Radiative torques: analytical model and basic properties

By A. Lazarian, Thiem C. Hoang. 2007

352 citations recorded by the source corpus.

We attempt to get a physical insight into grain alignment processes by studying basic properties of radiative torques (RATs). For this purpose we consider a simple toy model of a helical grain that reproduces well the basic features of RATs. The model grain consists of a spheroidal body with a mirror attached at an angle to it. Being very simple, the model allows analytical description of RATs that act upon it. We show a good correspondence of RATs obtained for this model and those of irregular grains calculated by DDSCAT. Our analysis of the role of different torque components for grain alignment reveals that one of the three RAT components does not affect the alignment, but induces only for grain precession. The other two components provide a generic alignment with grain long axes perpendicular to the radiation direction, if the radiation dominates the grain precession, and perpendicular to magnetic field, otherwise. The latter coincides with the famous predictions of the Davis-Greenstein process, but our model does not invoke paramagnetic relaxation. In fact, we identify a narrow range of angles between the radiation beam and the magnetic field, for which the alignment is opposite to the Davis-Greenstein predictions. This range is likely to vanish, however, in the presence of thermal wobbling of grains. In addition, we find that a substantial part of grains subjected to RATs gets aligned with low angular momentum, which testifies that most of the grains in diffuse interstellar medium do not rotate fast, that is, rotate with thermal or even subthermal velocities. This tendency of RATs to decrease grain angular velocity as a result of the RAT alignment decreases the degree of polarization, by decreasing the degree of internal alignment, that is, the alignment of angular momentum with the grain axes. For the radiation-dominated environments, we find that the alignment can take place on the time-scale much shorter than the time of gaseous damping of grain rotation. This effect makes grains a more reliable tracer of magnetic fields. In addition, we study a self-similar scaling of RATs as a function of λ/α eff . We show that the self-similarity is useful for studying grain alignment by a broad spectrum of radiation, that is, interstellar radiation field.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/7f8b600cce3a8e40c0f8a0ee6433fc1af2aadb6d

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Atomic and Molecular Physics, and Optics

Citations in the corpus

Planck intermediate results: XXXV. Probing the role of the magnetic field in the formation of structure in molecular clouds (2015, neutral): “A leading theory for the process of dust grain alignment involves radiative torques by the incident radiation (Lazarian & Hoang 2007; Hoang & Lazarian 2009; Andersson 2015).”
Radiative torque alignment: essential physical processes (2007, methodology): “We briefly described this in Lazarian (2007) and Lazarian & Hoang (2007a).”
The Efficiency of Grain Alignment in Dense Interstellar Clouds: a Reassessment of Constraints from Near-Infrared Polarization (2007, methodology): “A key point that arises from theory (Cho & Lazarian 2005; Lazarian & Hoang 2007) is that the efficiency of the RT mechanism increases systematically with grain size and is optimized for grains with dimensions comparable with the wavelength.”
Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust (2014, background): “…a consensus that the angular momentum of a grain spun up by photon-grain interactions (Dolginov & Mitrofanov 1976; Draine & Weingartner 1996, 1997; Lazarian & Hoang 2007; Hoang & Lazarian 2008) becomes aligned with the grain’s shortest axis, and then with the magnetic field via precession (e.g.,…”
AN IMPRINT OF MOLECULAR CLOUD MAGNETIZATION IN THE MORPHOLOGY OF THE DUST POLARIZED EMISSION (2013, background): “The mechanism which produces the alignment of the grains is still the subject of active research (Draine & Weingartner 1996, 1997; Weingartner & Draine 2003; Lazarian & Hoang 2007; Hoang & Lazarian 2009).”
Studies of Regular and Random Magnetic Fields in the ISM: Statistics of Polarization Vectors and the Chandrasekhar-Fermi Technique (2008, background): “It is currently believed that radiative torques play a major role on the alignment process and it strongly depends on AV (see Lazarian & Hoang 2007a).”