2. LITERATURE RESULTS

The literature was searched for determinations of dust scattering properties in objects possessing Milky Way interstellar dust. Astrophysical objects which satisfy this constraint are reflection nebulae, dark clouds, and the DGL.

For inclusion in this review, each study had to satisfy four criteria. These criteria are:

1. Determine albedo & g: This requires that specific values be quoted for the two quantities. Studies which were only sensitive to one of the two quantities were included as long as a realistic range of the other quantity was probed. Usually, the albedo (a) was the quantity determined with calculations performed for g values between zero and one. While this criteria means that the large number of early studies which only put limits on a and/or g are neglected, their limits are usually consistent with the values determined from later studies.
2. Determine uncertainties: The importance of this criteria cannot be understated. The uncertainties presented here are either the uncertainties quoted in the original paper or reflect the range of a & g values allowed by models presented in each study. This can mean that multiple models (with different assumptions) have been combined into a single measurement with larger uncertainties than a single model.
3. Refereed Publication: This ensures that the full details of the work have be presented. In addition to the usual journals (eg., ApJ, AJ, A&A, PASP, MNRAS, etc.), theses were counted as refereed publications.
4. Not be superseded by other work: A number of studies are superseded by newer work which usually used better radiative transfer models (especially the case for DGL studies). This usually results in significantly different determinations of a & g.

The studies satisfying the first three criteria are listed in Table 1, separated by astrophysical object studied. These tables give the study reference, a brief description, a model code, and whether the study satisfied the fourth criteria and, as such, was included in Figs. 1 - 4. Each model code is explained in Table 2 where the method of calculation, sources, and dust geometry are summarized. Ideally, the model used to interpret the observations of a specific object would include realistic illuminators (location and spectrum), a realistic dust distribution, and calculate the full multiple scattering of photons. This ideal is unlikely to be fully met by any specific model, but models which come closest have the best chance of producing good dust scattering properties. The original study reference should be consulted for the full details of each study.

The a and g values for the studies which satisfy all four criteria are plotted separate in Figs. 1 - 3 for reflection nebulae, dark clouds, and the DGL. In addition, predictions for different dust grain models are included in these figures (see Section 3.1. for model details). The results have been divided between object classes to highlight their differences. It would not be surprising to find real differences in the a and g values between object classes. For example, reflection nebulae and dark clouds possess, on average, larger dust grains than the DGL. This is seen from the different R_V values measured for these objects as R_V is a rough measure of the average grain size. Examining the results separately also allows for the impact of the different strengths and weaknesses of each object on the derived scattering parameters to be examined. The final reason to separate the results is to check if the assumptions in the modeling based on object type significantly affect the resulting scattering values. All of the a and g values have been plotted together in Fig. 4 to check the consistency of determinations between the three astrophysical object types.

Figure 1. Determinations of the albedo and g in reflection nebulae are plotted versus wavelength. In addition, predictions from dust grain models are plotted for comparison.

Figure 2. Determinations of the albedo and g in dark clouds are plotted versus wavelength. In addition, predictions from dust grain models are plotted for comparison.

Figure 3. Determinations of the albedo and g in the DGL are plotted versus wavelength. In addition, predictions from dust grain models are plotted for comparison.

Figure 4. The combined determinations of the albedo and g in reflection nebulae, dark clouds, and the DGL are plotted versus wavelength. The plot symbols are not identified and do not correspond to legends in previous figures, see Figs. 1 - 3 for information about specific studies. In addition, predictions from dust grain models are plotted for comparison.