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This review of dust scattering determinations has concentrated on giving a critical examination of results presented in the literature for reflection nebulae, dark clouds, and the DGL. Unlike previous such reviews, each study was required to pass four simple criteria for inclusion in this review. There were a total of 23 studies passing the four criteria with 7 on reflection nebulae, 7 on dark clouds, and 9 on the DGL.

A great deal of progress has been made since the last review of this area (Witt 1989). The uncertainty in far-ultraviolet albedo from DGL studies has been resolved with the help of better observations and more sophisticated modeling. The 2175 Å bump has been shown to be an absorption feature; earlier indications of scattering in the bump have been traced to bad data. The measurements in reflection nebulae, dark clouds, and the DGL are roughly consistent alleviating earlier worries about the effects of the modeling assumptions specific to the object being studied (eg. reflection versus the DGL).

While much progress has been made in measuring dust scattering properties, there are a number of questions which are outstanding.

  1. Does the assumption of the Henyey-Greenstein single parameter scattering phase function significantly bias the resulting dust scattering parameters (a and g)? This question is probably best answered with models of images of reflection nebulae and dark clouds which include either more complicated analytical phase functions (eg., a double Henyey-Greenstein phase function; eq. 4 of Witt 1977) or scattering phase functions computed from dust grain models.
  2. What are the a and g values in the near-infrared? Currently, there is only a measurements for a single dark cloud and no g measurements. The a and g values in the near-infrared probe the larger dust grains, information about which is difficult to determine from extinction curve measurements alone.
  3. What are the a and g values in the red-optical and near-infrared for reflection nebulae? Is it even possible to determine a and g in reflection nebulae for these wavelengths? There are reflection nebulae in which ERE has not been detected (Witt & Boroson 1990). Are there nebulae without near-infrared, non-equilibrium emission?
  4. Is it possible to measure dust scattering properties in the red-optical and near-infrared from DGL measurements?
  5. What does it mean to measure dust scattering properties in objects without ERE or near-infrared, non-equilibrium emission if the diffuse ISM has been shown to have ERE (Gordon, Witt, & Friedmann 1998) and probably also has near-infrared, non-equilibrium emission.
  6. Is it possible to use the full multiwavelength appearance of reflection nebulae and dark clouds to alleviate lingering concerns about the geometrical assumptions inherit in modeling such objects? Specifically, the inclusion of ultraviolet through far-infrared would provide direct measurements of the direct, scattered, and re-emitted light and thus require many fewer assumptions.
  7. Is it feasible to consider using the full multiwavelength appearance of the DGL to reduce number of assumption necessary to model the DGL? Or is this still too complex of a problem, especially in light of the need for accurate 3D star positions of all stars important for the DGL (eg., hot stars in the ultraviolet and cooler stars at longer wavelengths)?
  8. What are the real differences between dust scattering properties in reflection nebulae, dark clouds, and the DGL? The current evidence indicates they are consistent with each other at the ~ 20% level.
  9. Are the dust scattering properties in other galaxies different than those derived for Milky Way dust? The Large and Small Magellanic Clouds offer environments which have lower metallicities and high star formation rates than the Milky Way. In addition, dust extinction curves in the Magellanic Clouds have been measured to be quite different from Milky Way extinction curves (Gordon et al. 2003).

Finally, the author would like to encourage readers who find studies which have not been included in this review to email them to the author (currently The author is committed to continuing to update the web-based version of this review 1.

1 currently at

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