207 research outputs found
On gigahertz spectral turnovers in pulsars
Pulsars are known to emit non-thermal radio emission that is generally a
power-law function of frequency. In some cases, a turnover is seen at
frequencies around 100~MHz. Kijak et al. have reported the presence of a new
class of ''Gigahertz Peaked Spectrum'' (GPS) pulsars that show spectral
turnovers at frequencies around 1 GHz. We apply a model based on free-free
thermal absorption to explain these turnovers in terms of surrounding material
such as the dense environments found in HII regions, Pulsar Wind Nebulae
(PWNe), or in cold, partially ionized molecular clouds. We show that the
turnover frequency depends on the electron temperature of the environment close
to the pulsar, as well as the emission measure along the line of sight. We
fitted this model to the radio fluxes of known GPS pulsars and show that it can
replicate the GHz turnover. From the thermal absorption model, we demonstrate
that normal pulsars would exhibit a GPS-like behaviour if they were in a dense
environment. We discuss the application of this model in the context of
determining the population of neutron stars within the central parsec of the
Galaxy. We show that a non-negligible fraction of this population might exhibit
high-frequency spectral turnovers, which has implications on the detectability
of these sources in the Galactic centre.Comment: 7 pages, 3 figures, Accepted for publication in MNRA
A VLA Census of the Galactic H II Region Population
The Milky Way contains thousands of H II region candidates identified by
their characteristic mid-infrared morphology, but lacking detections of ionized
gas tracers such as radio continuum or radio recombination line emission. These
targets thus remain unconfirmed as H II regions. With only 2500 confirmed
H II regions in the Milky Way, Galactic surveys are deficient by several
thousand nebulae when compared to external galaxies with similar star formation
rates. Using sensitive 9 GHz radio continuum observations with the Karl G.
Jansky Very Large Array (VLA), we explore a sample of H II region candidates in
order to set observational limits on the actual total population of Galactic H
II regions. We target all infrared-identified "radio quiet" sources from the
WISE Catalog of Galactic H II regions between
with infrared diameters less than
80. We detect radio continuum emission from 50% of the
targeted H II region candidates, providing strong evidence that most of the
radio quiet candidates are bona fide HII regions. We measure the peak and
integrated radio flux densities and compare the inferred Lyman continuum fluxes
using models of OB-stars. We conclude that stars of approximately spectral type
B2 and earlier are able to create H II regions with similar infrared and radio
continuum morphologies as the more luminous H II regions created by O-stars.
From our 50% detection rate of "radio quiet" sources, we set a lower limit of
7000 for the H II region population of the Galaxy. Thus the vast majority
of the Milky Way's H II regions remain to be discovered.Comment: 11 pages of text, 146 infrared/radio images of faint HII region
candidate
Far-infrared and molecular line observations of Lynds 183 - studies of cold gas and dust
We have mapped the dark cloud L183 in the far-infrared at 100um and 200um
with the ISOPHOT photometer aboard the ISO satellite. The observations make it
possible for the first time to study the properties of the large dust grains in
L183 without confusion from smaller grains. The observations show clear colour
temperature variations which are likely to be caused by changes in the emission
properties of the dust particles. In the cloud core the far-infrared colour
temperature drops below 12K. The data allow a new determination of the cloud
mass and the mass distribution. The mass within a radius of 10 arcmin from the
cloud centre is 25 Msun. We have mapped the cloud in several molecular lines
including DCO+(2-1) and H13CO+(1-0). These species are believed to be tracers
of cold and dense molecular material and we detect a strong anticorrelation
between the DCO+ emission and the dust colour temperatures. In particular, the
DCO+(2-1) emission is not detected towards the maximum of the 100um emission
where the colour temperature rises above 15K. The H13CO+ emission follows the
DCO+ distribution but CO isotopes show strong emission even towards the 100um
peak. A comparison of the DCO+ and C18O maps shows sharp variations in the
relative intensities of the species. Morphologically the 200um dust emission
traces the distribution of dense molecular material as seen e.g. in C18O lines.
A comparison with dust column density shows that C18O is depleted by a factor
of 1.5 in the cloud core. We present results of R- and B-band starcounts. The
extinction is much better correlated with the 200um than with the 100um
emission. Based on the 200um correlation at low extinction values we deduce a
value of ~17mag for the visual extinction towards the cloud centre.Comment: to be published in A&
Complete genome sequence of Halorhabdus utahensis type strain (AX-2).
Halorhabdus utahensis Wainø et al. 2000 is the type species of the genus, which is of phylogenetic interest because of its location on one of the deepest branches within the very extensive euryarchaeal family Halobacteriaceae. H. utahensis is a free-living, motile, rod shaped to pleomorphic, Gram-negative archaeon, which was originally isolated from a sediment sample collected from the southern arm of Great Salt Lake, Utah, USA. When grown on appropriate media, H. utahensis can form polyhydroxybutyrate (PHB). Here we describe the features of this organism, together with the complete genome sequence, and annotation. This is the first complete genome sequence of the a member of halobacterial genus Halorhabdus, and the 3,116,795 bp long single replicon genome with its 3027 protein-coding and 48 RNA genes is part of the Genomic Encyclopedia of Bacteria and Archaea project
Complete genome sequence of Halorhabdus utahensis type strain (AX-2).
Halorhabdus utahensis Wainø et al. 2000 is the type species of the genus, which is of phylogenetic interest because of its location on one of the deepest branches within the very extensive euryarchaeal family Halobacteriaceae. H. utahensis is a free-living, motile, rod shaped to pleomorphic, Gram-negative archaeon, which was originally isolated from a sediment sample collected from the southern arm of Great Salt Lake, Utah, USA. When grown on appropriate media, H. utahensis can form polyhydroxybutyrate (PHB). Here we describe the features of this organism, together with the complete genome sequence, and annotation. This is the first complete genome sequence of the a member of halobacterial genus Halorhabdus, and the 3,116,795 bp long single replicon genome with its 3027 protein-coding and 48 RNA genes is part of the Genomic Encyclopedia of Bacteria and Archaea project
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