22,543 research outputs found
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 K. This is consistent with the available (R-I) and (I-K)
colours. If the parallax measured by Hawkins et al is correct, then the
implication is that D04 has a radius of , 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. We suggest that an equally valid
interpretation of the data is that D04 is a VB8 analogue at a distance of
parsecs.Comment: to appear in MNRAS, pink pages; 6 pages with 1 jpg, 1 postscript
figur
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
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
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
- âŠ