ABSTRACT
The thermal emission of dust is one of the most important tracers of the interstellar medium: multiwavelength photometry in the far-infrared (FIR) and submillimetre (submm) can be fitted ...with a model, providing estimates of the dust mass. The fit results depend on the assumed value for FIR/submm opacity, which in most models – due to the scarcity, until recently, of experimental measurements – is extrapolated from shorter wavelengths. Lab measurements of dust analogues, however, show that FIR opacities are usually higher than the values used in models and depend on temperature, which suggests that dust mass estimates may be biased. To test the extent of this bias, we create multiwavelength synthetic photometry for dusty galaxies at different temperatures and redshifts, using experimental results for FIR/submm dust opacity and then we fit the synthetic data using standard dust models. We find that the dust masses recovered by typical models are overestimated by a factor of 2–20, depending on how the experimental opacities are treated. If the experimental dust samples are accurate analogues of interstellar dust, therefore, current dust masses are overestimated by up to a factor of 20. The implications for our understanding of dust, both Galactic and at high redshift, are discussed.
The dependence of the polarization fraction p on total intensity I in polarized submillimeter emission measurements is typically parameterized as p ∝ I− ( ≤ 1) and used to infer dust grain alignment ...efficiency in star-forming regions, with an index = 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 ∼ 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, 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 m observations of three clumps in the Ophiuchus molecular cloud, finding that in the externally illuminated Oph A region, 0.34, while in the more isolated Oph B and C, despite their differing star formation histories, ∼ 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.
We present 850 m polarization observations of the L1689 molecular cloud, part of the nearby Ophiuchus molecular cloud complex, taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope ...(JCMT). We observe three regions of L1689: the clump L1689N which houses the IRAS 16293-2433 protostellar system, the starless clump SMM-16, and the starless core L1689B. We use the Davis-Chandrasekhar-Fermi method to estimate plane-of-sky field strengths of 366 55 G in L1689N, 284 34 G in SMM-16, and 72 33 G in L1689B, for our fiducial value of dust opacity. These values indicate that all three regions are likely to be magnetically transcritical with sub-Alfvénic turbulence. In all three regions, the inferred mean magnetic field direction is approximately perpendicular to the local filament direction identified in Herschel Space Telescope observations. The core-scale field morphologies for L1689N and L1689B are consistent with the cloud-scale field morphology measured by the Planck Space Observatory, suggesting that material can flow freely from large to small scales for these sources. Based on these magnetic field measurements, we posit that accretion from the cloud onto L1689N and L1689B may be magnetically regulated. However, in SMM-16, the clump-scale field is nearly perpendicular to the field seen on cloud scales by Planck, suggesting that it may be unable to efficiently accrete further material from its surroundings.
ABSTRACT
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 $\mu$m. 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 $\mu$m 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 $\mu$m) and short-wavelength (HAWC+, $\lesssim 200\, \mu$m) 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.
Abstract
We present the first 850
μ
m polarization observations in the most active star-forming site of the Rosette Molecular Cloud (
d
∼ 1.6 kpc) in the wall of the Rosette Nebula, imaged with the ...SCUBA-2/POL-2 instruments of the James Clerk Maxwell telescope, as part of the B-Fields In Star-forming Region Observations 2 (BISTRO-2) survey. From the POL-2 data we find that the polarization fraction decreases with the 850
μ
m continuum intensity with
α
= 0.49 ± 0.08 in the
p
∝
I
−
α
relation, which suggests that some fraction of the dust grains remain aligned at high densities. The north of our 850
μ
m image reveals a “gemstone ring” morphology, which is a ∼1 pc diameter ring-like structure with extended emission in the “head” to the southwest. We hypothesize that it might have been blown by feedback in its interior, while the
B
-field is parallel to its circumference in most places. In the south of our SCUBA-2 field the clumps are apparently connected with filaments that follow infrared dark clouds. Here, the POL-2 magnetic field orientations appear bimodal with respect to the large-scale Planck field. The mass of our effective mapped area is ∼174
M
⊙
, which we calculate from 850
μ
m flux densities. We compare our results with masses from large-scale emission-subtracted Herschel 250
μ
m data and find agreement within 30%. We estimate the plane-of-sky
B
-field strength in one typical subregion using the Davis–Chandrasekhar–Fermi technique and find 80 ± 30
μ
G toward a clump and its outskirts. The estimated mass-to-flux ratio of
λ
= 2.3 ± 1.0 suggests that the
B
-field is not sufficiently strong to prevent gravitational collapse in this subregion.
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.
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.
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.
The thermal emission of dust is one of the most important tracers of the interstellar medium: multi-wavelength photometry in the far-infrared (FIR) and submillimeter (submm) can be fitted with a ...model, providing estimates of the dust mass. The fit results depend on the assumed value for FIR/submm opacity, which in most models - due to the scarcity, until recently, of experimental measurements - is extrapolated from shorter wavelengths. Lab measurements of dust analogues, however, show that FIR opacities are usually higher than the values used in models and depend on temperature, which suggests that dust mass estimates may be biased. To test the extent of this bias, we create multi-wavelength synthetic photometry for dusty galaxies at different temperatures and redshifts, using experimental results for FIR/submm dust opacity, then we fit the synthetic data using standard dust models. We find that the dust masses recovered by typical models are overestimated by a factor 2 to 20, depending on how the experimental opacities are treated. If the experimental dust samples are accurate analogues of interstellar dust, therefore, current dust masses are overestimated by up to a factor 20. The implications for our understanding of dust, both Galactic and at high redshift, are discussed.
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