Astronomical instrumentation

Abstract:

We present near-infrared broad-band and H2 images of the quadrupolar source IRAS 04302+2247. High-resolution data at 3.8 μm show that the circumstellar envelope has a high degree of axisymmetry: the asymmetry at shorter wavelengths is almost certainly the result of extinction by dust in the cavity. Photometry of the flux peaks confirms that the source is variable but little change in appearance is observed between 1995 November and 1997 September. We report the first clear detection of the molecular outflow in H2 at 2.1 μm, revealing discrete streams which are inclined to the axis of the cavity. The motion of knots in the outflow remains the most likely cause of the variability.

Abstract:

We present near-infrared imaging polarimetry of three class I young stellar objects in the Taurus-Auriga dark cloud. We use Monte Carlo simulations to analyse the flux distributions and polarization patterns of these three sources and five others from an earlier paper. In addition, we present high-resolution polarimetry of HL Tau using the shift and add technique. Most young stellar objects in the sample display sharp, unresolved, peaks in the scattered light distribution. This is most simply explained by a strong concentration of matter in the centre, which we model by applying the ρ ∝ r-1.5 power law throughout the envelope. In terms of the Ulrich/Terebey, Shu and Cassen solution for the late stages of contraction of an initially spherical non-magnetic cloud, this corresponds to rc < 10 au. However, this almost spherically symmetric density distribution is inconsistent with observations of flattened, disc-like structures, so we conclude that this solution is not appropriate and different initial conditions apply. The multiple-scattering models with spherical grains do not reproduce some features of the observed polarization patterns, in particular the broad regions of aligned vectors seen in some sources. We interpret this as evidence for elongated aligned grains. The weak wavelength dependence of nebular morphology shows that the dust grains in circumstellar envelopes obey a much shallower extinction law than interstellar grains in the near-infrared, which we describe by the opacity ratio κ(J/K) = 1.8 ± 0.3, compared to the interstellar value of 3.25. We place an upper limit on albedo of ω < 0.6 from 1.25 to 2.2 μm and we find 0.1 < ω < 0.6 at 2.2 μm. Analysis of the quadrupolar source IRAS 04302+2247 indicates that the scattering function is not highly forward-throwing, which we express by the constraint g = 〈cos(θ)〉 < 0.4. With the addition of two more observables derived from the observed degrees of linear and circular polarization, we identify five empirical constraints on the properties of circumstellar dust. After calculating these observables for grain mixtures with a wide range of refractive indices and a variety of size distributions, we find that highly metallic spherical grains or highly elongated grains can satisfy these constraints and the size distribution extends to a ≳ 0.35μm. Amorphous carbon is the most plausible absorptive component, since graphite appears to be ruled out by its strong dispersion in the near-infrared.

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 an analysis of the redshift-space power spectrum, P(k), of rich clusters of galaxies based on an automated cluster catalogue selected from the APM Galaxy Survey. We find that P(k) can be approximated by a power law, P(k)∝kⁿ, with n ≈ -1.6 over the wavenumber range 0.04 < k < 0.1 h Mpc^-1. Over this range of wavenumbers, the APM cluster power spectrum has the same shape as the power spectra measured for optical and IRAS galaxies. This is consistent with a simple linear bias model in which different tracers have the same power spectrum as that of the mass distribution, but shifted in amplitude by a constant biasing factor. On larger scales, the power spectrum of APM clusters flattens and appears to turn over on a scale k ∼ 0.03 h Mpc^-1. We compare the power spectra estimated from simulated APM cluster catalogues with those estimated directly from cubical N-body simulation volumes, and find that the APM cluster survey should give reliable estimates of the true power spectrum at wavenumbers k ≳ 0.02 h Mpc^-1. These results suggest that the observed turnover in the power spectrum may be a real feature of the cluster distribution, and that we have detected the transition to a near-scale-invariant power spectrum implied by observations of anisotropies in the cosmic microwave background radiation. The scale of the turnover in the cluster power spectrum is in good agreement with the scale of the turnover observed in the power spectrum of APM galaxies.

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.