Étude sous-millimétrique de l’interaction entre le magnétisme et la turbulence dans les milieux interstellaires

L'astronomie sous-millimétrique est une fenêtre unique pour l'étude des propriétés physiques d'une grande variété d'environnements interstellaires, des pouponnières d'étoiles de notre Galaxie aux jets relativistes issus de noyaux galactiques actifs. Grâce en particulier aux observations polarimétriques, il est même possible d'étudier les effets de l'interaction entre le magnétisme et la turbulence sur la dynamique de ces milieux. Cette thèse présente les résultats d'une étude sous-millimétrique dont l'objectif est de caractériser les propriétés physiques et la dynamique d'une sélection de régions de formation d'étoiles et de jets extragalactiques à partir d'observations continues, spectroscopiques et polarimétriques obtenues au télescope James-Clerk-Maxwell (JCMT). Nous avons d'abord quantifié l'effet de la contamination moléculaire sur les observations du nuage moléculaire géant d'Orion A obtenues à 450 µm et 850 µm avec la caméra SCUBA-2. À l'aide de mesures spectroscopiques effectuées avec le spectromètre HARP de la raie moléculaire 12CO J=3-2, nous avons identifié un échantillon de 33 sources dont le flux à 850 µm est fortement contaminé par des flots moléculaires environnants. Nous avons finalement montré que cette contamination mène à une sous-estimation de l'indice spectral d'émissivité β obtenu à partir du ratio des flux mesurés à 450 µm et 850 µm. Dans le cadre du programme BISTRO au JCMT, nous avons utilisé le polarimètre POL-2 afin de caractériser le champ magnétique dans la région de formation d'étoiles Barnard 1 du complexe moléculaire de Persée. Nous avons d'abord déterminé l'orientation sur le plan du ciel du champ magnétique à partir de la carte de polarisation linéaire obtenue à 850 µm. Nous avons aussi calculé une valeur de 1.05 ± 0.92 pour le rapport entre les composantes turbulentes et ordonnées de l'énergie magnétique. Grâce aux observations de la raie moléculaire C18O J=1-0 obtenues au FCRAO, nous avons enfin appliqué la technique de Davis-Chandrasekhar-Fermi afin d'évaluer l'amplitude du champ magnétique à ~20 µG. Avec l'équipe de mise en marche de POL-2, nous avons détecté avec succès la polarisation à 850 µm dans le coeur protostellaire CB 68. Nous avons ainsi déterminé l'orientation dans le plan du ciel du champ magnétique à l'intérieur de cet objet, que nous avons ensuite comparé avec les données SCUPOL dans la littérature. Additionnellement, nous avons mesuré une diminution de la fraction de polarisation en fonction de l'intensité totale, ce qui pourrait être expliqué par des effets de dépolarisation le long de la ligne de visée. Finalement, nous avons mené la première campagne avec POL-2 afin d'étudier la variabilité temporelle de la polarisation linéaire à 850 µm vers quatre noyaux galactiques actifs: 3C 84, 3C 273, 3C 279 et 3C 454.3. Nous avons mesuré une variation significative de la fraction et de l'angle de polarisation pour 3C 84, 3C 273 et 3C 279 sur une période de 9 mois. Cette variabilité supporte la présence de cellules turbulentes magnétisées à l'intérieur de chocs permanents le long des jets relativistes issus de l'accrétion de matière sur les trous noirs supermassifs au centre de ces galaxies.Submillimetre astronomy is a unique window for the study of the physical properties of a large variety of interstellar environments, from the stellar nurseries of our Galaxy to the relativistic jets from active galactic nuclei. With polarimetric observations in particular, it is even possible to study the effects of the interaction between magnetism and turbulence on the dynamics of these environments. This thesis presents the results from a submillimetre study which goal was to characterise the physical and dynamical properties of a selection of star-forming regions and extragalactic jets using continuum, spectroscopic and polarimetric observations from the James Clerk Maxwell Telescope (JCMT). We have first quantified the effect of molecular contamination on SCUBA-2 observations at 450 µm and 850 µm of the Orion A giant molecular cloud. With spectroscopic measurements using the HARP spectrometer of the 12CO J=3-2 molecular line, we have identified a sample of 33 sources for which the 850 µm flux is highly contaminated by nearby molecular outflows. Finally, we have shown that this contamination leads to an underestimation of the emissivity spectral index β derived from the 450 µm to 850 µm flux ratio. As part of the BISTRO survey at the JCMT, we have used the POL-2 polarimeter in order to characterise the magnetic field in the Barnard 1 star-forming region in the Perseus molecular cloud complex. We have inferred the plane-of-sky orientation of the magnetic field from the linear polarisation map obtained at 850 µm. We have also calculated a value of 1.05 ± 0.92 for the turbulent-to-ordered magnetic energy ratio. With FCRAO observations of the C18O J=1-0 molecular line, we have also applied the Davis-Chandrasekhar-Fermi method in order to evaluate the amplitude of the magnetic field to be ~20 µG. With the POL-2 commissioning team, we have successfully detected the 850 µm polarisation in the CB 68 protostellar core. We have then inferred the plane-of-sky orientation of the magnetic field within this cloud that we have then compared to previously published SCUPOL observations. Additionally, we have measured a diminution in the fraction of polarisation as a function of total intensity, which could be explained by depolarisation effects along the line-of-sight. Finally, we have lead the first POL-2 campaign to study the temporal variability of the 850 µm linear polarisation towards four active galactic nuclei: 3C 84, 3C 273, 3C 279 and 3C 454.3. We report significant variability in the fraction and angle of polarisation for 3C 84, 3C 273 and 3C 279 over a period of 9 months. This variability supports the presence of magnetised turbulent cells within standing shocks along the relativistic jets originating from the accretion of matter on the central supermassive black holes of these galaxies

Most-Likely DCF Estimates of Magnetic Field Strength

The Davis-Chandrasekhar-Fermi (DCF) method is widely used to evaluate magnetic fields in star-forming regions. Yet it remains unclear how well DCF equations estimate the mean plane-of-the-sky field strength in a map region. To address this question, five DCF equations are applied to an idealized cloud map. Its polarization angles have a normal distribution with dispersion ${\sigma}_{\theta}$,and its density and velocity dispersion have negligible variation. Each DCF equation specifies a global field strength $B_{DCF}$ and a distribution of local DCF estimates. The "most-likely" DCF field strength $B_{ml}$ is the distribution mode (Chen et al. 2022), for which a correction factor ${\beta}_{ml}$ = $B_{ml}$/$B_{DCF}$ is calculated analytically. For each equation ${\beta}_{ml}$ < 1, indicating that $B_{DCF}$ is a biased estimator of $B_{ml}$. The values of ${\beta}_{ml}$ are ${\beta}_{ml}\approx$ 0.7 when $B_{DCF} \propto {{\sigma}_{\theta}}^{-1}$ due to turbulent excitation of Afv\'enic MHD waves, and ${\beta}_{ml}\approx$ 0.9 when $B_{DCF} \propto {{\sigma}_{\theta}}^{-1/2}$ due to non-Alfv\'enic MHD waves. These statistical correction factors ${\beta}_{ml}$ have partial agreement with correction factors ${\beta}_{sim}$ obtained from MHD simulations. The relative importance of the statistical correction is estimated by assuming that each simulation correction has both a statistical and a physical component. Then the standard, structure function, and original DCF equations appear most accurate because they require the least physical correction. Their relative physical correction factors are 0.1, 0.3, and 0.4 on a scale from 0 to 1. In contrast the large-angle and parallel-${\delta}B$ equations have physical correction factors 0.6 and 0.7. These results may be useful in selecting DCF equations, within model limitations.Comment: Accepted for publication in The Astrophysical Journa

Magnetic fields and outflows in the large Bok globule CB 54

We have observed the large Bok globule CB 54 in 850 μm polarised light using the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We find that the magnetic field in the periphery of the globule shows significant, ordered deviation from the mean field direction in the globule centre. This deviation appears to correspond with the extended but relatively weak 12CO outflow emanating from the Class 0 sources at the centre of the globule. Energetics analysis suggests that if the outflow is reshaping the magnetic field in the globule’s periphery, then we can place an upper limit of 0.1 pc

Magnetic fields and outflows in CB 54

We have observed the large Bok globule CB 54 in 850-μm polarized light using the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We find that the magnetic field in the periphery of the globule shows a significant, ordered deviation from the mean-field direction in the globule centre. This deviation appears to correspond with the extended but relatively weak 12CO outflow emanating from the Class 0 sources at the centre of the globule. Energetics analysis suggests that if the outflow is reshaping the magnetic field in the globule’s periphery, then we can place an upper limit of 0.1 pc

The magnetic field in the Flame nebula

Star formation is essential in galaxy evolution and the cycling of matter. The support of interstellar clouds against gravitational collapse by magnetic (B-) fields has been proposed to explain the low observed star formation efficiency in galaxies and the Milky Way. Despite the Planck satellite providing a 5-15' all-sky map of the B-field geometry in the diffuse interstellar medium, higher spatial resolution observations are required to understand the transition from diffuse gas to gravitationally unstable filaments. NGC 2024, the Flame Nebula, in the nearby Orion B molecular cloud, contains a young, expanding HII region and a dense filament that harbors embedded protostellar objects. Therefore, NGC 2024 is an excellent opportunity to study the role of B-fields in the formation, evolution, and collapse of filaments, as well as the dynamics and effects of young HII regions on the surrounding molecular gas. We combine new 154 and 216 micron dust polarization measurements carried out using the HAWC+ instrument aboard SOFIA with molecular line observations of 12CN(1-0) and HCO+(1-0) from the IRAM 30-meter telescope to determine the B-field geometry and to estimate the plane of the sky magnetic field strength across the NGC 2024. The HAWC+ observations show an ordered B-field geometry in NGC 2024 that follows the morphology of the expanding HII region and the direction of the main filament. The derived plane of the sky B-field strength is moderate, ranging from 30 to 80 micro G. The strongest B-field is found at the northern-west edge of the HII region, characterized by the highest gas densities and molecular line widths. In contrast, the weakest field is found toward the filament in NGC 2024. The B-field has a non-negligible influence on the gas stability at the edges of the expanding HII shell (gas impacted by the stellar feedback) and the filament (site of the current star formation).Comment: 36 pages, 26 figures Accepted for publication in Astronomy & Astrophysic

Tomographic Imaging of the Sagittarius Spiral Arm's Magnetic Field Structure

The Galactic global magnetic field is thought to play a vital role in shaping Galactic structures such as spiral arms and giant molecular clouds. However, our knowledge of magnetic field structures in the Galactic plane at different distances is limited, as measurements used to map the magnetic field are the integrated effect along the line of sight. In this study, we present the first-ever tomographic imaging of magnetic field structures in a Galactic spiral arm. Using optical stellar polarimetry over a $17' \times 10'$ field of view, we probe the Sagittarius spiral arm. Combining these data with stellar distances from the $Gaia$ mission, we can isolate the contributions of five individual clouds along the line of sight by analyzing the polarimetry data as a function of distance. The observed clouds include a foreground cloud ($d < 200$ pc) and four clouds in the Sagittarius arm at 1.23 kpc, 1.47 kpc, 1.63 kpc, and 2.23 kpc. The column densities of these clouds range from 0.5 to $2.8 \times 10^{21}~\mathrm{cm}^{-2}$. The magnetic fields associated with each cloud show smooth spatial distributions within their observed regions on scales smaller than 10 pc and display distinct orientations. The position angles projected on the plane-of-sky, measured from the Galactic north to east, for the clouds in increasing order of distance are $135^\circ$, $46^\circ$, $58^\circ$, $150^\circ$, and $40^\circ$, with uncertainties of a few degrees. Notably, these position angles deviate significantly from the direction parallel to the Galactic plane.Comment: Accepted for publication in Ap

Dust polarization in OMC-1

We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains (⁠>0.005−0.1μm), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion