Polarimetry in the Ultraviolet to Find Features in INterStellar dust

PUFFINS is a Pioneer-class, NASA, mission that will probe the ultraviolet (UV) polarization due to aligned dust grains in the interstellar medium. PUFFINS is a spectro-polarimeter and will cover the wavelength range ~1800 – 3000 Å at a resolving power of R~100.

Nominal launch in April 2030.

The mission Principal Investigator (PI) is Dr. Ramya Anche at the University of Arizona, and I am the Science-PI.

UV polarimetry of the ISM has been very poorly explored so far, primarily through the two flights of the Wisconsin Ultraviolet Photo-Polarization Experiment (WUPPE), on the Astro-1 and Astro-2 missions on the space shuttle in the early 1990ies. In addition, a few additional ISM lines of sight were observed with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) at about the same time.

A total of only 30 stars (and the ISM in front of them) have been probed. With PUFFINS we aim to (at least) triple this number. Not only is the WUPPE/FOS sample too small to allow a full understanding of the polarization effects in the ISM, but with the development of the “RAT alignment” paradigm, we now have a much better basis on which to define the observation plan, including hypothesis testing of what regions will best allow us to understand the physics of the small grains.

The two main questions raised by the WUPPE/FOS results are the origins and interpretation of the UV polarimetry are:

• Super-Serkowski UV Polarization
  For 22 of the 30 stars observed by WUPPE/FOS, if you fit a “Serkowski curve” to the optical data and extrapolate to the ultraviolet, the fitted curve and the observations agree. For the remaining 8, the UV polarization is significantly larger than would be predicted by the optical data. Why is that?
• Polarized 2175 Å extinction feature
  The well-known, but still somewhat mysterious enhancement in the interstellar extinction around 2175Å, does not show polarization in 28 of the 30 lines of sight studied by WUPPE/FOS. In 2 of them the feature is clearly polarized. What causes this dichotomy?
  
  PUFFINS will address the these questions, by observing carefully selected targets.
  Because of the very small sample probed by WUPPEE/FOS, and limited source selection, we expect that we will find additional effects and learn a lot about how the small dust behaves, and affects, the interstellar medium.

This page is under development… More soon!