The effect of magnetic fields and ambipolar diffusion on the column density probability distribution function in molecular clouds

Auddy, Sayantan; Basu, Shantanu; Kudoh, Takahiro

doi:10.1093/mnras/stx2740

Cited by 15 publications

(10 citation statements)

References 57 publications

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“…We also found that regions with shallower PDF tails show a broad range of global relative orientations of N H and B ⊥ , with the shallowest being those regions where N H and B ⊥ are closer to parallel. These results provide observational evidence of the effects of the magnetic fields in the N H distribution based on MHD simulations (see, e.g., Collins et al 2012;Molina et al 2012;Auddy et al 2018). However, further numerical studies are necessary to determine the physical mechanisms that shape these nearby MCs and properly account for observational effects, such as the mean orientation of the field with respect to the LOS.…”

Section: Discussionsupporting

confidence: 54%

“…More recent studies of MHD turbulence in self-gravitating clouds indicate that the slope of the N H PDF tail is significantly steeper in magnetically subcritical clouds than in supercritical clouds (Auddy et al 2018). These numerical experiments suggest that the magnetic fields act as a density cushion in turbulent gas, shaping the cloud by preventing the gas from reaching very high densities.…”

Section: Relation Of the Relative Orientation To The Distribution Of ...mentioning

confidence: 70%

“…Recent studies based on MHD simulations indicate that magnetic fields can affect N H PDFs. Specifically, MCs with subcritical mass-toflux ratios show significantly steeper power tails than supercritical MCs (Auddy et al 2018). A proof of concept of the study of the relative orientation of N H and B ⊥ and its relation with the N H PDFs in observations can be found in , where the authors considered the Serpens Main 2 region in the Aquila Rift.…”

Section: Introductionmentioning

confidence: 99%

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Using Herschel and Planck observations to delineate the role of magnetic fields in molecular cloud structure

Soler

2019

A&A

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We present a study of the relative orientation between the magnetic field projected onto the plane of sky (B ⊥ ) on scales down to 0.4 pc, inferred from the polarized thermal emission of Galactic dust observed by Planck at 353 GHz, and the distribution of gas column density (N H ) structures on scales down to 0.026 pc, derived from the observations by Herschel in submillimeter wavelengths, toward ten nearby (d < 450 pc) molecular clouds. Using the histogram of relative orientation technique in combination with tools from circular statistics, we found that the mean relative orientation between N H and B ⊥ toward these regions increases progressively from 0 • , where the N H structures lie mostly parallel to B ⊥ , with increasing N H , in many cases reaching 90 • , where the N H structures lie mostly perpendicular to B ⊥ . We also compared the relative orientation between N H and B ⊥ and the distribution of N H , which is characterized by the slope of the tail of the N H probability density functions (PDFs). We found that the slopes of the N H PDF tail are steepest in regions where N H and B ⊥ are close to perpendicular. This coupling between the N H distribution and the magnetic field suggests that the magnetic fields play a significant role in structuring the interstellar medium in and around molecular clouds. However, we found no evident correlation between the star formation rates, estimated from the counts of young stellar objects, and the relative orientation between N H and B ⊥ in these regions.

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Section: Discussionsupporting

confidence: 54%

Section: Relation Of the Relative Orientation To The Distribution Of ...mentioning

confidence: 70%

Section: Introductionmentioning

confidence: 99%

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Using Herschel and Planck observations to delineate the role of magnetic fields in molecular cloud structure

Soler

2019

A&A

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“…The PDFs show the expected, gravity-driven power-law behaviour at high N (e.g. Kainulainen et al 2009;Kritsuk et al 2011;Girichidis et al 2014;Schneider et al 2015;Auddy et al 2018). However, for a given run there are only marginal differences in the PDFs at N 10 20 cm −2 when considering a different LOS, with the only exception being the run SILCC-MC5.…”

Section: The Column Density Distributionmentioning

confidence: 65%

From parallel to perpendicular -- On the orientation of magnetic fields in molecular clouds

Seifried,

Walch,

Weis

et al. 2020

Preprint

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We present synthetic dust polarization maps of simulated molecular clouds with the goal to systematically explore the origin of the relative orientation of the magnetic field (B) with respect to the cloud sub-structure identified in density (n; 3D) and column density (N; 2D). The polarization maps are generated with the radiative transfer code POLARIS, which includes self-consistently calculated efficiencies for radiative torque alignment. The molecular clouds are formed in two sets of 3D magneto-hydrodynamical simulations: (i) in colliding flows (CF), and (ii) in the SILCC-Zoom simulations. In 3D, for the CF simulations with an initial field strength below ∼5 µG, B is oriented either parallel or randomly with respect to the n-structures. For CF runs with stronger initial fields as well as all SILCC-Zoom simulations, which have an initial field strength of 3 µG, a flip from parallel to perpendicular orientation occurs at high densities of n trans 10 2 -10 3 cm −3 . We suggest that this flip happens if the cloud's mass-to-flux ratio, µ, is close to or below the critical value of 1. This corresponds to a field strength around 3 -5 µG, close to the Galactic average. In 2D, we use the method of Projected Rayleigh Statistics (PRS) to study the relative orientation of B. If present, the flip in orientation occurs in the projected maps at N trans 10 21−21.5 cm −2 . This value is similar to the observed transition value from sub-to supercritical magnetic fields in the interstellar medium. However, projection effects can strongly reduce the predictive power of the PRS method: Depending on the considered cloud or line-of-sight, the projected maps of the SILCC-Zoom simulations do not always show the flip, although it is expected given the 3D morphology. Such projection effects can explain the variety of recently observed field configurations, in particular within a single cloud. Finally, we do not find a correlation between the observed orientation of B and the N-PDF.

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“…A PLT in the N-PDF is connected to the existence of a powerlaw distribution in volume density and is commonly attributed to the effects of the self-gravity of the gas in generating dense structures in the cloud (e.g., Klessen 2000;Dib & Burkert 2005;Kainulainen et al 2009;Kritsuk et al 2011;Ward et al 2014;Girichidis et al 2014;Schneider et al 2015;Donkov & Stefanov 2018;Corbelli et al 2018;Veltchev et al 2019). Another interpretation for the origin of the first, steep PLT has been proposed by Auddy et al (2018Auddy et al ( , 2019. These authors showed, using numerical simulations of molecular clouds with non-ideal magnetohydrodynamics (MHD), that in the case of a magnetically subcritical cloud, a steep PLT (slope ≈ -4) can emerge as a result of gravitational contraction driven by ambipolar diffusion.…”

Section: Column Density Distribution Functionsmentioning

confidence: 96%

The structure and characteristic scales of molecular clouds

Dib

Bontemps

Schneider

et al. 2020

A&A

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The structure of molecular clouds holds important clues regarding the physical processes that lead to their formation and subsequent dynamical evolution. While it is well established that turbulence imprints a self-similar structure onto the clouds, other processes, such as gravity and stellar feedback, can break their scale-free nature. The break of self-similarity can manifest itself in the existence of characteristic scales that stand out from the underlying structure generated by turbulent motions. In this work, we investigate the structure of the Cygnus-X North and Polaris Flare molecular clouds, which represent two extremes in terms of their star formation activity. We characterize the structure of the clouds using the delta-variance (Δ-variance) spectrum. In the Polaris Flare, the structure of the cloud is self-similar over more than one order of magnitude in spatial scales. In contrast, the Δ-variance spectrum of Cygnus-X North exhibits an excess and a plateau on physical scales of ≈0.5−1.2 pc. In order to explain the observations for Cygnus-X North, we use synthetic maps where we overlay populations of discrete structures on top of a fractal Brownian motion (fBm) image. The properties of these structures, such as their major axis sizes, aspect ratios, and column density contrasts with the fBm image, are randomly drawn from parameterized distribution functions. We are able to show that, under plausible assumptions, it is possible to reproduce a Δ-variance spectrum that resembles that of the Cygnus-X North region. We also use a “reverse engineering” approach in which we extract the compact structures in the Cygnus-X North cloud and reinject them onto an fBm map. Using this approach, the calculated Δ-variance spectrum deviates from the observations and is an indication that the range of characteristic scales (≈0.5−1.2 pc) observed in Cygnus-X North is not only due to the existence of compact sources, but is a signature of the whole population of structures that exist in the cloud, including more extended and elongated structures.

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The effect of magnetic fields and ambipolar diffusion on the column density probability distribution function in molecular clouds

Cited by 15 publications

References 57 publications

Using Herschel and Planck observations to delineate the role of magnetic fields in molecular cloud structure

Using Herschel and Planck observations to delineate the role of magnetic fields in molecular cloud structure

From parallel to perpendicular -- On the orientation of magnetic fields in molecular clouds

The structure and characteristic scales of molecular clouds

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