Internal and external alignment of carbonaceous grains within the radiative torque paradigm

Abstract

We study the internal and external alignment of carbonaceous grains, including graphite and hydrogenated amorphous carbon (HAC), in the interstellar medium (ISM) within the RAdiative Torque (RAT) paradigm. For internal alignment (IA), we find that HAC grains having nuclear paramagnetism due to hydrogen protons can have efficient nuclear relaxation, whereas both HAC and graphite grains can have efficient inelastic relaxation for grains aligned both at low−J and high−J attractors. For external alignment, HAC and graphite grains can align with the radiation direction (k-RAT) at low−J attractors but cannot have stable alignment at high−J attractors due to the suppression of radiative precession. HAC also has slow Larmor precession compared to the randomization by gas collisions and cannot be aligned with the magnetic field (B-RAT). Small HAC grains of a < 0.05 µm drifting through the diffuse ISM can be weakly aligned along the induced electric field (E-RAT) at high−J attractors due to its fast precession. Paramagnetic relaxation by nuclear magnetism is found inefficient for HAC grains due to the rapid suppression of nuclear susceptibility when grains rotate at high−J attractors. We then study the alignment of carbon dust in the envelope of a typical C-rich Asymptotic Giant Branch star, IRC+10216. We find that grains aligned at low−J attractors can occur via k-RAT with the wrong IA in the inner region but via B-RAT in the outer region. However, grains aligned at high−J attractors have the right IA alignment via k-RAT due to efficient inelastic relaxation. The polarization pattern observed toward IRC+10216 by SOFIA/HAWC+ can reproduced when only grains at low−J attractors are present due to removal of grains at high−J attractors by the RAT disruption.