M. G. Rawlings — Page 2

arXiv2004

ISO observations of 3 - 200 micron emission by three dust populations in an isolated local translucent cloud

We present ISOPHOT spectrophotometry of three positions within the isolated high latitude cirrus cloud G 300.2 - 16.8, spanning from the near- to far-infrared. The positions exhibit contrasting emission spectrum contributions from the UIBs, very small grains and large classical grains, and both semi-empirical and numerical models are presented. At all three positions, the UIB spectrum shapes are found to be similar, and the large grain emission may be fitted by an equilibrium temperature of ~17.5 K. The energy requirements of both the observed emission spectrum and optical scattered light are shown to be satisfied by the incident local ISRF. The FIR emissivity of dust in G 300.2 - 16.8 is found to be lower than in globules or dense clouds, and is even lower than model predictions for dust in the diffuse ISM. The results suggest physical differences in the ISM mixtures between positions within the cloud, possibly arising from grain coagulation processes.

astro-ph↗

arXiv2003

Infrared and Visual Interstellar Absorption Features toward Heavily- Reddened Field Stars

We present measurements of the 3.4-micron hydrocarbon dust absorption feature, and four visual diffuse interstellar bands, for twelve highly reddened (9.0<A(V)<15.8), early-type stars identified from the Stephenson (1992) catalogue, covering a range of Galactic longitude. The hydrocarbon feature is detected in eleven sightlines with optical depths of up to 0.05, and corresponding column densities are derived. The feature is weaker per unit A(V) than previously reported, further underlining earlier observations of its enhancement in the Galactic Centre. A comparison of the 3.4-micron feature substructure indicates organic material similar to that seen in earlier diffuse medium studies, suggesting a uniform formation history across the Galactic disc. The profile is well matched by both meteoritic material and several laboratory-prepared analogues. The diffuse interstellar bands (DIBs) measured were at 6177, 6203, 6284 and 6614 Angstroms; no strong correlations were detected, either between these bands themselves or between the DIBs and the 3.4-micron feature, although 6284 and 6614 appear to vary together. If the carriers of the DIBs are organic and molecular in nature and have cosmic carbon abundance requirements similar to those of the C-H stretch, these results imply that there is little direct carbon exchange between them and the aliphatic dust component. Measurements of the extinction to these objects suggest luminosity anomalies similar to that already seen in Cyg OB2 No. 12.

astro-ph↗