Nouveaux aperçus sur les propriétés des poussières à partir des données Planck en intensité et polarisation

Interstellar dust is a key component of the interstellar medium (ISM). Not only does it play an important role in thephysics and chemistry of the ISM, but its thermal emission can be used to trace the gas column density, and itspolarization angle to trace the magnetic field orientation projected on the plane of the sky.Different dust models have been built to reproduce the main dust observables: extinction curve and albedo, spectralenergy distribution (SED) from the near-infrared to the microwave continuum, polarization in extinction and emission,within cosmic elemental abundance constraints. Our understanding of interstellar dust is, however, still incomplete;among other things, we do not fully understand the local variations in the emission and extinction properties of dust.The variation of the dust far-infrared opacity from the diffuse ISM to molecular clouds is well established, and modelshave been proposed. With the Planck submillimeter survey we have, for the first time, a multi-wavelength, all-sky mapof dust emission allowing for a precise measure of dust temperature, and therefore of dust opacity variations in thediffuse ISM.This thesis, based on the comparison of Planck data with extinction measures toward stars and QSOs, makes use of bothdust models and data analysis to constrain the dust optical properties and evolution within the diffuse ISM, and toimprove our understanding on the interplay between grain alignment and dust optical properties in the emission ofpolarized thermal radiation.The first half of the thesis focuses on the total emission of dust in the diffuse ISM. The variations in the ratio of dustemission to extinction is used to constrain the variations of the dust optical properties. We fit the 20 SEDs normalizedper unit extinction of Planck intermediate results XXIX with three dust models (Draine & Li 2007, Compiegne et al2011, Jones et al 2013). The best agreement between model and observations is obtained for the model with the moreemissive grains (Jones 2013), with optical properties derived from recent laboratory data on silicates and amorphouscarbons. We develop a new estimator of the radiation field intensity G0, which combines the dust SED and theextinction on the same line of sight. We show that this new estimator is less biased than the one obtained through thefitting of the dust SED. With their fixed optical properties, none of the models can simultaneously reproduce thevariations of G0 and of the shape of the SED. With our new estimator of G0, we demonstrate that the variations in thedust optical properties and in the radiation field intensity give similar contributions to the scatter observed in the dustSED per unit extinction in the diffuse ISM.The second half of the thesis focuses on polarized dust extinction and emission in molecular clouds. By confrontingPlanck and stellar observations to a dust model, we attempt to disentangle the effects of variations in the dust opticalproperties from the effects of variations in the grain alignment. We find a correlation between the ratio of polarizedemission to polarized extinction, RP/p = P353/pv, and the wavelength of maximum polarization in extinction, λmax, whichtraces the typical size of the aligned grains. Using a new dust model for polarization based on Planck data, we show thatthe variation of the minimal size of aligned grains can reproduce the observed correlation, without any need for achange in the size distribution or in the optical properties of grains. This scenario is also compatible with the drop of thefractions of polarization with λmax. Alternative models cannot however be ruled out.Les poussières interstellaires sont une composante clé du milieu interstellaire (MIS). Elles jouent non seulement un rôleimportant dans la physique et la chimie du MIS, mais elles servent également de traceur, du gaz via leur émissionthermique, et du champ magnétique interstellaire via la polarisation de cette émission.De nombreux modèles de poussières reproduisent les principales observables sur les poussières (la courbe d'extinction,la distribution spectrale d'énergie (SED), la polarisation en extinction et en émission), tout en respectant les abundancecosmiques élémentaires. Notre compréhension des poussières reste cependant toujours incomplète, en particulier surl'origine physique des variations de l'extinction et de l'émission des poussières dans le MIS. Le changement d'opacitédes poussières entre le milieu diffus et les nuages moléculaires est bien établi, et des modèles physiques d'interprétationont été proposés. Avec ses cartes de l'émission submillimétrique de tout le ciel à plusieurs longueurs d'onde, le surveysubmillimétrique de Planck nous permet pour la première fois de mesurer la température des poussières, et d'étudierainsi les variations d'opacité des poussières dans le milieu diffus.Cette thèse, basée sur une comparaison des données Planck avec des mesures en extinction en direction d'étoiles et dequasars, combine modélisation et analyse de données, afin de contraindre les variations des propriétés optiques despoussières dans le MIS diffus, et d'estimer les contributions respectives de l'alignement et de l'évolution des poussières àleur émission polarisée.La première partie de la thèse se focalise sur l'émission non polarisée des poussières dans le MIS diffus. L'étude desvariations de l'émission par unité d'extinction permet de contraindre les variations des propriétées optiques despoussières. Nous fittons les 20 SEDs normalisées en extinction de Planck Intermediate Results XXIX à l'aide de troismodèles de poussière (Draine & Li 2007, Compiegne et al 2011, Jones etal 2013). Le meilleur accord entre modèle etobservations est obtenu pour le modèle utilisant les grains plus émissifs (Jones 2013), dont les propriétés optiques sontbasées sur des données de laboratoires portant sur les silicates et carbones amorphes. En combinant la mesure del'extinction et de la SED sur la même ligne de visée, nous obtenons un nouvel estimateur de l'intensité du rayonnementinterstellaire G0,, qui s'avère moins biaisé que celui obtenu par un fit de la SED. Aucun des modèles n'arrive àreproduire simultanément les variations de G0 et de la SED à propriétés optiques des poussières fixes. A l'aide de notreestimateur, nous démontrons que la variation des propriétées optiques et de l'intensité du rayonnement interstellaire ontdes contributions semblables aux variations observées des SED dans le MIS diffus.La seconde partie de la thèse se focalise sur l'extinction et l'émission polarisées dans les nuages moléculaires. Enconfrontant des données Planck et des observations stellaires à un modèle de poussières, nous tentons de séparer leseffets dûs aux variations de l'alignement des poussières des effets dûs aux variations de leurs propriétés optiques. Noustrouvons une corrélation entre le rapport de la polarisation en émission à la polarisation en extinction, RP/p = P353/pv, et lalongueur d'onde de polarisation maximale en extinction, λmax, qui trace la taille typique des grains alignés. A l'aide d'unnouveau modèle de poussières basé sur les données Planck, nous démontrons que la variation de la taille minimale desgrains alignés suffit à elle seule à reproduire la corrélation observée, sans avoir à modifier ni la distribution en taille niles propriétés optiques des poussières, et qu'elle est de plus compatible avec la chute observée des fractions depolarisation avec λmax. D'autres interprétations ne sont cependant pas exclues

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 \lambda_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

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.Comment: 14 pages, 6 figure

The JCMT BISTRO Survey: Multi-wavelength polarimetry of bright regions in NGC 2071 in the far-infrared/submillimetre range, with POL-2 and HAWC+

Polarized dust emission is a key tracer in the study of interstellar medium and of star formation. The observed polarization, however, is a product of magnetic field structure, dust grain properties and grain alignment efficiency, as well as their variations in the line of sight, making it difficult to interpret polarization unambiguously. The comparison of polarimetry at multiple wavelengths is a possible way of mitigating this problem. We use data from HAWC+/SOFIA and from SCUBA-2/POL-2 (from the BISTRO survey) to analyse the NGC 2071 molecular cloud at 154, 214 and 850 μm. The polarization angle changes significantly with wavelength over part of NGC 2071, suggesting a change in magnetic field morphology on the line of sight as each wavelength best traces different dust populations. Other possible explanations are the existence of more than one polarization mechanism in the cloud or scattering from very large grains. The observed change of polarization fraction with wavelength, and the 214-to-154 μm polarization ratio in particular, are difficult to reproduce with current dust models under the assumption of uniform alignment efficiency. We also show that the standard procedure of using monochromatic intensity as a proxy for column density may produce spurious results at HAWC+ wavelengths. Using both long-wavelength (POL-2, 850 μm) and short-wavelength (HAWC+, ≲200μm) polarimetry is key in obtaining these results. This study clearly shows the importance of multi-wavelength polarimetry at submillimeter bands to understand the dust properties of molecular clouds and the relationship between magnetic field and star formation

The JCMT BISTRO Survey: multiwavelength polarimetry of bright regions in NGC 2071 in the far-infrared/submillimetre range, with POL-2 and HAWC+

Polarized dust emission is a key tracer in the study of interstellar medium and of star formation. The observed polarization, however, is a product of magnetic field structure, dust grain properties, and grain alignment efficiency, as well as their variations in the line of sight, making it difficult to interpret polarization unambiguously. The comparison of polarimetry at multiple wavelengths is a possible way of mitigating this problem. We use data from HAWC+ /SOFIA and from SCUBA-2/POL-2 (from the BISTRO survey) to analyse the NGC 2071 molecular cloud at 154, 214, and 850 μm. The polarization angle changes significantly with wavelength over part of NGC 2071, suggesting a change in magnetic field morphology on the line of sight as each wavelength best traces different dust populations. Other possible explanations are the existence of more than one polarization mechanism in the cloud or scattering from very large grains. The observed change of polarization fraction with wavelength, and the 214-to-154 μm polarization ratio in particular, are difficult to reproduce with current dust models under the assumption of uniform alignment efficiency. We also show that the standard procedure of using monochromatic intensity as a proxy for column density may produce spurious results at HAWC+wavelengths. Using both long-wavelength (POL-2, 850 μm) and short-wavelength (HAWC+, ≲200μm) polarimetry is key in obtaining these results. This study clearly shows the importance of multi-wavelength polarimetry at submillimetre bands to understand the dust properties of molecular clouds and the relationship between magnetic field and star formation

JCMT BISTRO Survey Observations of the Ophiuchus Molecular Cloud: Dust Grain Alignment Properties Inferred Using a Ricean Noise Model

The dependence of the polarization fraction p on total intensity I in polarized submillimeter emission measurements is typically parameterized as p ∝ I −α (α ≤ 1) and used to infer dust grain alignment efficiency in star-forming regions, with an index α = 1 indicating near-total lack of alignment of grains with the magnetic field. In this work, we demonstrate that the non-Gaussian noise characteristics of the polarization fraction may produce apparent measurements of α ~ 1 even in data with significant signal-to-noise in Stokes Q, U, and I emission, and so with robust measurements of polarization angle. We present a simple model demonstrating this behavior and propose a criterion by which well-characterized measurements of the polarization fraction may be identified. We demonstrate that where our model is applicable, α can be recovered by fitting the p–I relationship with the mean of the Rice distribution without statistical debiasing of the polarization fraction. We apply our model to JCMT BISTRO Survey POL-2 850 μm observations of three clumps in the Ophiuchus molecular cloud, finding that in the externally illuminated Oph A region, α ≈ 0.34, while in the more isolated Oph B and C, despite their differing star formation histories, α ~ 0.6–0.7. Our results thus suggest that dust grain alignment in dense gas is more strongly influenced by the incident interstellar radiation field than by star formation history. We further find that grains may remain aligned with the magnetic field at significantly higher gas densities than has previously been believed, thus allowing investigation of magnetic field properties within star-forming clumps and cores

JINGLE, a JCMT legacy survey of dust and gas for galaxy evolution studies: II. SCUBA-2 850 μm data reduction and dust flux density catalogues

We present the SCUBA-2 850μm component of JINGLE, the new JCMT large survey for dust and gas in nearby galaxies, which with 193 galaxies is the largest targeted survey of nearby galaxies at 850 μm. We provide details of our SCUBA-2 data reduction pipeline, optimized for slightly extended sources, and including a calibration model adjusted to match conventions used in other far-infrared (FIR) data. We measure total integrated fluxes for the entire JINGLE sample in 10 infrared/submillimetre bands, including all WISE, Herschel-PACS, Herschel-SPIRE, and SCUBA-2 850 μm maps, statistically accounting for the contamination by CO(J = 3-2) in the 850 μm band. Of our initial sample of 193 galaxies, 191 are detected at 250 μm with a ≥5σ significance. In the SCUBA-2 850 μm band we detect 126 galaxies with ≥3σ significance. The distribution of the JINGLE galaxies in FIR/sub-millimetre colour-colour plots reveals that the sample is not well fit by single modified-blackbody models that assume a single dust-emissivity index (β). Instead, our new 850 μm data suggest either that a large fraction of our objects require β < 1.5, or that a model allowing for an excess of sub-mm emission (e.g. a broken dust emissivity law, or a very cold dust component ≲10 K) is required. We provide relations to convert FIR colours to dust temperature and β for JINGLE-like galaxies. For JINGLE the FIR colours correlate more strongly with star-formation rate surface-density rather than the stellar surface-density, suggesting heating of dust is greater due to younger rather than older stellar-populations, consistent with the low proportion of early-type galaxies in the sample

The JCMT BISTRO Survey: Revealing the diverse magnetic field morphologies in Taurus dense cores with sensitive sub-millimeter polarimetry

© 2021. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/We have obtained sensitive dust continuum polarization observations at 850 $\mu$m in the B213 region of Taurus using POL-2 on SCUBA-2 at the James Clerk Maxwell Telescope (JCMT), as part of the BISTRO (B-fields in STar-forming Region Observations) survey. These observations allow us to probe magnetic field (B-field) at high spatial resolution ($\sim$2000 au or $\sim$0.01 pc at 140 pc) in two protostellar cores (K04166 and K04169) and one prestellar core (Miz-8b) that lie within the B213 filament. Using the Davis-Chandrasekhar-Fermi method, we estimate the B-field strengths in K04166, K04169, and Miz-8b to be 38$\pm$14 $\mu$G, 44$\pm$16 $\mu$G, and 12$\pm$5 $\mu$G, respectively. These cores show distinct mean B-field orientations. B-field in K04166 is well ordered and aligned parallel to the orientations of the core minor axis, outflows, core rotation axis, and large-scale uniform B-field, in accordance with magnetically regulated star formation via ambipolar diffusion taking place in K04166. B-field in K04169 is found to be ordered but oriented nearly perpendicular to the core minor axis and large-scale B-field, and not well-correlated with other axes. In contrast, Miz-8b exhibits disordered B-field which show no preferred alignment with the core minor axis or large-scale field. We found that only one core, K04166, retains a memory of the large-scale uniform B-field. The other two cores, K04169 and Miz-8b, are decoupled from the large-scale field. Such a complex B-field configuration could be caused by gas inflow onto the filament, even in the presence of a substantial magnetic flux.Peer reviewe