10979(  1) - 04/03/06 11:20  - [  1]

                  HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 10979

Version: 1    Check-in Time: 03-Apr-2006 15:20:08

Title
Relating Extinction to Specific Grain Types in Individual Sight Lines
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Type             Cycle     Parallel Pointing Tolerance
AR               15        <none>
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Investigators
                                                                                     Contact?
    PI: Dr. Ulysses J. Sofia              Whitman College
   CoI: Dr. Michael J. Wolff              Space Science Institute                       N

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Abstract

We propose to use abundance constraints derived from archival GHRS and STIS data to
relate directly extinction to specific grain types. This will be the first attempt
to constrain dust models with extinction curves and full complements of important
abundances in individual sight lines. To this point, averaged interstellar
abundances have been used to constrain models of dust and their resulting extinction
effects. By using average abundances, any details concerning the relationship
between the grain composition and extinction is lost. The main reason that
individual sight lines have not been modeled in the past is the lack of an
appropriate data set, specifically sight lines where all of the most abundant
elements in dust, C, O, Si, Mg, and Fe, have been observed, and where extinction
curves are measured. We have identified seven sight lines that have recently met
those abundance and extinction criteria. For these sight lines we propose to
determine the abundances most important to dust composition. We will use the sight
line specific abundances to constrain model fits to the measured extinction toward
each star. The results of this study will help us to determine 1) the composition of
dust, 2) which grains types are responsible for extinction at various wavelengths,
3) the grain-size distribution of dust among sight lines, 4) the appropriate
abundance standard that represents the interstellar medium, and 5) how to estimate
more reliably reddening along sight lines.
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