6,390 research outputs found
Non-isothermal X-ray Emitting Gas in Clusters of Galaxies
We have analyzed X-ray spectra from six galaxy clusters which contain cooling
flows: A85, A478, A1795, A2142, A2147, & A2199. The X-ray spectra were taken
with the HEAO1-A2 Medium and High Energy Detectors and the Einstein Solid State
Spectrometer. For each cluster, we simultaneously fit the spectra from these
three detectors with models incorporating one or more emission components
comprised of either thermal or cooling flow models. Five of the clusters (all
but A2142) are better fit by a multi-component model (a cooling flow plus one
or two thermal components or a two thermal component model) than by isothermal
models. In four of the clusters (A85, A1795, A2147, & A2199), we find evidence
for cool gas outside of the canonical cooling flow region. These latter four
clusters can be characterized by three temperature components: a temperature
inversion in the central region, a hotter region with an emission-weighted
temperature which is higher than that of an isothermal model fit to the entire
cluster, and a cooler region with an emission-weighted temperature of ~1 keV.
The cool component outside the cooling flow region has a large minimum emission
measure which we attribute, in part, to diffuse cool gas in the outer cluster
atmosphere. If at least some of the cool exterior gas is virialized, this would
imply a radially decreasing temperature profile. Together with the density
profiles we have found, this leads to a baryon fraction in gas which increases
with radius and is larger than that for an isothermal cluster atmosphere.
Consequently, if clusters of galaxies trace the mass distribution in the
Universe, the gas mass fraction we have calculated for an isothermal gas (which
is ~15%) together with the nominal galaxy contribution (~5%) gives a baryon
fraction of ~20%. Using the upper limit to the baryon density derived from BigComment: gzipped tar file of 26 PostScript pages, including 2 figures, 7
tables. Also available at
http://www.astr.ua.edu/preprints/white/INDEX_READ_ME_1st.htm
Towards a Molecular Inventory of Protostellar Discs
The chemical environment in circumstellar discs is a unique diagnostic of the
thermal, physical and chemical environment. In this paper we examine the
structure of star formation regions giving rise to low mass stars, and the
chemical environment inside them, and the circumstellar discs around the
developing stars.Comment: 9 page PDF, 550 kbyte
Birds of remnant vegetation on the Mornington Peninsula, Victoria, Australia: the role of interiors, edges and roadsides
Habitat loss and fragmentation on the Mornington Peninsula, Victoria, Australia, has resulted in a mosaic of forest patches, forest edges abutted by agricultural land and linear habitat strips amidst a human-modified land matrix. To examine the use of forest elements by the avifauna in this landscape, bird populations were sampled along fixed transects established within forest interiors, on forest edges and along forested roadsides. A total of 60 species was recorded during this study, five of which were introduced. Species richness and diversity did not differ significantly between the three habitat elements, but avifaunal composition varied considerably. The species assemblages of all habitat elements differed significantly, with forest interiors and roadsides showing the greatest difference and forest interiors and forest edges showing the least degree of difference. Forest-dependent bird species used both interiors and edges. Interiors differed from edges and roadsides in having lower abundances of open country species, predatory species and introduced species. A clear gradient of change in bird communities from forest interiors to roadside vegetation was observed. This study suggests that the interiors of medium-sized (<1 000 ha) patches may play an important role in conserving bird biodiversity on a local level as they provide refuge for forest-dependent native species in extensively cleared landscapes.<br /
Spectral analysis of boundary layers in Rayleigh-Benard convection
A combined experimental and numerical study of the boundary layer in a 4:1
aspect-ratio Rayleigh-B\'{e}nard cell over a four-decade range of Rayleigh
numbers has been undertaken aimed at gaining a better insight into the
character of the boundary layers. The experiments involved the simultaneous
Laser Doppler Anemometry (LDA) measurements of fluid velocity at two locations,
i.e. in the boundary layer and far away from it in the bulk, for Rayleigh
numbers varying between and . In parallel,
direct numerical simulations (DNS) have been performed for the same
configuration for Rayleigh numbers between and . The temperature and velocity probability density functions and the power
spectra of the horizontal velocity fluctuations measured in the boundary layer
and in the bulk flow are found to be practically identical. Except for the
smallest Rayleigh numbers, the spectra in the boundary layer and in the bulk
central region are continuous and have a wide range of active scales. This
indicates that both the bulk and the boundary layers are turbulent in the
number range considered. However, molecular effects can still be
observed and the boundary layer does not behave like a classical shear-driven
turbulent boundary layer.Comment: 10 pages, 6 figures, Accepted for publication in Phys. Rev.
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Intense molecular emission from the Lagoon nebula, M8
The discovery is reported of the second strongest source of mm and submm wavelength CO line emission, towards M8, the Lagoon Nebula in Sagittarius. The ~31 M⊙ molecular core has dimensions ~0.2 x 0.3pc and is centred on the O7V star Herschel 36 (H36), near the Hourglass Nebula in the core of M8. Emission from the CO line wings extends to the north and south of the Hourglass, although a lack of near-IR H2 emission indicates that outflow activity is much less prominent than in many active star-formation regions, and suggests that the CO line wings may trace the expanding edge of a cavity around H36. The molecular line data are compared with new near-IR narrow-band, continuum-subtracted images in He I, H2, and H,+ (Brγ) lines and archival HST emission-line images in Hα, [O III], and [S II]. The optical and near-IR data are found to be broadly consistent with previous photo-ionisation models of the Hourglass, which is excited by H 36. However, there are variations in the He I/Brγ line ratio which are difficult to explain
The FIRST-2MASS Red Quasar Survey
Combining radio observations with optical and infrared color selection --
demonstrated in our pilot study to be an efficient selection algorithm for
finding red quasars -- we have obtained optical and infrared spectroscopy for
120 objects in a complete sample of 156 candidates from a sky area of 2716
square degrees. Consistent with our initial results, we find our selection
criteria -- J-K>1.7, R-K>4.0 -- yield a ~50% success rate for discovering
quasars substantially redder than those found in optical surveys. Comparison
with UVX- and optical color-selected samples shows that >~ 10% of the quasars
are missed in a magnitude-limited survey. Simultaneous two-frequency radio
observations for part of the sample indicate that a synchrotron continuum
component is ruled out as a significant contributor to reddening the quasars'
spectra. We go on to estimate extinctions for our objects assuming their red
colors are caused by dust. Continuum fits and Balmer decrements suggest E(B-V)
values ranging from near zero to 2.5 magnitudes. Correcting the K-band
magnitudes for these extinctions, we find that for K <= 14.0, red quasars make
up between 25% and 60% of the underlying quasar population; owing to the
incompleteness of the 2MASS survey at fainter K-band magnitudes, we can only
set a lower limit to the radio-detected red quasar population of >20-30%.Comment: 80 pages (single-column, preprint format) 20 figures, Accepted for
publicated in Ap
Heavy Quark Fragmentation to Baryons Containing Two Heavy Quarks
We discuss the fragmentation of a heavy quark to a baryon containing two
heavy quarks of mass . In this limit the heavy quarks
first combine perturbatively into a compact diquark with a radius small
compared to , which interacts with the light hadronic
degrees of freedom exactly as does a heavy antiquark. The subsequent evolution
of this diquark to a baryon is identical to the fragmentation of a
heavy antiquark to a meson. We apply this analysis to the production of baryons
of the form , , and .Comment: 9 pages, 1 figure included, uses harvmac.tex and epsf.tex, UCSD/PTH
93-11, CALT-68-1868, SLAC-PUB-622
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