Magnetic Fields in Massive Star-forming Regions (MagMaR). IX. Radiative Torque Alignment and Disruption in NGC 6334I

Abstract

Abstract Intense radiation from high-mass stars is expected to significantly affect dust grain alignment and evolution through radiative torques (RATs). We investigate this effect in a massive star-forming region, NGC 6334I, using 1.2 mm dust continuum polarization observations from the Atacama Large Millimeter/submillimeter Array. The polarization fraction spans from ≲1% to ∼10% and decreases with increasing column density, remaining below 2% in dense cores despite high temperatures (∼100 K), where efficient grain alignment by RATs is expected. We investigate how grain alignment, grain growth, grain disruption, B -field tangling, and local physical conditions affect the polarization properties of MM1, MM2, MM3, and their surroundings. Polarization-angle dispersion shows that B -field tangling contributes to depolarization at moderate densities but cannot fully explain the lowest-polarization fractions. Using RAT-based grain alignment and polarization modeling, we find that reduced alignment efficiency and high optical depth reproduce the low polarization in the densest regions. MM2 shows evidence of grain growth, with maximum grain sizes a max ∼ 0.35 – 1.0 μ m, while MM1 exhibits smaller values of ∼0.35–0.50 μ m. Accounting for optical depth increases the inferred grain sizes in MM1 to ∼1.0–2.0 μ m. Analytical estimates of RAT disruption from the intense outburst suggest that micron-sized grains in high-temperature, moderate-density regions can fragment into submicron grains. Alternatively, high optical depth may also explain the low polarization in the densest regions even in the presence of micron-sized grains. Incorporating the B -field inclination effect indicates a transition from predominantly plane-of-sky fields at low densities to more line-of-sight-aligned configurations at high densities.