Quantitative Theory of Grain Alignment: Probing Grain Environment and Grain Composition

While the problem of grain alignment was posed more than 60 years ago the quantitative model of grain alignment that can account for the observed polarization arising from aligned grains has been formulated only recently. The quantitative predictions of the radiative torque mechanism, which is currently accepted as the dominant mechanism of grain alignment, open avenues to tracing magnetic fields in various astrophysical environments, including diffuse and dense interstellar gas, molecular clouds, circumstellar environments, accretion disks, comet tails, Zodiacal dust etc. At the same time, measurements of the absolute value of polarization and its variations can, in addition, provide unique information about the dust composition and dust environment. In the review I describe the analytical model describing well radiative torques acting on irregular grains and discuss how the alignment induced by radiative torques varies in the presence of superparamagnetic inclusions and pinwheel torques, e.g. arising from the H2 formation over grain surface. I also describe observations that can establish whether grains are superparamagnetic and whether recoils from H2 formations are powerful enough to give rise to substantial uncompensated torques. Answering to these questions should allow for reliable modeling of astrophysical polarization with numerous important applications, from accounting for dust contribution in Cosmic Microwave Background polarization studies to obtaining magnetic field strength using Chandrasekhar-Fermi technique. 1. Attempts to Explain Alignment Polarization of starlight arising from aligned dust was discovered accidentally approximately 60 years ago (Hall 1949, Hiltner 1949). This gave rise to attempts to explain the alignment. In the years that followed various interactions from paramagnetic relaxation (Davis & Greenstein 1951) and streaming of grains (Gold 1951) to interaction with cosmic rays (Salpeter & Wickramasinghe 1969) and photons (Harwit 1970) have been explored. Many key insights into the dynamics of grains are associated with Lyman Spitzer and Edward Purcell who addressed the problem of grain alignment on a number of occasions. While we can refer the reader interested in the history of ideas on grain alignment to a review in Lazarian (2003), in this short publication we concentrate on the modern quantitative understanding of grain alignment. My communications with Lyman Spitzer revealed to me his vision of grain alignment being one of the fundamental problems of interstellar medium, which solution is required for getting the quantitative insight into the role of magnetic fields.