The Efficiency of Grain Alignment in Dense Interstellar Clouds: a Reassessment of Constraints from Near‐Infrared Polarization

Abstract

We present the results of a detailed study of interstellar polarization efficiency (as measured by the ratio p λ /τ λ ) toward molecular clouds, with the aim of discriminating between grain alignment mechanisms in dense regions of the interstellar medium. The data set includes both continuum measurements in the K (2.2 µm) passband and values based on ice and silicate spectral features. Background field stars are used to probe polarization efficiency in quiescent regions of dark clouds, yielding a dependence on visual extinction well-represented by a power law (p λ /τ λ ∝ [A V ] −0.52 ), in agreement with previous work. No significant change in this behavior is observed in the transition region between the diffuse outer layers and dense inner regions of clouds, where icy mantles are formed, and we conclude that mantle formation has little or no effect on the efficiency of grain alignment. The field-star data are used as a template for comparison with results for embedded young stellar objects (YSOs). The latter generally exhibit greater polarization efficiency compared with field stars at comparable extinctions, some displaying enhancements in p λ /τ λ by factors of up to ∼ 6 with respect to the power-law fit. Of the proposed alignment mechanisms, that based on radiative torques appears best able to explain the data. The attenuated external radiation field appears adequate to account for the observed polarization in quiescent regions for extinctions up to A V ∼ 10 mag. Radiation from the

Modelling grain alignment by radiative torques and hydrogen formation torques in reflection nebula

“…The simple, one-dimensional modeling of RAT alignment for a starless core in Whittet et al (2008) shows that the alignment in the dense cloud follows the model in which aligned grains are distributed in the outer layers, and the innermost region contains nonaligned grains. As a result, the fractional polarization is predicted to first decrease slowly with AV and then decline more rapidly according to a power-law A −1 V from some large AV ∼ 3.…”

“…Grain alignment by RATs in starless, dense clouds has been studied by a number of authors (Cho & Lazarian 2005;Bethell et al 2007;Whittet et al 2008). These studies dealt with the alignment of grains by RATs induced by the attenuated ISRF.…”

“…These studies dealt with the alignment of grains by the attenuated diffuse interstellar radiation field (ISRF). The observational data from Whittet et al (2008) show that the fractional polarization can be fitted with a power-law, pK/τK ∝ A −0.52±0.07 V , where pK and τK are the polarization and optical depth measured in the K band. A simple one-dimensional modeling of RAT alignment for a dense, uniform starless cloud in Whittet et al shows that the fractional polarization first decreases slowly with AV (with a slope shallower than the best-fit one) and then declines rapidly as A −1 V for AV > 3.…”

“…Grain alignment by RATs in molecular clouds has been studied extensively (Cho & Lazarian 2005;Bethell et al 2007;Whittet et al 2008;Pelkonen et al 2009). These studies dealt with the alignment of grains by the attenuated diffuse interstellar radiation field (ISRF).…”

“…Such a steep decline of pK/τK arises from a significant decrease in grain alignment due to the attenuation of ISRF toward the cloud center (see Whittet et al 2008). In the presence of magnetic field turbulence, the fractional polarization for sightlines above the decorrelation length of magnetic fields is considerably reduced, which is expected to produce a power-law A −0.5 V for moderate AV (Jones et al 1992), consistent with the best-fit slope in Whittet et al (2008). However, only the wandering of magnetic field lines seems to be insufficient to 1 The picture of thermal flipping was challenged by Weingartner (2009) who found that the grain does not experience thermal flipping as a result of internal relaxation, instead, it tends to be frozen at the separatrix (i.e., when the grain symmetry axis becomes perpendicular to the angular momentum).…”