck thermal dust emission. A three-component dust model is made publicly available. It consists of nanoparticles, amorphous grains, and micrometre-sized dust agglomerates, varying axial ratios, porosities, sizes, element abundances, and alignment efficiencies to match the observations. The diversity of reddening and polarisation spectra is well reproduced by prolate grains with typical axial ratios of two, a porosity of 10%, and high alignment efficiencies. Such efficiencies can be achieved with radiative torque alignment theory (RAT), but not with imperfect Davis-Greenstein (IDG) alignment, except when adjusting the magnetic-field orientation to maximise the polarisation. Micrometre-sized dust contributes wavelength-independent grey extinction in the optical, accounts for about one-third of the visual extinction, and carries one-third of the dust mass. A follow-up submillimetre survey with high-resolution polarimetry will further constrain grain shapes and alignment physics.

astro-ph.GA↗

arXiv2025

Variance of dust temperature and spectral index in Planck polarization data using spin-moment expansion

Thermal dust is the major polarized foreground hindering the detection of primordial cosmic microwave background (CMB) B-modes. Its signal exhibits complex behavior in frequency space, arising from the combined variation in our Galaxy of the orientation of magnetic fields and the spectral properties of dust grains aligned with magnetic field lines. In this work, we present a new framework for analyzing the thermal dust signal using polarized microwave data. We introduce residual maps, represented as complex quantities, which capture deviations of the local polarized spectral energy distribution (SED) from the mean complex SED averaged over the sky mask. We present simple predictions that relate the values of the statistical correlation and covariances between the residual maps to the physical properties of the emitting aligned grains. Testing these predictions provides valuable information about the nature of the dust signal. We evaluated our predictions using Planck data over a 97% mask excluding the inner Galactic plane. Despite its simplicity, our model captures a significant part of the statistical properties of the data. For the SRoll2 version of the data, the spectral dependence of the covariances between residual maps is compatible with a dust model that includes only temperature variations rather than spectral index variations. In contrast, for the PR4 Planck official release, it is incompatible with both models. Our methodology can be used to analyze future high-precision polarization data and to build more accurate dust models for use by the CMB community.

astro-ph.GA↗

arXiv2024

THEMIS 2.0: A self-consistent model for dust extinction, emission, and polarisation

Recent observations in emission, extinction, and polarisation have at least partially invalidated most of the astronomical standard grain models for the diffuse ISM. Moreover, lab measurements on interstellar silicate analogues have shown differences with the optical properties used in these standard models. To address these issues, our objective is twofold: (i) to update the optical properties of silicates and (ii) to develop the THEMIS dust model to allow the calculation of polarised extinction and emission. Based on optical constants measured in the lab for amorphous silicates and on observational constraints in mid-IR extinction and X-ray scattering, we defined new optical constants for the THEMIS silicates. Absorption and scattering efficiencies for spheroidal grains were then derived with the discrete dipole approximation. These new optical properties make it possible to explain the dust emission and extinction, both total and polarised. The model is not yet pushed to its limits since it does not require the perfect alignment of all grains to explain the observations and it therefore has the potential to accommodate the highest polarisation levels inferred from extinction measures. Moreover, the dispersion of the optical properties of the different lab silicates naturally explain the variations in both the total and polarised emission and extinction observed in the diffuse ISM. A single, invariant model calibrated on one single set of observations is obsolete for explaining contemporary observations. We are proposing a completely flexible dust model based entirely on lab measurements that has the potential to make major advances in understanding the nature of ISM grains and how they evolve as a function of their environment. Even if challenging, this is also relevant for future missions that will aim to perform precise measurements of the CMB spectral distortions and polarisation.

astro-ph.GA↗

arXiv2023

Fast methods for tracking grain coagulation and ionization. III. Protostellar collapse with non-ideal MHD

Dust grains influence many aspects of star formation, including planet formation, opacities for radiative transfer, chemistry, and the magnetic field via Ohmic, Hall, and ambipolar diffusion. The size distribution of the dust grains is the primary characteristic influencing all these aspects. Grain size increases by coagulation throughout the star formation process. We describe here numerical simulations of protostellar collapse using methods described in earlier papers of this series. We compute the evolution of the grain size distribution from coagulation and the non-ideal magnetohydrodynamics effects self-consistently and at low numerical cost. We find that the coagulation efficiency is mostly affected by the time spent in high-density regions. Starting from sub-micron radii, grain sizes reach more than 100 μm in an inner protoplanetary disk that is only 1000 years old. We also show that the growth of grains significantly affects the resistivities, and indirectly the dynamics and angular momentum of the disk.

astro-ph.SR↗

arXiv2022

Dust polarization spectral dependence from Planck HFI data. Turning point on CMB polarization foregrounds modelling

The search for the primordial B-modes of the cosmic microwave background (CMB) relies on the separation from the brighter foreground dust signal. In this context, the characterisation of the spectral energy distribution (SED) of thermal dust in polarization has become a critical subject of study. We present a power-spectra analysis of Planck data, which improves on previous studies by using the newly released SRoll2 maps that correct residual data systematics, and by extending the analysis to regions near the Galactic plane. Our analysis focuses on the lowest multipoles between l=4 and 32, and three sky areas with sky fractions of fsky = 80%, 90%, and 97%. The mean dust SED for polarization and the 353 GHz Q and U maps are used to compute residual maps at 100, 143 and 217 GHz, highlighting spatial variations of the dust polarization SED. Residuals are detected at the three frequencies for the three sky areas. We show that models based on total intensity data are underestimating by a significant factor the complexity of dust polarized CMB foreground. Our analysis emphasizes the need to include variations of polarization angles of the dust polarized CMB foreground. The frequency dependence of the EE and BB power spectra of the residual maps yields further insight. We find that the moments expansion to the first order of the modified black-body (MBB) spectrum provides a good fit to the EE power-spectra. This result suggests that the residuals could follow mainly from variations of dust MBB spectral parameters. However, this conclusion is challenged by cross-spectra showing that the residuals maps at the three frequencies are not fully correlated, and the fact that the BB power-spectra do not match the first order moment expansion of a MBB SED. This work sets new requirements for simulations of the dust polarized foreground and component separation methods (abridged)

astro-ph.CO↗

arXiv2022

Protostellar collapse simulations in spherical geometry with dust coagulation and fragmentation

We model the coagulation and fragmentation of dust grains during the protostellar collapse with our newly developed shark code. It solves the gas-dust hydrodynamics in a spherical geometry and the coagulation/fragmentation equation. It also computes the ionization state of the cloud and the Ohmic, ambipolar and Hall resistivities. We find that the dust size distribution evolves significantly during the collapse, large grain formation being controlled by the turbulent differential velocity. When turbulence is included, only ambipolar diffusion remains efficient at removing the small grains from the distribution, brownian motion is only efficient as a standalone process. The macroscopic gas-dust drift is negligible for grain growth and only dynamically significant near the first Larson core. At high density, we find that the coagulated distribution is unaffected by the initial choice of dust distribution. Strong magnetic fields are found to enhance the small grains depletion, causing an important increase of the ambipolar diffusion. This hints that the magnetic field strength could be regulated by the small grain population during the protostellar collapse. Fragmentation could be effective for bare silicates, but its modeling relies on the choice of ill-constrained parameters. It is also found to be negligible for icy grains. When fragmentation occurs, it strongly affects the magnetic resistivities profiles. Dust coagulation is a critical process that needs to be fully taken into account during the protostellar collapse. The onset and feedback of fragmentation remains uncertain and its modeling should be further investigated.

astro-ph.GA↗

arXiv2022

Frequency dependence of the thermal dust $E/B$ ratio and $EB$ correlation: insights from the spin-moment expansion

The change of physical conditions across the turbulent and magnetized interstellar medium (ISM) induces a 3D spatial variation of the properties of Galactic polarized emission. The observed signal results from the averaging of different spectral energy distributions (SED) and polarization angles, along and between lines of sight. As a consequence, the total Stokes parameters $Q$ and $U$ will have different distorted SEDs, so that the polarization angle becomes frequency dependent. In the present work, we show how this phenomenon similarly induces a different distorted SED for the three polarized angular power spectra $EE$, $BB$ and $EB$, implying a variation of the $EE/BB$ ratio with frequency. We demonstrate how the previously introduced spin-moment formalism provides a natural framework to grasp these effects, allowing us to derive analytical predictions for the spectral behaviors of the polarized spectra, focusing here on the example of thermal dust polarized emission. After a quantitative discussion based on a model combining emission from a filament with its background, we further reveal that the spectral complexity implemented in the dust models commonly used by the cosmic microwave background (CMB) community produce such effects. This new understanding is crucial for CMB component separation, in which an extreme accuracy is required in the modeling of the dust signal to allow for the search of the primordial imprints of inflation or cosmic birefringence. For the latter, as long as the dust $EB$ signal is not measured accurately, great caution is required about the assumptions made to model its spectral behavior, as it may not simply follow from the other dust angular power spectra.

astro-ph.CO↗

arXiv2022

Fast methods to track grain coagulation and ionization. II. Extension to thermal ionization

Thermal ionization is a critical process at temperatures T > 10 3 K, particularly during star formation. An increase in ionization leads to a decrease in nonideal magnetohydrodynamics (MHD) resistivities, which has a significant impact on protoplanetary disks and protostar formation. We developed an extension of the fast computational ionization method presented in our recent paper to include thermal ionization. The model can be used to inexpensively calculate the density of ions and electrons and the electric charge of each size of grains for an arbitrary size distribution. This tool should be particularly useful for the self-consistent calculation of nonideal MHD resistivities in multidimensional simulations, especially of protostellar collapse and protoplanetary disks.

astro-ph.SR↗

arXiv2021

Fast methods for tracking grain coagulation and ionization. I. Analytic derivation

Dust grains play a major role in many astrophysical contexts. They affect the chemical, magnetic, dynamical, and optical properties of their environment, from galaxies down to the interstellar medium, star-forming regions, and protoplanetary disks. Their coagulation leads to shifts in their size distribution and ultimately to the formation of planets. However, although the coagulation process is reasonably uncomplicated to numerically implement by itself, it is difficult to couple it with multidimensional hydrodynamics numerical simulations because of its high computational cost. We propose here a simple method for tracking the coagulation of grains at far lower cost. Given an initial grain size distribution, the state of the distribution at time t is solely determined by the value of a single variable integrated along the trajectory, independently of the specific path taken by the grains. Although this method cannot account for processes other than coagulation, it is mathematically exact, fast, inexpensive, and can be used to evaluate the effect of grain coagulation in most astrophysical contexts. It is applicable to all coagulation kernels in which local physical conditions and grain properties can be separated. We also describe another method for calculating the average electric charge of grains and the density of ions and electrons in environments that are shielded from radiation fields, given the density and temperature of the gas, the cosmic-ray ionization rate, and the average mass of the ions. The equations we provide are fast to integrate numerically and can be used in multidimensional numerical simulations to self-consistently calculate on the fly the local resistivities that are required to model nonideal magnetohydrodynamics.

astro-ph.GA↗

arXiv2020

A systematic study of radiative torque grain alignment in the diffuse interstellar medium

Context. Planck observations demonstrated that the grain alignment efficiency is almost constant in the diffuse ISM. Aims. We test if the Radiative Torque (RAT) theory is compatible with observational constraints on grain alignment. Methods. We combine a numerical simulation with the radiative transfer code POLARIS that incorporates a physical dust model and the detailed grain alignment physics of RATs. A dust model is designed to reproduce the spectral dependence of extinction of the ISM. From a RAMSES simulation of interstellar turbulence, we extract a cube representative of the diffuse ISM. We post-process the cube with POLARIS to get the grain temperature and RATs to simulate synthetic dust polarization maps. Results. In our simulation the grain alignment efficiency is correlated with gas pressure, but not with the RAT intensity. Because of the low dust extinction, the magnitude of RATs varies little, decreasing only for high column densities $N_H$. Comparing our maps with a uniform alignment efficiency, we find no systematic difference. The dependence of polarization fraction $p$ with $N_H$ or polarization dispersion $S$ is similar. The drop of RATs in dense regions barely affects the polarization pattern, the signal being dominated by the LOS and magnetic field geometry. If a star is inserted, the polarization increases, with no specific pattern around the star. The angle-dependence of RATs is not observed in the maps, and is weak using a uniform magnetic field. Conclusions. RATs are compatible with Planck data for the diffuse ISM such that both uniform alignment and RAT alignment lead to similar observations. To further test the predictions of RATs where an important drop of grain alignment is expected, polarization observations of dense regions must be confronted to numerical simulations sampling high column densities through dense clouds, with enough statistics.

astro-ph.IM↗

arXiv2019

Indirect evidence of significant grain growth in young protostellar envelopes from polarized dust emission

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.

astro-ph.GA↗

arXiv2018

Dust spectrum and polarisation at 850 um in the massive IRDC G035.39-00.33

The dust sub-millimetre polarisation of star-forming clouds carries information on dust and the role of magnetic fields in cloud evolution. With observations of a dense filamentary cloud G035.39-00.33, we aim to characterise the dust emission properties and the variations of the polarisation fraction. JCMT SCUBA-2/POL-2 data at 850um are combined with Planck 850um (353GHz) data to map polarisation fractions. With previous SCUBA-2 observations (450um and 850um) and Herschel data, the column densities are determined via modified blackbody fits and via radiative transfer modelling. Models are constructed to examine how the polarisation angles and fractions depend on potential magnetic field geometries and grain alignment. POL-2 data show clear changes in the magnetic field orientation. The filament has a peak column density of N(H2)~7 10^22 cm-2, a minimum dust temperature of T~12 K, and a mass of some 4300Msun for the area N(H2)> 5 10^21 cm-2. The estimated average value of the dust opacity spectral index is beta ~ 1.9. The ratio of sub-millimetre and J band optical depths is tau(250 um)/tau(J) ~ 2.5 10^-3, more than four times the typical values for diffuse medium. The polarisation fraction decreases as a function of column density to p ~ 1% in the central filament. Because of noise, the observed decrease of p(N) is significant only at N(H2)>2 10^22 cm-2. The observations suggest that the grain alignment is not constant. Although the data can be explained with a complete loss of alignment at densities above ~ 10^4 cm-3 or using the predictions of radiative torques alignment, the uncertainty of the field geometry and the spatial filtering of the SCUBA-2 data prevent strong conclusions. G035.39-00.33 shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts.

astro-ph.GA↗

arXiv2017

Interplay of dust alignment, grain growth and magnetic fields in polarization: lessons from the emission-to-extinction ratio

Polarized extinction and emission from dust in the interstellar medium (ISM) are hard to interpret, as they have a complex dependence on dust optical properties, grain alignment and magnetic field orientation. This is particularly true in molecular clouds. The data available today are not yet used to their full potential. The combination of emission and extinction, in particular, provides information not available from either of them alone. We combine data from the scientific literature on polarized dust extinction with Planck data on polarized emission and we use them to constrain the possible variations in dust and environmental conditions inside molecular clouds, and especially translucent lines of sight, taking into account magnetic field orientation. We focus on the dependence between λ_max -- the wavelength of maximum polarization in extinction -- and other observables such as the extinction polarization, the emission polarization and the ratio of the two. We set out to reproduce these correlations using Monte-Carlo simulations where the relevant quantities in a dust model -- grain alignment, size distribution and magnetic field orientation -- vary to mimic the diverse conditions expected inside molecular clouds. None of the quantities chosen can explain the observational data on its own: the best results are obtained when all quantities vary significantly across and within clouds. However, some of the data -- most notably the stars with low emission-to-extinction polarization ratio -- are not reproduced by our simulation. Our results suggest not only that dust evolution is necessary to explain polarization in molecular clouds, but that a simple change in size distribution is not sufficient to explain the data, and point the way for future and more sophisticated models.

astro-ph.GA↗

arXiv2015

Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium

We compare the performance of several dust models in reproducing the dust spectral energy distribution (SED) per unit extinction in the diffuse interstellar medium (ISM). We use our results to constrain the variability of the optical properties of big grains in the diffuse ISM, as published by the Planck collaboration. We use two different techniques to compare the predictions of dust models to data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared emission spectrum to recover the dust extinction and the intensity of the interstellar radiation field (ISRF). Second, we infer the ISRF intensity from the total power emitted by dust per unit extinction, and then predict the emission spectrum. In both cases, we test the ability of the models to reproduce dust emission and extinction at the same time. We identify two issues. Not all models can reproduce the average dust emission per unit extinction: there are differences of up to a factor $\sim2$ between models, and the best accord between model and observation is obtained with the more emissive grains derived from recent laboratory data on silicates and amorphous carbons. All models fail to reproduce the variations in the emission per unit extinction if the only variable parameter is the ISRF intensity: this confirms that the optical properties of dust are indeed variable in the diffuse ISM. Diffuse ISM observations are consistent with a scenario where both ISRF intensity and dust optical properties vary. The ratio of the far-infrared opacity to the $V$ band extinction cross-section presents variations of the order of $\sim20\%$ ($40-50\%$ in extreme cases), while ISRF intensity varies by $\sim30\%$ ($\sim60\%$ in extreme cases). This must be accounted for in future modelling.

astro-ph.GA↗

arXiv2014

The cycling of carbon into and out of dust

Observational evidence seems to indicate that the depletion of interstellar carbon into dust shows rather wide variations and that carbon undergoes rather rapid recycling in the interstellar medium (ISM). Small hydrocarbon grains are processed in photo-dissociation regions by UV photons, by ion and electron collisions in interstellar shock waves and by cosmic rays. A significant fraction of hydrocarbon dust must therefore be re-formed by accretion in the dense, molecular ISM. A new dust model (Jones et al., Astron. Astrophys., 2013, 558, A62) shows that variations in the dust observables in the diffuse interstellar medium (nH = 1000 cm^3), can be explained by systematic and environmentally-driven changes in the small hydrocarbon grain population. Here we explore the consequences of gas-phase carbon accretion onto the surfaces of grains in the transition regions between the diffuse ISM and molecular clouds (e.g., Jones, Astron. Astrophys., 2013, 555, A39). We find that significant carbonaceous dust re-processing and/or mantle accretion can occur in the outer regions of molecular clouds and that this dust will have significantly different optical properties from the dust in the adjacent diffuse ISM. We conclude that the (re-)processing and cycling of carbon into and out of dust is perhaps the key to advancing our understanding of dust evolution in the ISM.

astro-ph.GA↗