Prof Patrick Roche

Abstract:

Spectra are presented of 21 planetary nebulae spanning the well-known, but still unidentified, narrow emission features at 3.2-3.6 μm. The equivalent width of the 3.29-μm emission band is strongly correlated with the gas-phase carbon-to-oxygen ratio, as expected for a band origin in carbon-rich grains or molecules. It displays an approximately linear dependence upon the C/O ratio, with a threshold near C/O ∼0.6. The emission band is present in 18 of the 21 nebulae, being absent in three of the six oxygen-rich objects and only weakly present in a fourth. The profile of the , 3.29-μm band is closely similar in all of the nebulae and distinct from the band profile seen in some stellar envelopes. Weaker emission features in the 3.4-3.5 μm region are detected in nine objects, and are prominent only in nitrogen-rich type I nebulae. The intensity of the 3.4-μm feature in these objects is correlated with that of the 3.29-μm band, while the ratio of the two bands is strongly correlated with the nitrogen abundance. Possible explanations for this behaviour are discussed.

Abstract:

We present observations of solid-state absorption features due to water ice, CO ice, and silicate dust in the spectrum of the bright infrared source (1RS 1) associated with the Herbig-Haro nebula HH 100 in the R Coronae Australis dark cloud. These absorptions are shown to arise predominantly in the molecular cloud rather than in circumstellar matter associated with the infrared source itself, which we deduce to be a premain-sequence star obscured by ∼25 mag of visual extinction. In common with other lines of sight, the spectra indicate the presence of distinct hydrogen-rich (polar) and hydrogen-poor (nonpolar) phases in the ice toward HH 100-IR. The nonpolar phase is dominated by CO. The strength of the solid CO feature at 4.67 μm suggests that as much as ∼40% of all CO in the line of sight may be in the solid phase. Our data show a lack of significant absorption at 4.62 μm that might arise in CN-bearing molecules formed by energetic processing of the mantles. A previous report of structure in the profile of the 3 μm water-ice feature in HH 100-IR that might be attributed to the N-H resonance in condensed ammonia at ∼2.96 μm is not confirmed. The abundance ratio NH3:H2O in the ices is found to be no more than ∼8%, which is probably insufficient to account for the 3.3-3.6 μm "ice-band wing" in terms of ammonium hydrate. However, we propose that the wing might be explained by hydrates containing other bases in addition to ammonia. Further progress is likely to require new laboratory data for appropriate ice mixtures. © 1996. The American Astronomical Sodely. All rights reserved.

Abstract:

L1551-IRS5 is a classic example of a low-mass young stellar object. We present near-infrared images with a FWHM of 0.3 arcsec obtained using the 'Shift and Add' technique. These observations are combined with existing multiwaveband data to constrain Monte Carlo models of the system in scattered light. The models demonstrate that the observed flux distribution arises not from a disc but from a circumstellar envelope with an evacuated bipolar cavity. The envelope has a mass of 0.1-0.3 M⊙ within a radius of 670 au, and shows no evidence of a significant population of large dust grains.

Abstract:

We present spectropolarimetry of the 3.1 μm water-ice feature in the Becklin-Neugebauer (BN) object in OMC-1, with spectral resolution and signal-to-noise ratios substantially improved over previous observations. The well-known increase in polarization within the ice feature is interpreted in terms of a model for aligned graphite and silicate grains with ice mantles. We identify polarization structure in the long-wavelength (3.3-3.6 μm) wing of the ice profile, including a feature at 3.47 μm which matches closely the spectroscopic feature discovered in several protostars and attributed to carbonaceous material with diamond-like structure. We also show, for the first time, the occurrence of a systematic variation in the position angle of polarization across the ice feature in BN, indicating systematic differences in the relative numbers of core-mantle and unmantled grains along the line of sight, and a twist in the magnetic field orientation. © 1996. The American Astronomical Society. All rights reserved.

Abstract:

Spectra at 5-5.6 μm are presented of three planetary nebulae with strong unidentified infrared (UIR) emission bands. All three objects display the 5.25-μm emission band which is attributed to a combination band arising from out-of-plane C-H bending modes. The profiles of the 5.25-μm bands are closely similar to those of the 3.4- and 11.25-μm emission features, confirming that the 5.25-μm feature is a member of the UIR-band family. The origin of this band and its relationship to the UIR bands in the 3.4-3.6 and 11-13 μm spectral regions are discussed. An interpretation of these features in terms of combinations and overtones of the C-H out-of-plane modes is suggested, which, if correct, leads to a simplification of the UIR-band spectrum and suggests that the species that give rise to the bands must have a high degree of symmetry.