4. SUMMARY AND FUTURE
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.
- 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.
- 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.
- 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?
- Is it possible to measure dust scattering properties in the
red-optical and near-infrared from DGL measurements?
- 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.
- 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.
- 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)?
- 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.
- 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
kgordon@as.arizona.edu).
The author is committed to
continuing to update the web-based version of this review
1.
1 currently at
http://dirty.as.arizona.edu/~kgordon/Dust/Scat_Param/scat_data.html