Interstellar Extinction in the Milky Way Galaxy

Astrophysics of Dust ASP Conference Series, Vol. 309, 2004 Adolf N. Witt, GeoffreyC.Clayton&Bruce T. Draine, eds. Interstellar Extinction in the Milky Way Galaxy Edward L. Fitzpatrick Villanova University, 800 Lancaster Avenue, Villanova, PA 19085 Abstract. Ireview the basic properties of interstellar extinction in the Milky Way galaxy, focusing primarily on the wavelength dependence within the IR through UV spectral region. My primary goal is to review the evidence supporting the idea that Galactic extinction curves can be considered a 1-parameter family characterized by their value of RV ≡ AV /E(B − V ). Based on analysis of new (i.e., 2MASS)andold (i.e., IUE)datafor∼100 sightlines, I show 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 “pe- culiarity.” 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 vari- ability seen in Galactic extinction, and might also explain the “peculiar” extinction long-noted in the Magellanic Clouds. 1. Introduction This conference is a testament to the wide ranging impact of dust grains on the properties of the interstellar medium and to the rich variety of research opportunities in the interstellar dust field. Among these studies, interstellar extinction — the absorption and scattering of light by grains — often plays a central role. The ability of grains to transmit, redirect, and transmute electro- magnetic energy is enormously important to the physics of interstellar space. The wavelength dependence of extinction provides important diagnostic infor- mation about the physical properties of grain populations, and often serves as a first test for possible grain models. 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! In recent history there have been a number of excellent review articles about, or including, the basic properties of interstellar extinction in the Milky Way galaxy (e.g., Savage & Mathis 1979; Massa & Savage 1989; Draine 1995; and Draine 2003). In addition, the book by Whittet (2003) provides a valuable general reference. Since the pace of research in the extinction field is such that the astronomical community is not really demanding a new review article every 33 34 E. L. Fitzpatrick year, I have taken a liberal approach to the notion of a “review.” In this paper, my aim is principally to review an idea, rather than a field. Theideaisthat —despitethe wide range of spatial variations present — the λ-dependence of Galactic extinction can be viewed as a smoothly varying 1-parameter family and that the normalcy of an extinction sightline can be judged by how well it fits into this family. In a sense, this article is a natural follow-up to that by Massa &Savage (1989) presented at the Santa Clara dust meeting. Massa & Savage gave a comprehensive review of the detailed λ-dependence of extinction and included a discussion of the then-recent results of Cardelli, Clayton, & Mathis (1988, 1989) who first showed a relationship between extinction properties in the IR/optical and in the UV. It is these results which have given rise to the 1-parameter family scenario. Ibeginthis article with a brief description of the basic properties and termi- nology of Galactic extinction (§2) and then give a short history of the key devel- opments which provide the primary background for this paper (§3). In §4, I will examine the evidence for a multi-wavelength connection between IR, optical, and UV extinction, i.e., the issue of a 1-parameter family, based on a new analysis of UV, optical, and IR data. The analysis incorporates a threefold increase over past studies in the number of sightlines for which IR and UV data are combined, and utilizes a new technique for deriving extinction curves which promises to al- low precise extinction studies to be extended to more lightly-reddened sightlines than previously possible. Finally, in §5Iwill revert to classical“review-mode” and note a number of recent programs and results which are helping to clarify our view of the λ-dependence of Galactic extinction and the relationship between extinction properties and interstellar environment. 2. The Extinction Basics While the ultimate interpretation of results from extinction studies may be very complex, the measurements themselves for Galactic (and many extra-Galactic) sightlines are, in principle, straightforward. The most commonly used technique forderiving the λ-dependence of extinction, the “pair method,” is illustrated in idealized form in Figure 1. Measuring the extinction produced by an interstellar cloud requires photometric or spectrophotometric observations of the spectral energy distribution (SED) of some astronomical object — usually a star —locatedbehind the cloud and identical measurements of an identical object located at an identical distance, but unaffected by interstellar dust. The total extinction in magnitudes at each wavelength Aλ (which is essentially a measure of the optical depth τλ)isthensimply computed from the ratios of the SEDs as shown in Figure 1. Note that the assumption implicit in this discussion is that the star is located far from the interstellar cloud (as indicated in Figure 1) and that no photons are scattered into the line of sight. Such an assumption is not necessarily valid in the case of extinction produced by circumstellar material. Because reddened/unreddened star pairs are nearly never at the same distance and, in fact, the stellar distances are usually poorly determined, the total extinction Aλ is rarely computed directly. Instead, the stellar SEDs are usually normalized by the flux in a common wavelength region before computing the extinction. The usual choice for this is the optical V band. This normalized ex- Milky Way Extinction 35 Figure 1. A schematic illustration of the “Pair Method,” the princi- pal technique used to study Milky Way extinction. 0 0 tinction Aλ−V (computed by substituting Fλ/FV and Fλ /FV into the equation in Figure 1) is also known as the color excess E(λ − V )andisrelated to the total extinction by E(λ − V )=Aλ − AV . To effectively compare the λ-dependence of extinction among sightlines with very different quantities of interstellar dust, the color excess E(λ − V )itself must be normalized by some factor related to the amount of dust sampled. The optical color excess E(B − V )isusually employed forthispurpose. This second normalization yields the most commonly found form for observed “extinction curves”, namely E(λ − V )/E(B − V ). This normalized extinction is related to the total extinction Aλ throughtherelationE(λ − V )/E(B − V )=Aλ/E(B − V )−AV /E(B−V ). The quantity AV /E(B−V ), i.e., the ratio of total extinction to color excess in the opticalregion,is usually denoted RV .Ifitsvaluecanbe determined for a line of sight, then the easily-measured normalized extinction can be converted into total extinction. It has been notedoftenthatE(B−V )isaless-than-ideal normalization factor. Certainly a physically unambiguous quantity, such as the dust mass column density, would be preferred, or even a measure of the total extinction at some particular wavelength, such as AV .However, the issue is simply measurability. We have no model-independent ways to assess dust mass and total extinction requires either that we have precise stellar distances or can measure the stellar SEDs in the far-IR where extinction is negligible. While IR photometry is now available for many stars through the 2MASS survey, the determination of total extinction from these data still requires assumptions about the λ-dependence of extinction longward of 2µmandcanbecompromised by emission or scattering by dust grains near the stars. In this paper, all the observed extinction curves will be presented in the standard form of E(λ − V )/E(B − V ). Only in the case 36 E. L. Fitzpatrick of model curves, and for illustrative purposes, will alternate normalizations be employed. The quality of a pair method extinction curve clearly depends on how well a reddened object can be matched with an unreddened comparison object. Massa et al. (1983) discuss the effects and magnitude of “mismatch errors” on extinction curves. In general, mismatch effects become less important as the amount of extinction increases, and strongly limit our ability to probe the λ-dependence of extinction in lightly reddened sightlines, such as through the nearby interstellar medium and the galactic halo. In §4, I will present results based on the substitu- tion of stellar atmosphere models for the unreddened member of a pair method star team. Along with a number of other advantages, this technique promises to greatly improve the accuracy of extinction curves derived for low-reddening sightlines. Severaldecades of work utilizing pair method extinction curves spanning the near-IR through UV spectral domain have provided a good estimate of the “average” λ-dependence of extinction in the Milky Way galaxy. (I will defer describing what I mean by “average” until §3below). Figure 2 shows the familiar shape of this “Average Galactic Extinction Curve”, plotted as E(λ − V )/E(B − V )vs. inverse wavelength. This type of presentation was clearly invented by UV astronomers to emphasize that spectral region, but also has the advantage of being essentially a plot of normalized extinction cross section vs.

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