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

By D. Whittet, James Hough, A. Lazarian, Thiem C. Hoang. 2007

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We present the results of a detailed study of interstellar polarization efficiency (as measured by the ratio pλ/τλ) 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 (pλ/τλ ∝ [ AV]−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 pλ/τλ 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 AV ∼ 10 mag. Radiation from the embedded stars themselves may enhance alignment in the lines of sight to YSOs. Enhancements in pλ/τλ observed in the ice features toward several YSOs are of greatest significance, as they demonstrate efficient alignment in cold molecular clouds associated with star formation.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/ceeb83089b73c0c75cbd78fa6cb49052e67a8330

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Citations in the corpus

Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust (2014, background): “…of the polarization fraction toward large column densities on small (∼a few 0.1 pc) scales was reported previously in ground-based measurements of polarization both in emission (Ward-Thompson et al. 2000; Matthews & Wilson 2000) and extinction (e.g., Gerakines & Whittet 1995; Whittet et al. 2008).”
Planck2018 results (2018, background): “5 MJy sr−1 K−1 CMB given in Planck Collaboration III (2018). This CIB-subtracted total intensity map has a zero level that by construction is based on a correlation of the emission at high Galactic latitudes with the column density of the ISM traced by the 21-cm emission of Hi at low column densitie”
Studies of Regular and Random Magnetic Fields in the ISM: Statistics of Polarization Vectors and the Chandrasekhar-Fermi Technique (2008, background): “All in all, both observations (Arce et al. 1998; Whittet et al. 2008) and theory (Hoang & Lazarian 2008), suggest that there is a range of AV for which our assumptions are correct.”
Planck intermediate results: XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds (2015, unknown): “XIX 2014), when seen at small scales, might be interpreted in terms of a decrease of (cid:15) with increasing column density (Matthews et al. 2001; Whittet et al. 2008).”