Interstellar dust makes up about one percent of the visible mass of the Milky Way
and absorbs as much as half the ultraviolet light emitted by hot stars. How those
grains form, what they are made of, and how they evolve are still open problems.
Polarimetry is one of the few tools that can answer them directly.
Dust grains interact with the electric field of the light, absorbing and re-directing it.
How strongly depends on the size of the grain relative to the wavelength of the light. A
typical interstellar grain consists of a core of rocky material (silicates) or graphite,
surrounded by a mantle of ices. Typical grain sizes are 10−8 to
10−7 meters — a hundredth to a tenth of a micron, where a human hair is about
50 to 100 microns thick.
Carbon-rich grains (similar to soot) are diamagnetic and are not expected to align in most
environments, while silicate grains (similar to sand) do align. So the polarized signal and
the total extinction probe different populations. To use polarization to study the ISM we
need to understand why, where, and how the grains are — and are not — aligned.
Why the ultraviolet
The interstellar extinction curve shows that the grain size distribution follows a power law,
with the number density falling as n(a) ∝ a−3.5 — where
a is the effective grain radius — over a range of 0.01–0.3 μm in the diffuse
medium, with further growth in molecular clouds. That distribution is consistent with a
grain–grain collisional equilibrium after the supernova remnant phase. Because of the
wavelength dependence of small-grain scattering, understanding the grain population requires
observations from the UV to the infrared, with the small grains best studied in the ultraviolet
(λ < 3200 Å).
The gap
Twenty-eight sightlines in thirty years
The Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) flew on two NASA Space
Shuttle missions, ASTRO-1 and ASTRO-2, in 1990 and 1995, and the Hubble Faint Object
Spectrograph added a handful more. Between them they measured UV polarization along 28
lines of sight. No dedicated UV polarimetry mission has flown since.
Those few measurements produced two surprising results: enhanced UV polarization toward 25%
of the targets, and increased polarization in the 2175 Å extinction feature in 7% of them.
The sample was far too small — around 30 targets — to test grain alignment theory
conclusively. Paradigm-shifting developments in the theory and modeling of interstellar
grain alignment, together with comprehensive multi-band surveys for target selection, now
allow the origins and mechanisms of small-grain alignment to be tested properly.
UV (open circles) and optical (squares) data for HD 197770 reveal polarization in the 2175 Å
feature, as one of two known examples. The solid and dotted line shows the Serkowski fit, extrapolated into
the UV. The dashed curve adds a Drude function fit representing the 2175 Å feature
(Wolff et al.). Right: a model fit of the HD 197770 polarization curve, assuming a variably
aligned small-grain population for a > 0.005 μm and partially aligned PAHs. Further
observations are required to test these theoretical assumptions.
Credit: PUFFINS team.
Science objective 1
Super-Serkowski Polarization in the UV
Investigate the nature and extent of small grain alignment by measuring
SuSeP at wavelengths below 3200 Å.
Science objective 2
The 2175 Å feature
Determine the origin and prevalence of the polarization in the 2175 Å feature
Science objective 3
Survey of interstellar medium (ISM) UV polarization
Survey ISM UV polarization along a substantial number of lines of sight
with varying extinction, metallicity, depletion levels, and ISM and
radiation field characteristics.
A: Target and calibration star sample for PUFFINS are shown in galactic coordinates. The diamond
markers for the target candidates correspond to those stars that have been observed before by WUPPE.
B: Location/source of the target candidates chosen to address the three SOs
Credit: Anche et al. 2025, SPIE.
The targets
To achieve its science objectives, PUFFINS needs a substantial number of lines of sight
spanning a range of extinctions and distances, depletion levels, and ISM radiation field
characteristics. The target sample is a mixture of O, B and early-A spectral-type stars
with V magnitudes ranging from 2 to 11.
As the polarization observations are limited by the errors introduced because of instrumental effects,
regular calibrations are required to estimate the accurate value of the polarization. These calibrations
are performed by observing well-established targets from the polarized and unpolarized standard catalogs.
Importance of PUFFINS
Combined with ground-based optical spectropolarimetry carried out using SPOL and FiberPOL, the PUFFINS data give
a continuous polarization curve from the UV through the optical. The survey
also serves as a testbed for UV spectropolarimetry on the Habitable Worlds
Observatory, and helps characterise the Galactic dust foreground that limits
searches for primordial B-modes in the cosmic microwave background.
Papers and proceedings
Optical design and polarimetric performance of a SmallSat
UV polarimeter to study interstellar dust: PUFFINS
Anche et al., SPIE, 2025 · arXiv:2509.02994
