Prof Patrick Roche

Abstract:

We present 10 and 20-μm spectroscopic observations of a sample of F- and G-type carbon-rich post-AGB objects, three of which were known to exhibit the unidentified emission feature at 21 μm. We also find the F3I post-AGB object SAO 163075 to exhibit a (weak) 21-μm emission feature. We additionally obtained a 10-μm spectrum of IRAS 05341+0852, which has been reported to have a possible 21-μm emission band in its IRAS LRS spectrum, and obtained new 10 and 20-μm spectra of the carbon-rich bipolar post-AGB sources GL 2688 and GL 618, the extreme carbon star GL 3068, and the planetary nebulae IRAS 21282+5050 and He 2-447, in order to study the evolution of C-rich dust from the early post-AGB through to the planetary nebula (PN) phases. The 7.5-13-μm spectra of the 21-μm band objects exhibit broad plateau emission, shortwards of 9μm and from 10-13μm, superposed upon which, in addition to the well-known UIR-band at 11.3 μm, are several new features, at 7.9, 8.2, 10.6, 11.5 and 12.2 μm, differing from those observed in standard UIR-band spectra. An excellent match is found between the wavelengths of these new features and those of bands in the spectrum of chrysene (C18H12), one of the simplest PAH molecules. The absence of the new features in the spectra of earlier spectral-type post-AGB objects and PN is consistent with the expected complete dehydrogenation of any PAH molecules having less than 20-25 carbon atoms when exposed to the ultraviolet radiation fields of stars with spectral types earlier than F. Chrysene is not responsible for the 21-μm emission bands observed in the spectra of the cool post-AGB objects. Possible identifications for the 21-μm band in terms of highly hydrogenated 2-dimensional PAH molecules or 3-dimensional fulleranes (hydrogenated fullerenes) are discussed. The mid-infrared spectrum of GL 2688 is largely featureless, apart from two broad weak emission features between 9.5-10.5 and 10.5-12.2 μm. The profiles of the broad 10-13-μm absorption features in the spectra of the extreme carbon star GL 3068 and the C-rich bipolar post-AGB object GL 618 are compared. For GL 3068 the profile shape and the wavelength of peak absorption, near 11 μm, are consistent with absorption by silicon carbide particles. However, the absorption observed towards GL 618 is considerably broader and peaks at 12 μm. Its profile is very similar to that of the 10-13-μm emission plateau observed in the spectra of the 21-μm band objects, suggesting that it arises from absorption by a large column of highly-hydrogenated PAH-type species.

Abstract:

We present linear spectropolarimetry in the wavelength range 4.5-4.8 μm of the embedded infrared source W33A. Our observations show for the first time the presence of polarization associated with the CO and XCN ice features, demonstrating that the absorbers reside in or on aligned grains. Both narrow and broad components of the solid CO feature near 4.67 μm are polarized. The detection of polarization associated with the narrow CO component is particularly significant, as the ices responsible are thought to exist only in dense, cold regions of molecular clouds, where gas and grain temperatures are expected to be closely coupled and traditional grain alignment mechanisms should become ineffective. We discuss the significance of this result with regard to current grain alignment theories. Mechanisms in which grain rotational energy is enhanced by interaction with cosmic rays merit further investigation. © 1996. The American Astronomical Society. All rights reserved.

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.