Far-infrared Polarization Properties of Nearby Star-forming Regions: A New Compendium of SOFIA/HAWC+ Observations

The gist The study presents a comprehensive polarimetric study of 26 nearby molecular clouds in four far-infrared bands using 52 archival SOFIA/HAWC+ datasets to probe the plane-of-sky magnetic field.

Observations and Data Compilation

The research utilizes 52 archival SOFIA/HAWC+ datasets covering 26 star-forming regions spanning distances from 130 to 2620 pc The compilation includes data at wavelengths of 70, 100, 160, and 250 µm from the Herschel Space Observatory to derive temperature and column density maps The study analyzes the data at common angular (25′′) and common physical (0.052 pc and 0.32 pc) resolutions to determine the effects of resolution and distance on polarization properties The data were reduced using the SOFIA Redux pipeline, resulting in maps of linear Stokes parameters (I, Q, and U) from which percent polarization (p) and polarization angle (ϕ) are computed.

Derived Properties

The researchers compute three derived quantities: column density, temperature, and polarization angular dispersion Column density is estimated by fitting a modified blackbody curve to the Herschel data to estimate the column density, dust temperature, and dust emissivity index The intensity is modeled as Iν = (1 − e −τ(ν))Bν(T), where τ (ν) is defined as τ ≡ ϵ(ν / ν0) β. The analysis involves fitting for a different β value for each region that is then used for every pixel in that region, and a Markov-Chain Monte Carlo (MCMC) method is used to estimate the uncertainty on β.

Percent Polarization Trends

The analysis of percent polarization (p) reveals scale-dependent trends across different wavelengths and resolutions In Bands A and C, there is generally a negative trend between p and distance, where the largest p are in the closest regions The distributions of p in the close regime do not change greatly when smoothed to a common physical resolution (0.052 pc) Conversely, the far regime data changes more when smoothed to a common physical resolution.

Polarization Spectra and Wavelength Dependence

The study investigates polarization spectra, which is the change in p with wavelength In the close regime, p is generally higher in Bands A and C, approximating a falling polarization spectrum (decreasing p with increasing wavelength), though the behavior is not strictly monotonic The behavior of the far regime is distinct, with a median p that is approximately flat or increasing with wavelength. The transition from a falling spectrum in the close regime to the flat spectrum in the far regime may be caused by the physical scales resolved by our common physical resolutions.

Magnetic Field Orientation and Depolarization

The analysis of magnetic field orientation shows no preferred alignment between the mean plane-of-sky magnetic field in our regions and larger-scale magnetic fields The average HAWC+ magnetic field angles reported in Figure 1 do not show a preferential alignment with ϕB = 90◦, nor any other orientation. The p − S correlation is uniformly negative, and more consistent across regions than the p − NH2 correlation This suggests that beam depolarization is generally a stronger source of depolarization than effects related to NH2.

Conclusion

The study concludes that the two shortest wavelengths of HAWC+ (53 µm and 89 µm) exhibit qualitatively distinct properties from the two longer wavelengths The falling spectrum in the close regime can be from environmental factors such as shielding of colder grains, or it can be from beam depolarization in these shortest wavelengths. The transition from a falling spectrum in the close regime to the flat spectrum in the far regime may be caused by the physical scales resolved by our common physical resolutions. The study illustrates that beam depolarization is generally a stronger source of depolarization than effects related to NH2. This work enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. The results illustrate the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. Future far-infrared instruments such as PRIMA (Glenn et al. 2025), with its instrument PRIMAger (Ciesla et al. 2025), will be an excellent tool for measuring dust polarization in molecular clouds, with similar resolution and higher sensitivity than HAWC+. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths. This work illustrates the importance of observing star-forming regions at both shorter wavelengths (≲100 µm) and longer wavelengths (≳ 100 µm) and resolving sub-parsec structures in the dust polarization in order to interpret the observations. The compilation enables statistical investigations of dust polarization properties in star-forming regions across far-infrared wavelengths.

1. The AI system can perform scale-dependent trend analysis by comparing polarization spectra at different physical resolutions, specifically noting that We find a 'falling' spectrum at the 0.052 pc resolution, but find a 'flat' spectrum at the 0.32 pc resolution, suggesting that resolution plays an important role in the observed polarization spectra.

2. The system can model and interpret magnetic field geometry by identifying regions where warm dust grain emission in small-scale structures (≲ 0.1 pc) traces different magnetic field geometries only resolved in our close regime data, enabling it to infer localized magnetic field structures beyond the resolution limits of large-scale datasets.

3. The AI system can differentiate between physical drivers of polarization spectra by comparing the Cohen’s d statistic results, noting that "most bins in both T and NH2 at all resolutions result in a d < 0.8, while highlighting that the transition from a falling spectrum in the close regime to the flat spectrum in the far regime may be caused by the physical scales resolved by our common physical resolutions."

4. The system can assess magnetic field orientation relative to Galactic structures, noting that The average magnetic field angles of our HAWC+ data reported in Figure 1 do not show a preferential alignment with ϕB = 90◦, indicating that the magnetic fields in the starforming regions we analyze are decoupled from the large-scale Galactic magnetic field.

5. The AI system can quantify the dominance of beam depolarization versus column density effects by analyzing the p-S correlation, finding that p shows a stronger negative correlation with S than NH2, meaning a significant amount of the depolarization is likely from beam depolarization.

We present a comprehensive polarimetric study of 26 nearby molecular clouds in four far-infrared bands (53 μ m to 214 μ m) using 52 archival SOFIA/HAWC+ datasets, over half of which are unpublished. Far-infrared dust polarization observations probe the plane-of-sky magnetic field. To investigate scale-dependent trends, we group the molecular clouds by distance and analyze the data at common angular (25'') and common physical (0.052 pc and 0.32 pc) resolutions. The two shorter wavelengths are more impacted by smoothing, exhibiting a larger decrease in percent polarization. We analyze the polarization spectrum --- the polarization fraction as a function of wavelength --- and find that it depends more strongly on column density than dust temperature. We find a ``falling" spectrum at the 0.052 pc resolution, but find a ``flat" spectrum at the 0.32 pc resolution, suggesting that resolution plays an important role in the observed polarization spectra. We propose that warm dust grain emission in small-scale structures (0.1 pc) traces different magnetic field geometries only resolved in our near regime data. There is no preferred magnetic field orientation across our data, which suggests that the magnetic field in our about parsec scale regions is decoupled from the large-scale field that is primarily parallel to the Galactic plane. The relationship between percent polarization and column density varies between clouds, but the correlation between percent polarization and polarization angle dispersion is consistent across regions. This compendium highlights the value of observing at multiple far-infrared wavelengths and enables additional population-level studies of magnetic fields and dust across star-forming environments.