Magnetic Field Diagnostics Based on Far-Infrared Polarimetry: Tests Using Numerical Simulations

By F. Heitsch, E. Zweibel, M. Mac Low, P. Li, M. Norman. 2001

136 citations recorded by the source corpus.

The dynamical state of star-forming molecular clouds cannot be understood without determining the structure and strength of their magnetic fields. Measurements of polarized far-infrared radiation from thermally aligned dust grains are used to map the orientation of the field and estimate its strength, but the accuracy of the results has remained in doubt. In order to assess the reliability of this method, we apply it to simulated far-infrared polarization maps derived from three-dimensional simulations of supersonic magnetohydrodynamical turbulence, and we compare the estimated values to the known magnetic field strengths in the simulations. We investigate the effects of limited telescope resolution and self-gravity on the structure of the maps. Limited observational resolution affects the field structure such that small-scale variations can be completely suppressed, thus giving the impression of a very homogeneous field. The Chandrasekhar-Fermi method of estimating the mean magnetic field in a turbulent medium is tested, and we suggest an extension to measure the rms field. Both methods yield results within a factor of 2 for field strengths typical of molecular clouds, with the modified version returning more reliable estimates for slightly weaker fields. However, neither method alone works well for very weak fields, missing them by a factor of up to 150. Taking the geometric mean of both methods estimates even the weakest fields accurately within a factor of 2.5. Limited telescope resolution leads to a systematic overestimation of the field strengths for all methods. We discuss the effects responsible for this overestimation and show how to extract information on the underlying (turbulent) power spectrum.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/98cb7f5733b39cf8601eba05eac49cf391366166

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Citations in the corpus

The James Clerk Maxwell Telescope Legacy Survey of Nearby Star‐forming Regions in the Gould Belt (2007, methodology): “The gas dynamics from HARP C18O/13CO observations will be used with the polarization maps to test predictions of magnetized simulations of turbulence (e.g., Heitsch et al. 2001; Ostriker et al. 2001; Padoan & Nordlund 2002).”
Control of star formation by supersonic turbulence (2000, methodology): “The models of driven, self-gravitating turbulence by Klessen et al. (2000) and Heitsch et al. (2001) described above (Section II.”
Studies of Regular and Random Magnetic Fields in the ISM: Statistics of Polarization Vectors and the Chandrasekhar-Fermi Technique (2008, methodology): “Heitsch et al. (2001) presented an extended analysis using a larger number of models, with different physical parameters and numerical resolutions (including 1model with 5123 resolution).”
Physical Processes in the Interstellar Medium (2014, background): “However, converting polarization into magnetic field strength is very difficult (Heitsch et al., 2001b).”
Planck intermediate results: XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds (2015, unknown): “This is quantiﬁed as half the ratio of the gas pressure to the mean-ﬁeld magnetic pressure (Ostriker et al. 2001; Heitsch et al. 2001), with the resulting turbulence ranging from sub-Alfv´enic to super-Alfv´enic.”
Statistical Properties of Galactic Starlight Polarization (2001, methodology): “(Hildebrand et al. 1999 ; Novak et al. 2000 ; see also Hildebrand et al. 2000 ; Heitsch et al. 2001 for recent reviews) but they reÑect only rather local distortions of the large-scale magnetic Ðeld.”
AN IMPRINT OF MOLECULAR CLOUD MAGNETIZATION IN THE MORPHOLOGY OF THE DUST POLARIZED EMISSION (2013, background): “…& Li 2008), elongated structures along the magnetic field lines when the medium is strongly compressed supersonically (Ostriker et al. 2001; Heitsch et al. 2001; Falceta-Gonçalves et al. 2008), and linear correlation of the magnetic field strength with column density for supersonic…”
SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores (2003, methodology): “The Heitsch et al. (2001) simulations that were used for the resolution studies only had 1283 computational cells.”