A UNIFIED MODEL OF GRAIN ALIGNMENT: RADIATIVE ALIGNMENT OF INTERSTELLAR GRAINS WITH MAGNETIC INCLUSIONS

Journal Article · Thu Nov 10 00:00:00 EST 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/831/2/159· OSTI ID:22667225

Hoang, Thiem ^[1]; Lazarian, A. ^[2]

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1. Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8 (Canada)
2. Department of Astronomy, University of Wisconsin-Madison (United States)

The radiative torque (RAT) alignment of interstellar grains with ordinary paramagnetic susceptibilities has been supported by earlier studies. The alignment of such grains depends on the so-called RAT parameter q {sup max}, which is determined by the grain shape. In this paper, we elaborate on our model of RAT alignment for grains with enhanced magnetic susceptibility due to iron inclusions, such that RAT alignment is magnetically enhanced, which we term the MRAT mechanism. Such grains can be aligned with high angular momentum at the so-called high- J attractor points, achieving a high degree of alignment. Using our analytical model of RATs, we derive the critical value of the magnetic relaxation parameter δ {sub m} to produce high- J attractor points as functions of q {sup max} and the anisotropic radiation angle relative to the magnetic field ψ . We find that if about 10% of the total iron abundance present in silicate grains is forming iron clusters, this is sufficient to produce high- J attractor points for all reasonable values of q {sup max}. To calculate the degree of grain alignment, we carry out numerical simulations of MRAT alignment by including stochastic excitations from gas collisions and magnetic fluctuations. We show that large grains can achieve perfect alignment when the high- J attractor point is present, regardless of the values of q {sup max}. Our obtained results pave the way for the physical modeling of polarized thermal dust emission as well as magnetic dipole emission. We also find that millimeter-sized grains in accretion disks may be aligned with the magnetic field if they are incorporated with iron nanoparticles.

• OSTI ID: 22667225
• Journal Information: Astrophysical Journal, Vol. 831, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABUNDANCE
ACCRETION DISKS
ANGULAR MOMENTUM
COMPUTERIZED SIMULATION
COSMIC DUST
EMISSION
EXCITATION
FLUCTUATIONS
INCLUSIONS
INTERSTELLAR GRAINS
IRON
MAGNETIC DIPOLES
MAGNETIC FIELDS
MAGNETIC SUSCEPTIBILITY
PARAMAGNETISM
POLARIZATION
STOCHASTIC PROCESSES
UNIFIED MODEL