883 research outputs found
An AT Mutational Bias in the Tiny GC-Rich Endosymbiont Genome of Hodgkinia
The fractional guanine + cytosine (GC) contents of sequenced bacterial genomes range from 13% to 75%. Despite several decades of research aimed at understanding this wide variation, the forces controlling GC content are not well understood. Recent work has suggested that a universal adenine + thymine (AT) mutational bias exists in all bacteria and that the elevated GC contents found in some bacterial genomes is due to genome-wide selection for increased GC content. These results are generally consistent with the low GC contents observed in most strict endosymbiotic bacterial genomes, where the loss of DNA repair mechanisms combined with the population genetic effects of small effective population sizes and decreased recombination should lower the efficacy of selection and shift the equilibrium GC content in the mutationally favored AT direction. Surprisingly, the two smallest bacterial genomes, Candidatus Hodgkinia cicadicola (144 kb) and Candidatus Tremblaya princeps (139 kb), have the unusual combination of highly reduced genomes and elevated GC contents, raising the possibility that these bacteria may be exceptions to the otherwise apparent universal bacterial AT mutational bias. Here, using population genomic data generated from the Hodgkinia genome project, we show that Hodgkinia has a clear AT mutational bias. These results provide further evidence that an AT mutational bias is universal in bacteria, even in strict endosymbionts with elevated genomic GC contents
Structural dynamic analysis of the Space Shuttle Main Engine
This structural dynamic analysis supports development of the SSME by evaluating components subjected to critical dynamic loads, identifying significant parameters, and evaluating solution methods. Engine operating parameters at both rated and full power levels are considered. Detailed structural dynamic analyses of operationally critical and life limited components support the assessment of engine design modifications and environmental changes. Engine system test results are utilized to verify analytic model simulations. The SSME main chamber injector assembly is an assembly of 600 injector elements which are called LOX posts. The overall LOX post analysis procedure is shown
A Neutral Hydrogen Self-Absorption Cloud in the SGPS
Using data from the Southern Galactic Plane Survey (SGPS) we analyze an HI
self-absorption cloud centered on l = 318.0 deg, b = -0.5 deg, and velocity, v
= -1.1 km/s. The cloud was observed with the Australia Telescope Compact Array
(ATCA) and the Parkes Radio Telescope, and is at a near kinematic distance of
less than 400 pc with derived dimensions of less than 5 x 11 pc. We apply two
different methods to find the optical depth and spin temperature. In both
methods we find upper limit spin temperatures ranging from 20 K to 25 K and
lower limit optical depths ~ 1. We look into the nature of the HI emission and
find that 60-70% originates behind the cloud. We analyze a second cloud at the
same velocity centered on l = 319 deg and b = 0.4 deg with an upper limit spin
temperature of 20 K and a lower limit optical depth of 1.6. The similarities in
spin temperature, optical depth, velocity, and spatial location are evidence
the clouds are associated, possibly as one large cloud consisting of smaller
clumps of gas. We compare HI emission data with 12CO emission and find a
physical association of the HI self-absorption cloud with molecular gas.Comment: 33 pages, 17 figures, 5 tables; Accepted for publication in ApJ. A
version with higher quality images availabe at
http://www.astro.umn.edu/~dkavars/ms.p
HI Narrow Line Absorption in Dark Clouds
We have used the Arecibo telescope to carry out an survey of 31 dark clouds
in the Taurus/Perseus region for narrow absorption features in HI (
21cm) and OH (1667 and 1665 MHz) emission. We detected HI narrow
self--absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH
emission, observed simultaneously are remarkably well correlated. Spectrally,
they have the same nonthermal line width and the same line centroid velocity.
Spatially, they both peak at the optically--selected central position of each
cloud, and both fall off toward the cloud edges. Sources with clear HINSA
feature have also been observed in transitions of CO, \13co, \c18o, and CI.
HINSA exhibits better correlation with molecular tracers than with CI.
The line width of the absorption feature, together with analyses of the
relevant radiative transfer provide upper limits to the kinetic temperature of
the gas producing the HINSA. Some sources must have a temperature close to or
lower than 10 K. The correlation of column densities and line widths of HINSA
with those characteristics of molecular tracers suggest that a significant
fraction of the atomic hydrogen is located in the cold, well--shielded portions
of molecular clouds, and is mixed with the molecular gas.
The average number density ratio [HI]/[\h2] is . The
inferred HI density appears consistent with but is slightly higher than the
value expected in steady state equilibrium between formation of HI via cosmic
ray destruction of H and destruction via formation of H on grain
surfaces. The distribution and abundance of atomic hydrogen in molecular clouds
is a critical test of dark cloud chemistry and structure, including the issues
of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.Comment: 40 pages, 10 figures, accepted by Ap
Beyond the InsiderâOutsider Perspective: The Study of Religion as a Study of Discourse Construction
This essay reflects on contemporary theorizing of religion which embodies an explicit critique of the imperial project, seeing that by most common consent the scholarly disciplinary field of religious studies (history of religion, phenomenology of religion, Religionswissenschaft) is a late nineteenth century invention that coincides with the emergence of anthropology and ethnography as epiphenomena of the colonial project (whether as Orientalism or as exoticism the Other is rendered manageable subjects). The scholarly study of religion is, therefore, simultaneously a study of the history of theory and concept formation, and the social, cultural, and political work performed by such study and theorizing. The metatheory of the study of religion is a main focus of the essay. Alongside that, the essay focuses more pointedly on the concept of discourse, and considers the extraordinary situation where the same methodological vocabulary that functions in religious studies also functions in critical theological studies, which relativizes the division of âinsiderâ and âoutsiderâ perspectives. Yet both are conventionally practised either in isolation from each other as distinct theoretical and disciplinary bounded/defined study fields, orâthe other and almost direct oppositeâreligious studies being performed in the context of theological study, situated in and offered by theological faculties. An overview of recent debates in the field of religious studies serves to highlight the continued struggle to demarcate the boundaries between the study of religion and the study of theologyâin some of the recent, very strident debates mainstream religious studies is labelled as nothing more than theology. This contribution, then, aims at a kind of metatheoretical reflection on the study of religion and theology both as discourses that serve mythmaking, identity formation, culturally strategic purposes. That is, from the discourse perspective that is proposed here, it is possible to move beyond the definitional divide between religious studies and theologyâeven beyond âreligionâ itselfâto focus on the mundanely material practices that constitute that which is called religion. In the way in which the terms are used it is clear that the terminologies themselves bear the imprint of historical social discourses that occasioned the rise of their use. This essay, then, is something of a metacritique of the language of the study of religionâbeyond religion, and beyond the study of religion and theology
Non-equilibrium entangled steady state of two independent two-level systems
We determine and study the steady state of two independent two-level systems
weakly coupled to a stationary non-equilibrium environment. Whereas this
bipartite state is necessarily uncorrelated if the splitting energies of the
two-level systems are different from each other, it can be entangled if they
are equal. For identical two-level systems interacting with two bosonic heat
baths at different temperatures, we discuss the influence of the baths
temperatures and coupling parameters on their entanglement. Geometric
properties, such as the baths dimensionalities and the distance between the
two-level systems, are relevant. A regime is found where the steady state is a
statistical mixture of the product ground state and of the entangled singlet
state with respective weights 2/3 and 1/3
An Improved Technique for Measurement of Cold HI in Molecular Cloud Cores
The presence of atomic gas mixed with molecular species in a "molecular"
cloud may significantly affect its chemistry, the excitation of some species,
and can serve as probe of the cloud's evolution. Cold neutral atomic hydrogen
(HI) in molecular clouds is revealed by its self absorption of background
galactic HI 21-cm emission. The properties of this gas can be investigated
quantitatively through observation of HI Narrow Self-Absorption (HINSA). In
this paper, we present a new technique for measuring atomic gas physical
parameters from HINSA observations that utilizes molecular tracers to guide the
HINSA extraction. This technique offers a significant improvement in the
precision with which HI column densities can be determined over previous
methods, and it opens several new avenues of study of relevance to the field of
star formation.Comment: Accepted for Astrophycal Journal 8/200
Understanding the Spectral Energy Distributions of the Galactic Star Forming Regions IRAS 18314-0720, 18355-0532 & 18316-0602
Embedded Young Stellar Objects (YSO) in dense interstellar clouds is treated
self-consistently to understand their spectral energy distributions (SED).
Radiative transfer calculations in spherical geometry involving the dust as
well as the gas component, have been carried out to explain observations
covering a wide spectral range encompassing near-infrared to radio continuum
wavelengths. Various geometric and physical details of the YSOs are determined
from this modelling scheme. In order to assess the effectiveness of this
self-consistent scheme, three young Galactic star forming regions associated
with IRAS 18314-0720, 18355-0532 and 18316-0602 have been modelled as test
cases. They cover a large range of luminosity ( 40). The modelling of
their SEDs has led to information about various details of these sources, e.g.
embedded energy source, cloud structure & size, density distribution,
composition & abundance of dust grains etc. In all three cases, the best fit
model corresponds to the uniform density distribution.Comment: AAMS style manuscript with 3 tables (in a separate file) and 4
figures. To appear in Journal of Astronophysics & Astronom
Far and mid infrared observations of two ultracompact H II regions and one compact CO clump
Two ultracompact H II regions (IRAS 19181+1349 and 20178+4046) and one
compact molecular clump (20286+4105) have been observed at far infrared
wavelengths using the TIFR 1 m balloon-borne telescope and at mid infrared
wavelengths using ISO. Far infrared observations have been made simultaneously
in two bands with effective wavelengths of ~ 150 and ~ 210 micron, using liquid
3He cooled bolometer arrays. ISO observations have been made in seven spectral
bands using the ISOCAM instrument; four of these bands cover the emission from
Polycyclic Aromatic Hydrocarbon (PAH) molecules. In addition, IRAS survey data
for these sources in the four IRAS bands have been processed using the HIRES
routine. In the high resolution mid infrared maps as well as far infrared maps
multiple embedded energy sources have been resolved. There are structural
similarities between the images in the mid infrared and the large scale maps in
the far infrared bands, despite very different angular resolutions of the two.
Dust temperature and optical depth (tau_150 um) maps have also been generated
using the data from balloon-borne observations. Spectral energy distributions
(SEDs) for these sources have been constructed by combining the data from all
these observations. Radiation transfer calculations have been made to
understand these SEDs. Parameters for the dust envelopes in these sources have
been derived by fitting the observed SEDs. In particular, it has been found
that radial density distribution for three sources is diffrent. Whereas in the
case of IRAS 20178+4046, a steep distribution of the form r^-2 is favoured, for
IRAS 20286+4105 it is r^-1 and for IRAS 19181+1349 it the uniform distribution
(r^0). Line ratios for PAH bands have generally been found to be similar to
those for other compact H II regions but different from general H II regions.Comment: To appear in Astronomy & Astrophysics; (19 pages including 14 Figures
and 6 Tables
Quantum modulation of a coherent state wavepacket with a single electron spin
The interaction of quantum objects lies at the heart of fundamental quantum
physics and is key to a wide range of quantum information technologies.
Photon-quantum-emitter interactions are among the most widely studied.
Two-qubit interactions are generally simplified into two quantum objects in
static well-defined states . In this work we explore a fundamentally new
dynamic type of spin-photon interaction. We demonstrate modulation of a
coherent narrowband wavepacket with another truly quantum object, a quantum dot
with ground state spin degree of freedom. What results is a quantum modulation
of the wavepacket phase (either 0 or {\pi} but no values in between), a new
quantum state of light that cannot be described classically.Comment: Supplementary Information available on reques
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