Prof Patrick Roche

Abstract:

Thermal emission from magnetically aligned dust grains produces the observed mid-infrared polarization in the northern arm and east - west bar of SgrA West; recent arcsecond-resolution imaging polarimetry at 12.5 μm of these ionized filaments is presented, which confirms and extends previous studies. A lower limit ∼2 mG is found for the magnetic field in the northern arm and the IRS16 complex appears to be displaced from the northern arm by ∼ 0.15 pc along the line of sight. It is shown that the physical conditions in the ionized filaments of the central parsec lead to a very uniform grain alignment that is directed along the local magnetic field. The position angle of polarized emission will then be at right angles to the projection of the field direction on the plane of the sky and its amplitude a measure of the component of field along the line of sight; this makes possible a partial reconstruction of the field in three dimensions. We present the first application of the use of polarimetry in this way. This partial reconstruction is compared with the H92α observations of Roberts et al. and the implications are that the northern arm and east - west bar do not define either an orbital path or a spiral arm but rather represent a tidally stretched structure in free fall about SgrA(Black star) with significant deviations from a single plane, and most likely represent the inner ionized rim of a more extended neutral cloud.

Abstract:

We present high spatial resolution (∼0.8 arcsec) diffraction-limited 12.8-μm Ne II fine-structure emission line and 12.5-μm continuum images of the bright southern compact H II region G333.6-0.2, taken with the mid-infrared imaging polarimeter NIMPOL. The two images show remarkably similar, compact, yet asymmetric, flux distributions. The [Ne II] image shows a complex structure near the ionizing source(s) which we interpret in terms of the ionization structure of the H II region. It is found that G333.6-0.2 is more likely to be excited by a cluster of O and B stars than by a single star.

Abstract:

We present high-resolution, near-infrared images of a sample of five low-mass young stellar objects in the Taurus-Auriga dark cloud, mostly observed with the 'shift and add' technique, together with deep polarimetric images. These systems are at the 'Class I' phase of evolution, prior to the T Tauri phase. Monte Carlo models tailored to the individual sources support the view that the flux distribution of the circumstellar nebulosity arises from an envelope with a bipolar cavity, but show that dust in outflows can also be prominent in the near-infrared. The source IRAS 04302 + 2247 is analysed in detail, since it displays a unique quadrupolar morphology, which is explained in terms of a system oriented precisely edge-on with a perpendicular outflow. The masses of the envelopes derived from the models are in good agreement with estimates obtained from submillimetre thermal fluxes and estimates derived from CS(J = 2 - 1) emission. Constraints are placed on envelope and cavity structure, indicating higher density near the equatorial plane, smooth, continuous density profiles and physically thin inner discs. Three of the five systems show evidence for two bipolar cavity/outflow systems, perhaps indicating that these are close binaries. The very high (up to 80 per cent) degrees of polarization observed in the envelopes show that the scattering is dominated by small, interstellar-type dust grains, and the morphologies and high polarizations of the cores show that the protostars themselves are usually obscured at this phase of evolution, even at 3.7 μm. © 1997 RAS.

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.