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Abstract
Interstellar dust plays an important role in the physics of the
interstellar medium, as well as the formation and evolution of stars. The
presence of dust is often indicated in optical images by dark lanes which
bisect spiral galaxies, or seen directly as reflection nebulosity around
stars or emission nebulosity if sufficient heating is present. Of interest
in this thesis is the dust that is associated with bipolar outflow sources.
Bipolar outflows can occur in either evolved stars or in young stellar
objects, and are so named because they consist of two lobes which are
thought to be due to out-flowing dust and gas, with a dark lane between
them due to thick dust in a circumstellar disk or shell which often blocks
the light from the central star.
The spatial distribution of the properties of dust around bipolar outflow
sources has been examined using a combination of theoretical and
observational techniques. To aid the interpretation of observations of
bipolar outflow sources, we have modelled the wavelength dependence of
light from 0.36 to 22\um, scattered by dust particles with varying characteristics. The results were then presented in the form of colour excess ratios. These model ratios can be applied to observations if the contribution due to the central star is able to be removed, such that all that remains in the image is the effect of the dust particles.
The scattering of light by dust particles was modelled by varying six
different characteristics: grain material, size (particle radius from 0.002 to 0.75\um), mantle temperatures and thicknesses, shape, and orientation. Of those characteristics, the largest variation in the colour excess ratios resulted from varying grain composition and size. Different scattering angles also produce a noticeable variation in the colour excess ratios, however the effect is difficult to distinguish from the general extinction due to dust around the source. Water ice mantles were also found to significantly change the colour excess ratios. Grain shape and orientation produced only small variations in the
colour excess ratios.
Three bipolar outflow sources were studied as part of this thesis, two
evolved objects, OH~231.8+4.2 and Mz\,3, and the young T-Tauri object
Rno\,91. The observations involved multi-wavelength imaging in the
infrared, from which colours and colour excess ratios were obtained at
various points of the bipolar outflows and then compared to the predictions
made in the modelling.
The most extensive data set analysed was seven images of the object
OH~231.8+4.2 which were used in a multi-wavelength study in the infrared H
to N bands (1.25--12\um). The central source position of the object has
been confined to less than an arcsec using the longer wavelength images and
an L--M colour image. The two peaks which dominate the lobes in the shorter
wavelength images were found to be scattering peaks where the light from
the central source is scattered from the walls of the lobes. The spatial
distribution of water ice in the nebula has also been constrained to the
circumstellar disk which has a torus or disk shape rather than being a
spherical shell. The colour excess ratios derived for the nebula from the
images also suggest slightly different dust properties between the
circumstellar disk, lobe walls and within the lobe cavities.
The young T-Tauri star Rno\,91 also contains ice, and was observed between
J and L. The central star which illuminates the nebula was shown to be
coincident with the brightest point in these images. Using colour excess
ratio results for various parts of the nebula, it was shown that the dust
close to the central star is likely to contain larger grains than the
diffuse ISM, but with a similar composition. Moving away from the central
star, the dust becomes more like that observed in the diffuse ISM. The
presence of water ice on dust close to the central star was confirmed using
images centred in the ice band.
The protoplanetary bipolar outflow source Mz\,3 is slightly more evolved
than OH 231.8+4.2, and does not have evidence of any water ice in the
circumstellar disk. Images of this object were obtained between J and
10\um. The presence of warm dust throughout the inner bipolar lobes of this
object is noticeable by the brightness of the lobes in the image at 10\um.
Line profiles through the position of the central source of the 10\um\
image demonstrate that there is a circumstellar shell close to the central
source which has an inner radius of $\lta\,375$\,AU. Colour excess ratio
results for the bipolar lobes suggest that the dust associated with Mz\,3
is generally smaller than that found in the diffuse ISM. The properties of
the dust in the bipolar lobes were also observed to be different to the
dust closer to the central source and lying in the circumstellar disk. The
small sizes for dust in Mz\,3 is consistent with the high velocity outflows
that have been associated with the object.