Prof Patrick Roche

Abstract:

We report on advances in the study of the cores of NGC 6302 and 6537 using infrared grating and echelle spectroscopy. In NGC 6302, emission lines from species spanning a large range of ionization potential, and in particular [Si IX] 3.934 μm, are interpreted using photoionization models (including CLOUDY), which allow us to re-estimate the temperature of the central star to be about 250 000 K. All of the detected lines are consistent with this value, except for [Al V] and [Al VI]. Aluminium is found to be depleted to one hundredth of the solar abundance, which provides further evidence for some dust being mixed with the highly ionized gas (with photons harder than 154 eV). A similar depletion pattern is observed in NGC 6537. Echelle spectroscopy of IR coronal ions in NGC 6302 reveals a stratified structure in ionization potential, which confirms photoionization to be the dominant ionization mechanism. The lines are narrow (<22km s-1 FWHM), with no evidence of the broad wings found in optical lines from species with similar ionization potentials, such as [Ne V] 3426 Å. We note the absence of a hot bubble, or a wind-blown bipolar cavity filled with a hot plasma, at least on 1 arcsec and 10 km s-1 scales. The systemic heliocentric velocities for NGC 6302 and 6537, measured from the echelle spectra of IR recombination lines, are found to be -34.8±1 km s-1 and -17.8±3 km s-1. We also provide accurate new wavelengths for several of the infrared coronal lines observed with the echelle.

Abstract:

We present 8-13 μm spectropolarimetry of 55 sources and 16-22 μm spectropolarimetry of six of these. This represents a substantial fraction of star formation regions that can be observed in this way with current technology on 4-m class telescopes (i.e. brighter than about 20 Jy at 10 μm in a 4-arcsec beam). Most of the sources are embedded young stellar objects (YSOs), H II regions containing sites of star formation or bipolar protoplanetary nebulae (PPN), although a few other sources (e.g. NGC 1068, MWC 349) are also included. The majority have oxygen-rich chemistry but there are three carbon-rich sources. Many of the oxygen-rich sources show deep silicate absorption overlying featureless or optically thin silicate emission. Absorptive polarization with polarization per optical depth (pa/τ) ≃ 1-3 per cent is common, and many also show evidence for an emissive polarization component as well, although pure polarization in emission is rare. The observed ranges of pa/τ and pe are very similar, rather surprising in view of their origin from very different environments. Typically the absorptive polarization profiles are similar to the archetypal silicate polarization found in OMC1 BN, but an exception is AFGL 2591, which displays an additional narrow polarization feature at 11.2 μm, which has been attributed to annealed silicates. Many of the intensity absorption spectra also show an inflection near 11.2 μm, which might also be attributable to annealed silicates. The carbon-rich sources have nearly featureless polarizations in the 0.5-1 per cent range, which we ascribe to dichroism in carbon-based grains; this is the first evidence that such grains can be aligned. In two of these sources the polarization appears to be caused by absorption by SiC. A few of the polarization spectra have no straightforward interpretation.

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.