Characteristics of 2175 a Extinction

We propose to observe a number of lines of sight which appear to exhibit unusually weak 2175 A feature strengths as determined from ANS extinction excesses. Recent work has indicated that the width of the feature (fwhm; gamma) varies while the central position (lambda0) appears to stay fixed. These results suggest that if the feature is caused by graphite grains, they are very small (a < 50 A). The broadest observed features appear to be associated with dense, quiescent regions and are also largely represented by relative feature strengths that are weaker than average. We therefore feel that examining lines of sight with extremely weak feature strengths may lead to more extreme limits on gamma and lambda 0 Such results would certainly lead to tighter restrictions on the origin of the feature and seriously test the recent theoretical premise that the feature is the result of extinction by hydrogen-free carbon grains of the order a < 50 A.