SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores

Abstract

We have mapped linearly polarized dust emission from L183 with the JCMT SCUBA polarimeter and have analyzed these and our previously published data for the prestellar cores L183, L1544, and L43 in order to estimate magnetic field strengths in the plane of the sky, B pos . The analysis used the Chandrasekhar-Fermi technique, which relates the dispersion in polarization position angles to B pos . We have used these estimates of the field strengths (neglecting the unmeasured line-of-sight component) to find the mass-to-magnetic flux ratios λ (in units of the critical ratio for magnetic support). Results are B pos ≈ 80 µG and λ ≈ 2.6 for L183, B pos ≈ 140 µG and λ ≈ 2.3 for L1544, and B pos ≈ 160 µG and λ ≈ 1.9 for L43. Hence, without correction for geometrical biases, for all three cores the mass-to-flux ratios are supercritical by a factor of ∼ 2, and magnetic support cannot prevent collapse. However, a statistical mean correction for geometrical bias may be up to a factor of three; this correction would reduce the 1 crutcher@uiuc.edu 2 David.Nutter@astro.cf.ac.uk 3 Derek.Ward-Thompson@astro.cf.ac.uk 4 jmkirk@astro.uiuc.edu -2 -individual λ's to λ cor ≈ 0.9, 0.8, and 0.6, respectively; these values are approximately critical or slightly subcritical. These data are consistent with models of star formation driven by ambipolar diffusion in a weakly turbulent medium, but cannot rule out models of star formation driven by turbulence.

First Results from BISTRO: A SCUBA-2 Polarimeter Survey of the Gould Belt

“…However, a measure of the field strength can be derived from the commonly used Chandrasekhar-Fermi (C-F) method (Chandrasekhar & Fermi 1953), and modern variants thereof (e.g., Hildebrand et al 2009;Houde et al 2009), using dispersion in polarization half-vectors (where high dispersion indicates a highly turbulent velocity field and a weak mean B-field component; "halfvector" refers to the ±180°ambiguity in B-field direction), the line widths estimated from spectroscopic data, and the density from the SCUBA-2 flux densities (e.g., Crutcher et al 2004;Kirk et al 2006). Simulations show that this estimate can be corrected for a statistical ensemble of objects to yield realistic estimates of the field strength (Heitsch et al 2001;Ostriker et al 2001;Falceta-Gonçalves et al 2008).…”

Fossil magnetic field of accretion disks of young stars

“…Typical values are B c = 10 −5 Gs, n c = 10 5 cm −3 (Crutcher et al (2004)), so estimation of frozen-in magnetic field at the distance 1 AU is (adopting ρ ≃ 10 −13 g cm −3 )…”

Polarized dust emission of magnetized molecular cloud cores

“…1). In some cases, however, like in the dark cloud L183 (Crutcher et al 2004) or L1544 (Ward-Thomson et al 2000), the observed p−I relation is too steep to be explained only on the basis of the field morphology and inclination. A full model for the polarization of the emitted radiation probably requires additional mechanisms (such as an increase in size and sphericity of dust grains near the core centre) to reduce the polarizing efficiency α at high values of density or extinction.…”

“…This casts doubt on the use of the Chandrasekhar-Fermi formula to estimate the magnetic field strength in molecular cloud cores, as this formula assumes that observed deviations of polarization angles from a given direction (of the order of σ χ ≈ 10 • −15 • in starless cores, see e.g. Crutcher et al 2004) are solely due to the presence of a turbulent (or better, "wavy") component of the field.…”