In searching for the intrinsic polarization in the atmosphere of hot stars, Hiltner and Hall discovered that interstellar extinction is birefringent. Interstellar grains were elongated and evidently rather efficiently oriented by some mechanism that even today, some 45 years later, is not fully understood. The interstellar extinction increases greatly towards the UV, while the polarization falls off towards the UV. In addition to the far UV rise in extinction, a striking, order of magnitude, increase occurs centered at 2175A. Among the prominent questions prior to Astro were, does the UV polarization continue to decrease and if so, why? Does the polarization increase across the 2200A bump? The answer to the first question is yes, it continues to decrease. The answer to the second question is that, with the possible exception of two stars, there is no polarization signature corresponding to the extinction bump. This simple fact results in one very important conclusion, namely that the particles responsible for the 2200A extinction peak are different than those responsible for the general UV extinction. Those grain models that attempt to account for both are eliminated. The fact that along two lines of sight there may be a component of polarization attributable to the bump particles is interesting. This would mean that the bump particle can be non-spherical (perhaps a flake) and that these particles can be aligned.
Interstellar polarization is telling us about the alignment mechanism. The wavelength dependence of interstellar polarization peaks in the visual. The extremes of Lambdamax range from 3600A to 8900A with a mean of approximately 5500A. The small particles predominate shortward of Lambdamax and the larger particles become increasingly important as one moves longward of Lambdamax. WUPPE results showed that the UV polarization increased with decreasing Lambdamax. It is in fact possible to predict the UV polarization from Lambdamax and pmax, the value of polarization at Lambdamax.
One of the most interesting features of these discoveries is the fact that the wavelength dependence of interstellar polarization from the UV to the IR can be characterized by the single parameter Lambdamax. This is analogous to the finding that the wavelength dependence of extinction can be characterized by the ratio of selective-to-total extinction, Rv. There is a well-known relationship between Rv and Lambdamax, which means that Rv can be found from polarization measurements on the blue side of Lambdamax. This may prove to be a powerful technique for determining stellar distances within our galaxy. The uncertainty in extinction corrections can cause the largest error in distance determinations of distant stars. It is necessary to know the appropriate value of Rv. Rv is difficult to determine. It is found by determining a complete reddening curve extending as far into the infrared as possible, or in clusters with variable reddening from the slope of the apparent distance modulus vs. color excess. Neither technique is possible for single distant stars. It seems likely that a polarization index can be developed that will permit extinction corrections to be determined. This has some exciting ramifications for future developments in polarimetry.
The WUPPE results have important implications for alignment mechanisms; it indicates that the small particles are not efficiently aligned; whereas larger particles become nearly completely aligned. An interesting result is that if polarization magnitude divided by extinction is plotted vs. wavelength, a linear relation from 1300A to beyond 10000A except for a dip at 2200A is found. Does this mean that the major contributor to extinction over all this region are the same kind of particle modified only by the size distribution? There are currently active studies being carried out on alignment mechanisms and on grain models invoking the new results from WUPPE. These results strongly support the super paramagnetic inclusion model over other alignment theories and by using the discreet dipole approximation to study grain models, it appears that the results favor particles with voids over simple solid grains.
During the Astro-2 mission, WUPPE observed 20 new interstellar medium related sightlines and reobserved 6 objects. The data are currently being analyzed.
- A. D. Code