165 research outputs found
Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138
We present three epochs of VLBA observations of Galactic HI absorption toward
the quasar 3C~138 with resolutions of 20 mas (~ 10 AU). This analysis includes
VLBA data from observations in 1999 and 2002 along with a reexamination of 1995
VLBA data. Improved data reduction and imaging techniques have led to an order
of magnitude improvement in sensitivity compared to previous work. With these
new data we confirm the previously detected milliarcsecond scale spatial
variations in the HI opacity at the level of Delta(tau_{max}) =0.50 \pm 0.05.
The typical size scale of the optical depth variations is ~ 50 mas or 25 AU. In
addition, for the first time we see clear evidence for temporal variations in
the HI opacity over the seven year time span of our three epochs of data. We
also attempted to detect the magnetic field strength in the HI gas using the
Zeeman effect. From this analysis we have been able to place a 3 sigma upper
limit on the magnetic field strength per pixel of ~45 muG. We have also been
able to calculate for the first time the plane of sky covering fraction of the
small scale HI gas of ~10%. We also find that the line widths of the
milliarcsecond sizescale HI features are comparable to those determined from
previous single dish measurements toward 3C~138, suggesting that the opacity
variations cannot be due to changes in the HI spin temperature. From these
results we favor a density enhancement interpretation for the small scale HI
structures, although these enhancements appear to be of short duration and are
unlikely to be in equilibrium.Comment: 34 pages, 8 figures. Figures 3 & 4 are in color. Accepted to A
Galactic HI on the 50-AU scale in the direction of three extra-galactic sources observed with MERLIN
We present MERLIN observations of Galactic 21-cm HI absorption at an angular
resolution of c. 0.1-0.2 arcsec and a velocity resolution of 0.5 km/s, in the
direction of three moderately low latitude (-8< b <-12 deg) extragalactic radio
sources, 3C111, 3C123 and 3C161, all of which are heavily reddened. HI
absorption is observed against resolved background emission sources up to c. 2
arcsec in extent and we distinguish details of the opacity distribution within
1-1.5 arcsec regions towards 3C~123 and 3C~161. This study is the second MERLIN
investigation of small scale structure in interstellar HI (earlier work probed
Galactic HI in the directions of the compact sources 3C138 and 3C147). The
0.1-arcsec scale is intermediate between HI absorption studies made with other
fixed element interferometers with resolution of 1 to 10 arcsec and VLBI
studies with resolutions of 10-20 milli-arcsec. At a scale of 1 arcsec (about
500 AU), prominent changes in Galactic HI opacity in excess of 1-1.5 are
determined in the direction of 3C161 with a signal-to-noise ratio of at least
10 sigma. Possible fluctuations in the HI opacity at the level of about 1 are
detected at the 2.5-3 sigma level in the direction of 3C123.Comment: 23 pages, 16 figures Accepted by MNRAS 2008 April
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Polymer-Nanodiscs as a Novel Alignment Medium for High-Resolution NMR-Based Structural Studies of Nucleic Acids
Residual dipolar couplings (RDCs) are increasingly used for high-throughput NMR-based structural studies and to provide long-range angular constraints to validate and refine structures of various molecules determined by X-ray crystallography and NMR spectroscopy. RDCs of a given molecule can be measured in an anisotropic environment that aligns in an external magnetic field. Here, we demonstrate the first application of polymer-based nanodiscs for the measurement of RDCs from nucleic acids. Polymer-based nanodiscs prepared using negatively charged SMA-EA polymer and zwitterionic DMPC lipids were characterized by size-exclusion chromatography, 1H NMR, dynamic light-scattering, and 2H NMR. The magnetically aligned polymer-nanodiscs were used as an alignment medium to measure RDCs from a 13C/15N-labeled fluoride riboswitch aptamer using 2D ARTSY-HSQC NMR experiments. The results showed that the alignment of nanodiscs is stable for nucleic acids and nanodisc-induced RDCs fit well with the previously determined solution structure of the riboswitch. These results demonstrate that SMA-EA-based lipid-nanodiscs can be used as a stable alignment medium for high-resolution structural and dynamical studies of nucleic acids, and they can also be applicable to study various other biomolecules and small molecules in general
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
Interstellar absorptions towards the LMC: Small-scale density variations in Milky Way disc gas
Observations show that the ISM contains sub-structure on scales less than 1
pc, detected in the form of spatial and temporal variations in column densities
or optical depth. Despite the number of detections, the nature and ubiquity of
the small-scale structure in the ISM is not yet fully understood. We use UV
absorption data mainly from FUSE and partly from STIS of six LMC stars
(Sk-67{\deg}111, LH54-425, Sk-67{\deg}107, Sk-67{\deg}106, Sk-67{\deg}104, and
Sk-67{\deg}101), all located within 5 arcmin of each other, and analyse the
physical properties of the Galactic disc gas in front of the LMC on sub-pc
scales. We analyse absorption lines of a number of ions within the UV spectral
range. Most importantly, interstellar molecular hydrogen, neutral oxygen, and
fine-structure levels of neutral carbon have been used in order to study
changes in the density and the physical properties of the Galactic disc gas
over small angular scales. While most species do not show any significant
variation in their column densities, we find an enhancement of almost 2 dex for
H_2 from Sk-67{\deg}111 to Sk-67{\deg}101, accompanied by only a small
variation in the OI column density. Based on the formation-dissociation
equilibrium, we trace these variations to the actual density variations in the
molecular gas. On the smallest spatial scale of < 0.08 pc, between
Sk-67{\deg}107 and LH54-425, we find a gas density variation of a factor of
1.8. The line of sight towards LH54-425 does not follow the relatively smooth
change seen from Sk-67{\deg}101 to Sk-67{\deg}111, suggesting that
sub-structure might exist on a smaller spatial scale than the linear extent of
our sight-lines. Our observations suggest that the detected H_2 in these six
lines of sight is not necessarily physically connected, but that we are
sampling molecular cloudlets with pathlengths < 0.1-1.8 pc and possibly
different densities.Comment: 17 pages, 8 figures. Accepted for publication in A&
Dynamical friction on cold fractal gas clouds,applications to disc formation
It is likely that cold molecular clouds form at high redshift, before
galaxies. Considering these cold clouds instead of hot homogeneous gas as the
main baryonic component of proto-galactic halos may affect several issues
during galaxy formation. In particular, the baryonic matter loses angular
momentum through dynamical friction on the dark matter halo. In numerical
simulations using hot gas for baryonic matter, this gives rise to the so-called
angular momentum problem. In this work, we study the dynamical friction exerted
on cold fractal gas by a collisionless background (dark matter) through
high-resolution numerical simulations. First, we find that, for values of the
parameters relevant during galaxy formation, the friction does not alter the
morphology of the fractal, which is mainly driven by internal dynamics. Then,
we show that the presence of substructures and inhomogeneities in a body has
little effect on the global value of the friction. Parameters such as branching
ratio and fractal dimension also have little effect. In fact, we find out that
the main effect comes from the deformations and fluctuations of the fractal
structure in cold gas clumps. If the deformation time is of the same order of
magnitude or shorter than the typical build-up time of the friction, the
friction is weakened. We argue that this effect is relevant for galaxy
formation, and that the angular momentum problem should not be worsened by
including the small scale inhomogeneities of the cold gas which occur at a
resolution out of reach of present simulations.Comment: 19 pages, 8 figures, Accepted for publication in A&
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
The changing role of china in the global illegal cigarette trade
This study explores the history of the illegal production, distribution, and smuggling of cigarettes in mainland China. Data were obtained from a content analysis of 931 media reports retrieved from LexisNexis for the time period 1975 until 2010, and from other open sources. The illegal cigarette trade first emerged in the form of violations of state tobacco monopoly regulations. In the course of the restructuring of the legal tobacco sector, which occurred under external political pressure to open the Chinese market to foreign competition, an illegal cigarette industry emerged which at first primarily produced fake Chinese brand cigarettes for the domestic black market. At the same time, China became a destination country for smuggled genuine Western brand cigarettes. It was only after effective crackdowns against cigarette smuggling and domestic distribution channels in the late 1990s that the Chinese illegal cigarette industry shifted to exporting large numbers of counterfeit Western brand cigarettes to black markets abroad. China’s current role as a leading supplier of counterfeit cigarettes is a result of the contradictions of the economic reform process and of external licit and illicit forces that worked toward opening up the Chinese tobacco sector to the outside world
Effects of Heavy Metals and Arbuscular Mycorrhiza on the Leaf Proteome of a Selected Poplar Clone: A Time Course Analysis
Arbuscular mycorrhizal (AM) fungi establish a mutualistic symbiosis with the roots of most plant species. While receiving photosynthates, they improve the mineral nutrition of the plant and can also increase its tolerance towards some pollutants, like heavy metals. Although the fungal symbionts exclusively colonize the plant roots, some plant responses can be systemic. Therefore, in this work a clone of Populus alba L., previously selected for its tolerance to copper and zinc, was used to investigate the effects of the symbiosis with the AM fungus Glomus intraradices on the leaf protein expression. Poplar leaf samples were collected from plants maintained in a glasshouse on polluted (copper and zinc contaminated) or unpolluted soil, after four, six and sixteen months of growth. For each harvest, about 450 proteins were reproducibly separated on 2DE maps. At the first harvest the most relevant effect on protein modulation was exerted by the AM fungi, at the second one by the metals, and at the last one by both treatments. This work demonstrates how importantly the time of sampling affects the proteome responses in perennial plants. In addition, it underlines the ability of a proteomic approach, targeted on protein identification, to depict changes in a specific pattern of protein expression, while being still far from elucidating the biological function of each protein
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