The relation between the column density structures and the magnetic field orientation in the Vela C molecular complex

Abstract

We statistically evaluated the relative orientation between gas column density structures, inferred from Herschel submillimetre observations, and the magnetic field projected on the plane of sky, inferred from polarized thermal emission of Galactic dust observed by the Balloon-borne Large-Aperture Submillimetre Telescope for Polarimetry (BLASTPol) at 250, 350, and 500 µm, towards the Vela C molecular complex. First, we find very good agreement between the polarization orientations in the three wavelength-bands, suggesting that, at the considered common angular resolution of 3. 0 that corresponds to a physical scale of approximately 0.61 pc, the inferred magnetic field orientation is not significantly affected by temperature or dust grain alignment effects. Second, we find that the relative orientation between gas column density structures and the magnetic field changes progressively with increasing gas column density, from mostly parallel or having no preferred orientation at low column densities to mostly perpendicular at the highest column densities. This observation is in agreement with previous studies by the Planck collaboration towards more nearby molecular clouds. Finally, we find a correspondence between (a) the trends in relative orientation between the column density structures and the projected magnetic field; and (b) the shape of the column density probability distribution functions (PDFs). In the sub-regions of Vela C dominated by one clear filamentary structure, or "ridges", where the high-column density tails of the PDFs are flatter, we find a sharp transition from preferentially parallel or having no preferred relative orientation at low column densities to preferentially perpendicular at highest column densities. In the sub-regions of Vela C dominated by several filamentary structures with multiple orientations, or "nests", where the maximum values of the column density are smaller than in the ridge-like sub-regions and the high-column density tails of the PDFs are steeper, such a transition is also present, but it is clearly less sharp than in the ridge-like sub-regions. Both of these results suggest that the magnetic field is dynamically important for the formation of density structures in this region.

An application of an optimal statistic for characterizing relative orientations

“…as derived in Soler et al (2017) . While simple in form and execution, this shape parameter cannot be the optimal statistic for characterising relative orientations.…”

“…As with the HRO statistic, the PRS can be used to test for preferentially parallel or perpendicular alignment, in a modification of the classic Rayleigh test widely used in circular statistics (see e.g., Batschelet 1981;Glimm 1996;Mardia & Jupp 1999). We compare the two approaches using simulated models, and for the Vela C region using the same BLASTPol and Herschel observations, as described in Soler et al (2017). From our results, we argue that the PRS is the optimal statistic for this type of analysis, although with open issues regarding the choice of weighting scheme.…”

“…Furthermore, Planck Collaboration XXXV, (2016) determined that within ten nearby (d < 450 pc) Gould Belt molecular clouds, the relative orientations of the column density, N H inferred from the Planck dust emission maps, and the B ⊥ inferred from the Planck 353-GHz polarization maps at 10 arcmin resolution changed progressively from preferentially parallel to perpendicular with increasing N H . Soler et al (2017) examined the relationship between the magnetic field and column density morphology by applying the HRO statistic to the magnetic field inferred from the Balloon-borne Large-Aperture Submillimetre Telescope for Polarimetry (BLASTPol) observations in three different wavelength-bands (250, 350, and 500 µm) for the Vela C molecular cloud.…”

Statistical analysis of the interplay between interstellar magnetic fields and filaments hostingPlanckGalactic cold clumps

“…Simulations show that such a change could be related to the degree of magnetization of the cloud, particularly when magnetic energy is in equipartition with turbulent energy (Hennebelle 2013; Soler & Hennebelle 2017;Chen et al 2016). In a detailed study of three nearby filaments, Planck Collaboration Int.…”

Filaments and striations: anisotropies in observed, supersonic, highly magnetized turbulent clouds

“…Magnetic fields also may play a role in feedback, for example, by driving protostellar jets that inject mass, momentum and energy into the medium, possibly stirring turbulent motions (Frank et al 2014;Gerrard et al 2019;Krumholz & Federrath 2019;. They also seem to play a role in the orientation of filamentary structures and striations in the MCs which may not directly influence the star-forming potential of the cloud, but certainly influences the global structure of the cloud through the density PDF (Planck Collaboration et al 2016a,b;Cox et al 2016;Malinen et al 2016;Soler et al 2017;Tritsis & Tassis 2016;Soler 2019). High-density filamentary structures house the local conditions necessary to overcome the turbulent motions, and observations find protostars located at junctions (or hubs) between intersecting filaments (André et al 2010;Men'shchikov et al 2010;Schneider et al 2013;Arzoumanian et al 2018;Tokuda et al 2018;Trevino-Morales et al 2019;Roy et al 2019;Xu et al 2019).…”