The Impact of WUPPE and the Future of Spectropolarimetry
Prior to Astro-1, WUPPE was the future of UV spectropolarimetry; there were no data and many questions. WUPPE was designed to measure all four Stokes parameters with a spectral resolution of the order of 4A in the region from 1400A to 3300A. The sensitivity for point sources is competitive with the HST Faint Object Spectrograph and of course for diffuse objects considerably more sensitive because of WUPPE's larger field of view. In-flight calibration yielded an instrumental linear polarization averaging 0.05% and observational agreement to about the same value for brighter stars.
Actually WUPPE observations started some two years before the launch of Astro-1 and continue to this time. The reason is that for many sources the time dependence is an important part of the diagnostic process. The WUPPE data obtained in space give one or a few snapshots of the UV behavior; which sheds light on the behavior shortward of the Balmer jump, provides data to separate intrinsic from interstellar polarization, measures polarization in the important UV resonance lines, and provides other information unique to the vacuum UV. WUPPE data obtained from the ground extends the spectral coverage and adds the important time development history to the study. At the Pine Bluff Observatory in Wisconsin, over 2000 hours of spectropolarimetry have been carried out in support of this program; and for those objects in the southern hemisphere we have received both observing time and collaborative support.
The division between UV and visual is, of course, mandated by atmospheric absorption and not by technical or scientific requirements. WUPPE was intended to provide astronomers with their first look at the polarized flux in the UV. It was, however, specifically designed to obtain high precision low resolution spectropolarimetry. The attempt to incorporate polarimetry within a more general purpose facility such as HST has been less successful for a host of reasons. Based on what we have learned, future investigations should have wider spectral coverage (i.e. visual through ultraviolet), IR and X-ray polarimetry will require separate instrumentation but might be bore sighted. The full data cube includes the time domain thus requiring an extended observational life time. The next step has in fact been taken and takes the form of a Wide-Field Imaging Survey Polarimeter (WISP) which was launched in a sounding rocket configuration in December 1994 and November 1995. This provides 15 arc sec images over a 1.7 x 5 degree field of view down to the UV sky limit primarily directed towards polarimetry maps of reflection nebulae, diffuse galactic light, supernova remnants, etc. Both the scattered intensity and the polarization are larger in the UV. High galactic latitude clouds that are too faint to see in the visual and too cool to observe in the IR may be easily observed in the UV. The extension of this concept is a small explorer class satellite that will add time studies and complete sky coverage to the menu.
Some of the objectives of future UV polarimetric measures in space are the following. A map of the sky can provide an integrated view of the magnetic field distribution and the correlation with jets and flows in evolving objects. The polarized flux images (these are polarization divided by total flux and show just the polarized radiation, i.e. just the scattered flux) taken at different epochs will result in maps of time varying and probably exotic objects. Of course polarization measures always result in photometry too. In regions like Lockman's window we can study the extragalactic background. UV polarization provides two important advantages, anmely reduced galactic starlight background (both intensity and polarization) and reduced interstellar polarization.
Beyond a survey, one would expect instruments designed to provide higher spatial and spectral resolution which would be applied to questions such as radiation driven flows in Herbig Haro objects. Which of the HH objects are forming stars which are holes in the dust? Polarization allows one to discriminate between scattering and other mechanisms and by covering the UV through visible it is possible to determine the nature of the scattering medium. Are arcs or rings, whether associated with supernovae or with galaxies, matter or light echoes? If light echoes, what is the scattering medium and is it behind or in front of the source? Polarization tells you about the geometry; the position angle tells you the direction to the source and the variation of the degree of polarization with wavelength and time allows you to determine the scattering angle and thus the distance. Usually when we carry out theoretical investigations we do not calculate the polarization characteristics of the observed radiation because such observations are infrequent and difficult but often polarization provides some important discriminant. For example, it is believed that in the strong magnetic fields of white dwarfs, cyclotron radiation is generated which if so should have a cutoff in the UV. Does it cutoff? What is the maximum field strength?
It may be possible to shed light on some of these questions with the Astro-2 data but in any event we have come a long way, from simply wondering what the ultraviolet polarized sky is like to asking detailed questions about specific mechanisms. This is one of the legacies of WUPPE; others include the accumulated space and ground based WUPPE data that is growing every day.
- A. D. Code