Characteristics of Weak 2175 a Extinction
Abstract
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 (lambda 0) appears to stay relatively fixed. These results suggest that if the feature is caused by graphite grains, they are very small (a < 50 A). In general, 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 y and lambda 0. Such results would certainly lead to tighter restrictions on the origin of the feature and seriously test the recent theoretical promise that the feature is the result of extinction by hydrogen-free carbon grains of the order a < 50 A. These studies may reveal lines of sight having extreme physical conditions which are ideally suited for studying dust-gas phase interactions and interstellar molecular processes. Such processes are fundamental to both the evolution of the interstellar medium and star formation.
- Publication:
-
IUE Proposal
- Pub Date:
- 1989
- Bibcode:
- 1989iue..prop.3388C