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Theses Year : 2021

Constraining the dust grain alignment mechanisms responsible for the (sub-) millimeter dust polarization observed in class 0 protostellar cores

Caractérisation des mécanismes responsables de l’émission polarisée des grains de poussières dans les proto-étoiles de Classe 0

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Abstract

With the aim of characterizing the role played by magnetic fields in the formation of young protostars, several recent studies have revealed unprecedented features toward high angular resolution ALMA dust polarization observations of Class 0 protostellar cores. Observations of polarized dust emission allow us to investigate the physical processes involved in the Radiative Alignment Torques (RATs) acting on dust grains from the core to disk scales, that align the angular momentum of grains with magnetic field. We find that the dust polarization is enhanced along the cavity walls of bipolar outflows, which are subject to high irradiation from the reprocessed radiation field emanating from the center of the protostar. In addition, highly polarized dust thermal emission has been detected in region most likely linked with the infalling envelope, in the form of filamentary structure being potential magnetized accretion streamer. Notably, we propose that the polarized emission we see at millimeter wavelengths along the irradiated cavity walls can be reconciled with the expectations of RAT theory if the aligned grains present in these cavities have grown larger than what is typically expected in young protostellar cores. To approach an estimation of the efficiency of dust alignment in protostars, we gathered a large sample of ALMA dust polarization observations of Class 0 protostars in order to perform a statistical analysis examining the trend between the dispersion of polarization position angles and the fractional polarization. We report a significant correlation between these two quantities, whose power-law index differs significantly from the one observed by Planck in star-forming clouds, confirming the different nature for the disorganized component of magnetic fields at the scales of protostellar envelopes. The grain alignment efficiency, is surprisingly constant across three orders of magnitude in envelope column density. Synthetic observations of non-ideal magneto-hydrodynamic simulations of protostellar cores implementing RATs, show that the ALMA values of grain alignment efficiency lie among those predicted by a perfect alignment of grains, and are significantly higher than the ones obtained with RATs. Ultimately, our results suggest dust alignment mechanism(s) are efficient at producing polarized dust emission in the 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. We performed further detailed modelling of the protostellar inner envelope physical conditions, alongside tentative comparisons between ALMA molecular line observations of UV-sensitive chemical tracers and dust polarization observations. We found that indeed, grains with super-paramagnetic inclusions, significant irradiation conditions (qualitatively comforted by the chemical observations), and large grains (10 micron) of compact structure are necessary to reproduce the observed grain alignment efficiency. However, further studies leading to a better characterization of dust grain characteristics, and additional grain alignment mechanisms, will be required to investigate deeper the cause of strong polarized dust emission located in regions of the envelope where alignment conditions are not favorable.
Dans le but de caractériser le rôle du champ magnétique dans la formation des jeunes protoétoiles, plusieurs études ont révélé de surprenants résultats au sein d’observations de l’émission polarisée des grains de poussières dans les cœurs proto-stellaires de Classe 0 à hautes résolution angulaire réalisées avec ALMA. Ces observations de polarisation de l’émission thermique des grains nous permettent d’investiguer les processus physiques impliqués dans les couples radiatifs d’alignement (RATs) agissant sur les grains depuis les échelles du cœur jusqu’à celles du disque, qui alignent le moment angulaire des grains avec le champ magnétique. Nos observations montrent que l’émission polarisée des grains est accentuée le long des parois des flots de cavités bipolaires, qui sont sujets à une forte irradiation, causée par le champ de radiation émanant du centre de la protoétoile. De plus, de fortes émissions polarisées ont été détectées au sein de régions probablement liées à l’enveloppe en effondrement, sous la forme de structures filamentaires, possiblement des streamers d’accrétion magnétisés. Notamment, nous émettons l’hypothèse que cette émission polarisée, observées aux longueurs millimétriques le long des parois de cavités irradiées, peut s’expliquer avec la théorie des RATs si les grains alignés responsables de la polarisation ont atteint des tailles supérieures à ce qui est typiquement attendu dans les jeunes cœurs proto-stellaires. Pour approcher une estimation de l'efficacité de l'alignement de la poussière dans les protoétoiles, nous avons rassemblé un grand échantillon d'observations ALMA en polarisation de plusieurs protoétoiles de Classe 0 afin d'effectuer une analyse statistique examinant la corrélation entre la dispersion des angles de polarisation et la polarisation fractionnelle
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Dates and versions

tel-03601487 , version 1 (08-03-2022)

Identifiers

  • HAL Id : tel-03601487 , version 1

Cite

Valentin Le Gouellec. Constraining the dust grain alignment mechanisms responsible for the (sub-) millimeter dust polarization observed in class 0 protostellar cores. Astrophysics [astro-ph]. Université Paris Cité, 2021. English. ⟨NNT : 2021UNIP7052⟩. ⟨tel-03601487⟩
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