B-fields In STar-forming Region Observations (BISTRO): Population-Level Universality and Environmental Structuring of 450/850 $\mu$m Polarization Signatures

Abstract

We observed 69 molecular cloud regions at 450 and 850 $\mu$m with the POL-2 polarimeter on the James Clerk Maxwell Telescope and matched the maps to a common $12.6^{\prime \prime }$ beam. Polarization fractions are typically lower at 450 $\mu$m. The median of 30 measured ratios is $P_{450}/P_{850}=0.69\pm 0.02_{\mathrm{sys}}\pm 0.03_{\mathrm{stat}}$. Across 35 regions with reliable spectral indices of total ($\alpha$) and polarized ($\gamma$) intensity, the medians over sources are $⟨\alpha ⟩=3.33$ and $⟨\gamma ⟩=2.85$-$3.04$. The median $\gamma -\alpha$ is $-0.57$ to $-0.45$, with $\gamma <\alpha$ in 24-27 regions. These ranges span the two extreme placements of $3\sigma$ limits for polarized intensity below the noise floor. The position angles at the two wavelengths agree within $10^{\circ }$ in 10 of 13 regional medians, disfavoring a geometric origin that would rotate the angles. The medians of $\gamma -\alpha$ in 9 low-mass, 19 high-mass and 7 Central Molecular Zone (CMZ) regions are not ordered low-mass $<$ high-mass $<$ CMZ (Jonckheere-Terpstra $p=0.43$-$0.70$). A difference among classes is not robustly detected. The Kruskal-Wallis $p=0.056$-$0.078$ becomes $p=0.026$-$0.041$ when the Rosette Molecular Cloud alone is excluded, although the high-mass class has the lowest median in every treatment. The classes follow a distance sequence (the beam spans 0.008-0.51 pc), coupling environment and physical scale. Within clouds, the 450 $\mu$m polarization fraction decreases with Herschel column density in all 7 fields with sufficient dynamic range. Coefficients from pooled pixels mix trends within clouds, differences between clouds, and the influence of OMC 1. They therefore do not define a single law for individual pixels. The wavelength dependence across sources provides a benchmark for grain alignment. We cannot order it by environment and do not propose a unique mechanism.

arXiv comment161 pages, 88 figures, 21 tables. Submitted to PASJ. Paper S2 of a series of three (Papers T, S1, and S2); Papers T and S1 are arXiv:2610.29273 and arXiv:2610.29285. The submitted versions of all three papers are available at https://github.com/450um1G/PASJ2026