Interstellar Medium

Fitzpatrick delivered an invited review talk at the Astrophysics of Dust 2003 Symposium held in May 2003 at Estes Park, Colorado. The talk, entitled "Interstellar Extinction in the Milky Way Galaxy", reviewed the current state of our characterization of the wavelength dependence of absorption and scattering (i.e., ''extinction'') by interstellar dust grains. The ability of dust grains to transmit, redirect, and transmute electromagnetic radiation as it travels through interstellar space has a great impact on the physical processes occurring in interstellar space and on the resultant physical conditions there. Understanding the ambient radiation field in space requires a detailed knowledge of the extinction properties of the dust. In addition, the wavelength dependence of extinction provides important diagnostic information about the physical properties of the dust grains themselves, and often serves as a first test for possible grain models. Finally, and perhaps most far-reaching, interstellar extinction profoundly limits our ability to study the universe. It is certainly fair to say that many more astronomers care about extinction, than care about the dust grains that produce it! The primary goal of the talk was to review the evidence supporting the idea that Galactic extinction in the UV through IR region can be considered a 1-parameter family characterized by their value of R $_{\rm V}\equiv {\rm A_{V}}/E(B-V)$. Based on analysis of new (i.e., 2MASS) and old (i.e., IUE) data for $\sim 100$ sightlines, it was shown that the UV, optical, and IR wavelength regimes do display coherent variations, but with too much intrinsic scatter to be considered truly correlated. A 1-parameter family can be constructed which illustrates these broad trends, but very few individual sightlines are actually well-reproduced by such a family and disagreement with the mean trends is not a sufficient condition for considering a sightline to be ``peculiar.'' Only a very small number of extinction sightlines stand out as truly peculiar. It is likely that simple variations in the mean grain size from sightline to sightline are responsible for much of the coherent variability seen in Galactic extinction, and might also explain the ''peculiar'' extinction long-noted in the Magellanic Clouds. A paper based on this talk will appear in the Astrophysics of Dust 2003 conference proceedings, to be published in 2004.

Fitzpatrick and Derck Massa (SGT, Inc.) presented a poster paper at the Astrophysics of Dust 2003 Symposium, entitled ''Extinction Without Standards and UV Extinction Curves for Stars with Small Color Excesses'' The poster illustrated a new technique for deriving the wavelength dependence of interstellar extinction, utilizing stellar atmosphere models rather than unreddened ''standard stars'', the classical pair method technique. The benefits to this technique are numerous, including: 1) the elimination of the subjective process of determining the pair method standard; 2) a large reduction in mismatch error in the resultant extinction curves; 3) the ability to determine reliable extinction curves for lower E(B-V) sightlines than possible previously; and 4) the ability to determine reliable UV extinction curves for later type stars (i.e., as cool as late-B) than previously possible. The poster described the new analysis and provided examples of its application. Particularly important was the demonstration that the new technique greatly reduces the scatter among the extinction curves derived from lightly reddened, late-B stars in the young galactic cluster IC 4665. This result verified that the technique does indeed provide dramatic improvements over classical methods of analysis. Fitzpatrick and Massa are preparing a reanalysis of the entire UV extinction database, based on the new analysis.