Heavily-reddened lines of sight in the galaxy

This thesis presents an investigation into the reddening effects of the diffuse ISM using a subset of stars from the Stephenson (1992) objective prism survey. The survey contained approximately 400 stars, which were originally reported to be heavily-reddened, early-type objects. (B)VRJ photometry of 101 of the stars is presented. Intrinsic colours are inferred, based on the spectral types identified via subsequent optical spectroscopy. These are then used to obtain colour excesses and hence visual extinctions (Ar), using a standard diffuse interstellar reddening law (R= 3.05). Subsequent 6100 - 6900 A spectra of 193 of the Stephenson (1992) stars are then presented. 87% of the spectroscopic sample are K - M stars, 3% are S-type, and 8% are of genuinely early type. Visual extinctions of these fifteen early-type stars are found to lie between 8 and 18'". In addition, measurements of the strengths of four Diffuse Interstellar Bands (DIBs) towards these stars suggest a previously unseen correlation between fl6284 and 6614. II genuine, this correlation does not show a strong dependence on A. Spectroscopy at 3.4 pm of twelve of the genuinely early-type sources is also presented. Aliphatic C - H stretch absorption band strengths at 3.4 pm are listed for eleven of the sources, and found to be consistently weaker per unit extinction than previously measured (e.g. Sandford et al. 1995). This result supports earlier suggestions of 3.4 pm band enhancement towards the Galactic Centre. No global dependence on Galactic longitude is evident in the 3.4 pm band strengths towards the Stephenson stars. Profile fining of the hydrocarbon band towards these sources indicates some variation between lines of sight, but appears to preclude several previously proposed carriers (e.g. photolysed ice residues). No correlations are found between the hydrocarbon band strengths and those of the DIBs, suggesting that if the DIBs carriers are indeed carbonaceous, there is little direct interaction between them and the solid-phase, aliphatic material in dust grains