Radiative torques: analytical model and basic properties

Abstract

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/a eff . We show that the self-similarity is useful for studying grain alignment by a broad spectrum of radiation, that is, interstellar radiation field.

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

“…where QΓ is the RAT efficiency, which can be decomposed into three components Qe1, Qe2 and Qe3 in a scattering reference frame defined by unit vectorsê1,ê2,ê3 withê1 k,ê2 ⊥ê1 andê3 = e1 ×ê2 (Draine & Weingartner 1996;Lazarian & Hoang 2007a). In general, the magnitude of RAT efficiency QΓ depends on the radiation field, grain shape, size and its orientation relative to k. When the anisotropic direction of the radiation field is parallel to the axis of maximum moment of inertiaâ1, LH07 found that the magnitude of RAT efficiency can be approximated by a power-law as:…”

“…The quantitative study of RAT alignment was initiated in a series of papers, by our group, starting with Lazarian & Hoang (2007a) (henceforth LH07) where an analytical model of RAT alignment was introduced. The analytical model was the basis for further theoretical studies in (Lazarian & Hoang 2007b;, Hoang & Lazarian 2009bsee also reviews in Lazarian 2007;Lazarian, Andersson, & Hoang 2015;Andersson, Lazarian, & Vaillancourt 2015).…”

Angle-dependent radiative grain alignment

“…Radiative alignment torque (RAT) theory has matured over the last decade and is now providing a number of specific, testable, predictions (Lazarian & Hoang 2007). One such prediction, which is especially attractive for probing the validity of the theory, since it is unique to RAT theory, is that the alignment efficiency, but not the polarization angle, should vary with the angle between the magnetic field and the radiation field anisotropy (Ψ; see Lazarian & Hoang 2007).…”

“…Draine & Weingartner 1997;Lazarian & Hoang 2007). Based on early work by Dolginov & Mitrofanov (1976) they showed that an irregular grain with a net helicity will be spun up by the differential scattering of the left and right-hand circular components in an external light source.…”

“…One such prediction, which is especially attractive for probing the validity of the theory, since it is unique to RAT theory, is that the alignment efficiency, but not the polarization angle, should vary with the angle between the magnetic field and the radiation field anisotropy (Ψ; see Lazarian & Hoang 2007). One way to observationally probe this prediction is to find dust grains for which the dominant radiation source is a localized source (a star) and measure the alignment efficiency for background stars projected at different position angles around the source star.…”