B-fields In STar-forming Region Observations (BISTRO): Dual-Wavelength Constraints on Size-Dependent Grain Alignment in Star-Forming Regions

Abstract

We compared 450 and 850 $\mu$m POL-2 observations from the James Clerk Maxwell Telescope at $12.6^{\prime \prime }$ resolution. The median absolute difference in polarization angle between bands is $6.7^{\circ }$ across 21 regions. For total intensity $I\propto {\nu }^{\alpha }$ and polarized intensity $PI\propto {\nu }^{\gamma }$, 24-27 of 35 regions have $\gamma <\alpha$, allowing for $3\sigma$ limits on weak polarized intensities. The median $\gamma -\alpha$ is $-0.51$, ranging from $-0.57$ to $-0.45$ under these limits. The ratio of polarization fractions is $P_{450}/P_{850}=0.69\pm 0.02_{\mathrm{sys}}\pm 0.03_{\mathrm{stat}}$, with no detected dependence on the angle difference. We suggest that the dependence of alignment on grain size offers the simplest explanation of these findings. For optically thin components sharing a polarization angle and opacity law, temperature mixing reproduces the median $-0.51$ only if the warm dust is polarized less efficiently than the cold dust. If the cold dust is at 10 K or warmer and the warm dust at 40 K or cooler, the median requires a ratio of warm to cold effective polarization efficiencies $r\lesssim 0.18$. We tested mixtures with opacity index $\beta =2$ in four fields against Herschel dust temperatures and Green Bank Ammonia Survey kinetic temperatures. We assumed equal gas and dust temperatures, ammonia tracing cold gas, and model ranges of 3-20 K for cold dust and 25-40 K for warm dust carrying at most half the mass. Mixtures matching both temperatures cannot reproduce $\gamma -\alpha$ within $1\sigma$ in three fields, including OMC 2 tentatively, even for $r=0$. Allowing warm dust at 10-100 K and up to 95% of the mass permits solutions only for $r\le 0.041$. To explain these three fields by temperature mixing, we need wider model ranges and a very strong contrast in polarization efficiency. The origin of this contrast remains undetermined.

arXiv comment45 pages, 13 figures, 7 tables. Submitted to PASJ. Paper S1 of a series of three (Papers T, S1, and S2); Paper T is posted at the same time, and Paper S2 will be posted in November 2026. The submitted versions of all three papers are available at https://github.com/450um1G/PASJ2026