Prof Patrick Roche

Abstract:

We present ISOSWS, ISOPHOT_S and 8-13 μm observations of Centaurus A which show prominent PAH and silicate features. These and other data are used to construct a model for the infrared continuum. We find that, in a small nuclear aperture (∼4 arcsec, ∼60 pc), the spectral energy distribution is characteristic of emission from a starburst and dusty AGN torus; in larger apertures, additional components of cirrus and starburst emission are required. The model components are based on the radiative transfer models of Efstathiou et al. which include multiple scattering and the radiative effects of a dust-embedded source with a distribution of grain species and sizes. The torus component is modelled in terms of a tapered dusty disc centrally illuminated by a quasar-like source. The cirrus and starburst components are, respectively, modelled in terms of diffuse dust illuminated by the interstellar medium and an ensemble of optically thick molecular clouds centrally illuminated by hot stars. These latter components additionally include emission from small graphite particles and PAHs. Based on our overall model, the torus diameter is estimated to be 3.6pc and the best inclination angle of the torus is 45°. We present independent observational evidence for this structure. This result has implications for the detectability of tori in low-power AGN and for the use of the IRAS 60/25-μm flux ratio as an indicator of the torus inclination.

Abstract:

We present mid-infrared images and an 8-13 μm spectrum of the southern H II region RCW 38. We determine the dust colour temperature from both our spectrum and images at 10 and 20 μm, and deduce the gas excitation from an image in the [S IV] fine-structure line, as well as spectra of the [Ar III], [S IV] and [Ne II] fine-structure lines. Our observations are consistent with a complex of sources associated with the RCW 38 IRS1 region, which represent knots of material in a shell, or ridge, surrounding a cavity of about 0.1 pc in radius, which is itself created by the stellar wind of the hot young source IRS2. The dust temperature does not peak closest to IRS2, but rather along the centre of the ridge, and is remarkably uniform over the extent of our image. From photoionization models for the observed line ratios at IRS1 we deduce a stellar effective temperature and gas density of about 43 000-48 000 K and 104 cm-3 respectively. Whilst the star, or star cluster, IRS2 is ultimately responsible for the observed thermal and ionic emission, the relatively uniform dust temperature implies that the bulk of the dust heating in the region is provided by resonantly trapped Lyman α photons, rather than direct stellar photons. This then also implies that the dust is depleted with respect to the gas by a factor of at least 100 from its normal interstellar value. The small-scale spatial variations in the continuum emission and temperature can be explained by changes in the density and/or gas-to-dust mass ratio.

Abstract:

Spectropolarimetry of the 3.4 μm aliphatic C-H stretch feature, generally attributed to carbonaceous dust in the diffuse interstellar medium, has been carried out in the line of sight from the Galactic center source Sagittarius A IRS 7. The feature is unpolarized (Δp/Δτ < 0.2): the upper limit for polarization is well below that expected on the basis of a model in which the carrier molecules are associated with the aligned silicate component of interstellar dust, for example, as an organic or carbonaceous mantle on a silicate core. The simplest explanation is that the 3.4 μm carrier resides in a population of small, non-polarizing carbonaceous grains, physically separate from the silicates and sharing many characteristics with the carriers of the 217.5 nm extinction bump.

Abstract:

The properties of the carbon and oxygen chemical balance in planetary nebulae (PNe) are analysed through mid infrared spectroscopy of warm dust emission features in a sample of 72 objects. The adoption of a statistical distance scale shows that the galactic disk distribution of warm dust types in PNe is rather homogeneous with height over the galactic plane, and that there is a trend for the proportion of PNe with O rich dust signatures to decrease with galactocentric radius. Models of the galactic distribution of PNe compositions require a minimum progenitor mass of 1.2 M., although the observational constraints suffer from the smallness of the sample. This initial investigation is however an incentive to pursue the use of warm dust emission in PNe to study their progenitor population in various galactic environments.