Smear fitting: a new deconvolution method for interferometric data

A new technique is presented for producing images from interferometric data. The method, ``smear fitting'', makes the constraints necessary for interferometric imaging double as a model, with uncertainties, of the sky brightness distribution. It does this by modelling the sky with a set of functions and then convolving each component with its own elliptical gaussian to account for the uncertainty in its shape and location that arises from noise. This yields much sharper resolution than CLEAN for significantly detected features, without sacrificing any sensitivity. Using appropriate functional forms for the components provides both a scientifically interesting model and imaging constraints that tend to be better than those used by traditional deconvolution methods. This allows it to avoid the most serious problems that limit the imaging quality of those methods. Comparisons of smear fitting to CLEAN and maximum entropy are given, using both real and simulated observations. It is also shown that the famous Rayleigh criterion (resolution = wavelength / baseline) is inappropriate for interferometers as it does not consider the reliability of the measurements.Comment: 16 pages, 38 figures (some have been lossily compressed for astro-ph). Uses the hyperref LaTeX package. Accepted for publication by the Monthly Notices of the Royal Astronomical Societ

Optical spectroscopy of a brown dwarf candidate

We have used the Low-Resolution Imaging Spectrograph on the Keck II telescope to observe the brown dwarf candidate D04 (Hawkins et al, 1998). The spectrum matches that of a spectral-type M7 dwarf, implying a photospheric temperature of $\approx 2600$K. This is consistent with the available (R-I)$_C$ and (I-K) colours. If the parallax measured by Hawkins et al is correct, then the implication is that D04 has a radius of $\sim 0.035 R_\odot$, or one-third that of Jupiter. This contradicts the predictions made by current stellar models that electron degeneracy leads to nearly constant radii for stars and brown dwarfs at masses below 0.1 M$_\odot$. We suggest that an equally valid interpretation of the data is that D04 is a VB8 analogue at a distance of $\approx 150$ parsecs.Comment: to appear in MNRAS, pink pages; 6 pages with 1 jpg, 1 postscript figur

The distance to NGC 6397 by M-subdwarf main-sequence fitting

Recent years have seen a substantial improvement both in photometry of low luminosity stars in globular clusters and in modelling the stellar atmospheres of late-type dwarfs. We build on these observational and theoretical advances in undertaking the first determination of the distance to a globular cluster by main-sequence fitting using stars on the lower main sequence. The calibrating stars are extreme M subdwarfs, as classified by Gizis (1997), with parallaxes measured to a precision of better than 10%. Matching against King et al's (1998) deep (V, (V-I)) photometry of NGC 6397, and adopting E_{B-V}=0.18 mag, we derive a true distance modulus of 12.13 +- 0.15 mag for the cluster. This compares with (m-M)_0=12.24 +- 0.1 derived through conventional main-sequence fitting in the (V, (B-V)) plane. Allowing for intrinsic differences due to chemical composition, we derive a relative distance modulus of delta (m-M)_0=2.58 mag between NGC 6397 and the fiducial metal-poor cluster M92. We extend this calibration to other metal-poor clusters, and examine the resulting RR Lyrae (M_V, [Fe/H]) relation.Comment: 19 pages, AASTeX, to appear in the December 1998 A

Spatial variability in the hyporheic zone refugium of temporary streams

A key ecological role hypothesized for the hyporheic zone is as a refugium that promotes survival of benthic invertebrates during adverse conditions in the surface stream. Many studies have investigated use of the hyporheic refugium during hydrological extremes (spates and streambed drying), and recent research has linked an increase in the abundance of benthic invertebrates within hyporheic sediments to increasing biotic interactions during flow recession in a temporary stream. This study examined spatial variability in the refugial capacity of the hyporheic zone in two groundwater-dominated streams which flow permanence varied over small areas. Two non- insect taxa, Gammarus pulex and Polycelis spp. Were common to both streams and were investigated in detail. Hydrological conditions in both streams comprised a four- month period of flow recession and low flows, accompanied by reductions in water depth and wetted width. Consequent declines in submerged benthic habitat availability were associated with increases in population densities of mobile benthic taxa, in particular G. pulex

Housing professionalism in the United Kingdom: the final curtain or a new age?

The unusually large, predominantly municipal, housing sector in the UK has provided the context for a large occupational grouping of "housing managers" that has claimed professional status. However, within the post-1945 British welfare state this professional project enjoyed limited success and social housing remained a fragile professional domain. This article explores the consequences for housing professionalism of the recent displacement of the bureau-professional "organisational settlement" by that characterising an emerging "managerial state". Managerialism constitutes a clear challenge to established forms of "professionalism", especially a weak profession such as housing management. However, professionalism is temporally and culturally plastic. Hence, the demands of managerialism, within the specific context of New Labour's quest for "community" cohesion, may be providing opportunities for a new urban network professionalism founded on claims to both generic and specific skills and also a knowledge base combining abstraction with local concreteness. The prominence in these networks of erstwhile "housing" practitioners may become the basis for a new, quite different, professional project. This argument is developed through both conceptual exploration and reference to empirical research. The latter involves reference to recent work by the authors on, first, the perception of housing employers of the changing nature and demands of "housing" work and its consequences for professionalism and, secondly, the professional project implications of the increasing prominence of neighbourhood management.</p

High-resolution Spectra of Very Low-Mass Stars

We present the results of high-resolution (1-0.4A) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I-K > 3.5 (MBol > 12.8) and those of more massive M-dwarfs (MBol = 7-10). A spectral atlas at high (0.4A) resolution for M8-M9+ stars is provided, and the equivalent widths of CsI, RbI and Halpha lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of ``inactive, rapid rotators''. Such objects seem to be common at and below the hydrogen burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism which heats the chromosphere, or the mechanism which generates magnetic fields, is greatly suppressed.Comment: 19 pages, 12 figure files. MNRAS style file. Accepted for publication in MNRAS, August 199