New Model Explains Dust Grain Alignment Across Ultraviolet to Infrared Wavelengths
Researchers have developed a model combining two grain-alignment mechanisms — radiative torque (RAT) alignment and paramagnetic (DG) relaxation — to explain both standard and anomalous 'super-Serkowski' interstellar dust polarisation spectra across infrared to far-ultraviolet wavelengths. About 25% of observed polarisation spectra show excess polarisation at mid-UV wavelengths that the standard Serkowski relation cannot account for, a long-standing puzzle in interstellar dust physics. The model's ability to reproduce observations from infrared to far-UV makes it a potentially valuable tool for interpreting data from future UV spectrophotometry space missions.
Interstellar dust grains aligned by magnetic and radiation fields polarise starlight in a characteristic wavelength-dependent pattern described by the empirical Serkowski relation, but roughly 25% of observed sightlines show anomalous excess polarisation at mid-ultraviolet wavelengths — a phenomenon called 'super-Serkowski' polarisation. In a new study accepted to Astronomy & Astrophysics, researchers modelled four well-studied stars (HD 30614, HD 204827, HD 37903, and HD 161056) using a framework that combines radiative torque (RAT) alignment and paramagnetic (Davis-Greenstein, DG) relaxation alignment. For the two super-Serkowski cases (HD 30614 and HD 204827), the model shows that enhanced RAT alignment driven by radiation from a nearby B-type star at wavelengths below the Lyman limit (912 Å) can explain the UV excess, with DG alignment improving the fit at the shortest wavelengths. For the standard Serkowski cases (HD 37903 and HD 161056), RAT alignment alone under a typical interstellar radiation field in a cold neutral medium suffices, with DG alignment playing only a negligible role. The study demonstrates that a unified physical framework can reproduce polarisation spectra spanning infrared to far-UV, positioning it as a promising interpretive tool for upcoming UV-capable space observatories.
What's missing
The model is tested on only four sightlines; it remains unclear how well it generalises across the full diversity of interstellar environments and dust compositions.
What different sources said
- arXiv astro-phCenter
Grain Alignment and Dust Evolution Physics with Polarisation (GRADE-POL). II. On the physical basis of Serkowski and super-Serkowski polarisation spectra
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