The measurement of polarization provides a powerful diagnostic tool for the study of stars, interstellar matter and extragalactic objects. It yields information on the geometric structure of evolving astronomical sources that can not be obtained in any other way. Polarimetry adds another dimension to the traditional techniques of spectroscopy and imaging. We describe some of these unique features of polarization measurements, why measurements in the ultraviolet as well as the visual are required and why WUPPE was designed to pioneer these adventures.
Traditional astronomy, the study of the thermal universe, is characterized by steady state configurations in hydrostatic and local thermodynamic equilibrium. Modern observations have, however, revealed a dynamic evolving universe often accompanied by violent events. This thermal universe is essentially unpolarized, the dynamic universe is polarized.
The light from astronomical sources becomes polarized as a result of a departure from spherical symmetry (i.e. if a direction is assigned by rotation, a magnetic field or a binary orbit). The source may be intrinsically polarized as in the case of beamed emission in strong magnetic fields, synchrotron emission or plasma oscillations; or the radiation may be polarized by scattering from clouds, jets, disks or blobs. The polarization properties can also be modified by the intervening interstellar medium. Thus measurements of the polarization provide unique information on the nature of astronomical sources.
There are numerous examples of the great value of determining the polarization as well as the intensity and time dependence of the observed radiation. For example, in extragalactic astronomy, the discovery and delineation of the important BL Lac object class is largely due to polarimetric measurements and the relationship among different classes of active galactic nuclei is becoming clarified due to the geometric models based upon polarization data. In stellar astronomy the geometry and dynamics of stellar winds, disks and jets revealed by polarimetry have yielded unique insight into mass loss processes and hence stellar evolution and enrichment of the interstellar medium. Polarization measurements of spectroscopic binaries permit the determination of the orbital inclination and hence masses, while the discovery of magnetic white dwarfs and other stellar magnetic fields have been found from polarimetric observations. In the study of the interstellar medium polarization maps have shown the detailed small scale structure of galactic magnetic fields and have provided important constraints on the composition and structure of interstellar grains. Polarization provides the additional information necessary to determine the albedo of dust grains, a parameter vital to the discussion of star formation.
Despite the obvious value of fully characterizing the radiation received from celestial sources, the application of polarimetric data is still rare. There are several reasons for this. Polarimetry is more difficult; since often the polarization signature is less than 1%, the signal-to-noise ratio needs to be 10 times or more greater than that required for spectroscopy. This in turn means that exposure times need to be 100 to 1000 times as long. It also means that the instrumental polarization and instrumental stability must be at least in the 0.1 - 0.05% range. another difficulty is that the polarimetric backgrounds in the visible due to light pollution, zodiacal light, integrated star background, and interstellar polarization greatly complicates the observations and analysis. Finally, polarization is often accompanied by variability, a common feature of non-thermal states, and synoptic observations are required to fully exploit the diagnostic nature of polarization. For these reasons the data base is relatively sparse and until the observations of WUPPE during the Astro-1 mission, ultraviolet spectropolarimetry was nonexistant.
There are two main scientific drivers for carrying out spectropolarimetric observations in the ultraviolet. The first is that for many sources observed from the ground, the polarization continued to increase towrds the ultraviolet atmospheric limit. Does it continue to increase? Competing theories make differing predictions with respect to the behavior in the vacuum UV, and polarization can be the decisive desiderata. Another reason for observing in the ultraviolet is that the polarized background effects that limit visual light polarimetry all decrease significantly in the UV.
- A. D. Code