Prof Patrick Roche

Abstract:

We present imaging polarimetry of the central 30 arcsec of OMC-1 at 12.5 and 17 μm with arcsecond resolution, together with complementary spectropolarimetry in the ranges 8-13 and 16-22 μm at selected positions, and polarimetry at 800 μm over an approximately 1-arcmin field. The polarization is due to the dichroism of aligned grains in emission in the far-infrared, and predominantly due to absorption in the mid-infrared. The images reveal large variations of polarization fraction and position angle in BNKL, the central region, and these can explain the low fractional polarization observed when this region is unresolved, as in the far-infrared. The mid-infrared polarization indicates that a substantial component of magnetic field is aligned with the plane of the disc-like structures inferred from millimetre-wave studies, and suggests the presence of a toroidal field within the disc. © 1997 RAS.

Abstract:

We present near infrared observations of IRAS 04302+2247, a Class I protostar in the Taurus-Auriga molecular cloud complex whose equatorial plane is inclined precisely edge-on to the line of sight (i = 90 degrees +/- 4 degrees). This system displays a unique quadrupolar morphology, which had not been previously predicted in any simulations of a single protostar. We use the Monte Carlo method to tailor a model to our imaging and polarimetric data, and we explain the quadrupolar structure in terms of a dusty jet or outflow which lies perpendicular to the equatorial plane. We constrain the circumstellar structure to the form of an equatorially condensed envelope with a concave bipolar cavity. The circumstellar disk is not seen, which requires that it is physically thin. With its convenient orientation and proximity (d = 140pc) this system should yield many insights into the Class I phase of evolution.

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.