8,438 research outputs found
Ion dynamics and the magnetorotational instability in weakly-ionized discs
The magnetorotational instability (MRI) of a weakly ionized, differentially
rotating, magnetized plasma disk is investigated in the multi-fluid framework.
The disk is threaded by a uniform vertical magnetic field and charge is carried
by electrons and ions only. The inclusion of ion dynamics causes significant
modification to the conductivity tensor in a weakly ionized disk. The parallel,
Pedersen and Hall component of conductivity tensor become time dependent
quantities resulting in the AC and DC part of the conductivity. The
conductivity may change sign leading to the significant modification of the
parameter window in which MRI may operate.
The effect of ambipolar and Hall diffusion on the linear growth of the MRI is
examined in the presence of time dependent conductivity tensor. We find that
the growth rate in ambipolar regime can become somewhat larger than the
rotational frequency, especially when the departure from ideal MHD is
significant. Further, the instability operates on large scale lengths. This has
important implication for the angular momentum transport in the disk.Comment: 13 pages, 12 figure
Hall instability of solar flux tubes
The magnetic network which consists of vertical flux tubes located in
intergranular lanes is dominated by Hall drift in the photosphere-lower
chromosphere region (). In the internetwork regions, Hall drift
dominates above in the photosphere and below in the
chromosphere. Although Hall drift does not cause any dissipation in the ambient
plasma, it can destabilise the flux tubes and magnetic elements in the presence
of azimuthal shear flow. The physical mechanism of this instability is quite
simple: the shear flow twists the radial magnetic field and generates azimuthal
field; torsional oscillations of the azimuthal field in turn generates the
radial field completing feedback loop. The maximum growth rate of Hall
instability is proportional to the absolute value of the shear gradient and is
dependent on the ambient diffusivity. The diffusivity also determines the most
unstable wavelength which is smaller for weaker fields.
We apply the result of local stability analysis to the network and
internetwork magnetic elements and show that the maximum growth rate for
kilogauss field occurs around and decreases with increasing altitude.
However, for a field, the maximum growth rate remains almost constant
in the entire photosphere-lower chromosphere except in a small region of lower
photosphere. For shear flow gradient , the Hall growth time
is 10 minute near the footpoint. Therefore, network fields are likely to be
unstable in the photosphere, whereas internetwork fields could be unstable in
the entire photosphere-chromosphere. Thus the Hall instability can play an
important role in generating low frequency turbulence which can heat the
chromosphere.Comment: 8 page, 4 figure
VLA Observations of a Complete Sample of Radio Loud Quasars between redshifts 2.5 and 5.28: I. high-redshift sample summary and the radio images
We present high resolution (arcsecond or better) observations made with the
Karl G. Jansky Very Large Array of 123 radio-loud quasars with redshifts in the
range that form a complete flux limited sample ( mJy at 1.4 GHz or 5 GHz). Where possible, we used previous high resolution
VLA observations (mainly A array at 1.4, 5 and 8 GHz) from the NRAO archive and
re-imaged them (43 sources). For the remainder, new observations were made in
the A array at 1.4 and 5 GHz. We show images of the 61 resolved sources, and
list structural properties of all of them. Optical data from the SDSS are
available for nearly every source. This work represents a significant increase
in the number of high redshift quasars with published radio structures, and
will be used to study the properties and evolution of luminous radio sources in
the high redshift universe
Studies Toward the Total Synthesis of Antibiotic 593A
In 1970 art article was published in the Journal of Antibiotics in which the isolation of a substance from the south African soil microorganism Streptomyces griseolutus designated 593A (NSC-135758) was reported. It was reported to inhibit the growth of human tumors in the chick embryo. Arison and Beck in 1973 proposed the structure to be 3,6-bis-(5-chloro-2 -piperidyl-)-2 ,5-piperizinedione based on spectral data.
In the ensuing years a number of further studies were made on the activity of 593A, It was found to be active against a number of solid tumors, several variants of leukemia, and in selectively inhibiting DNA synthesis. A possible mode of operation was suggested in which the Antibiotic would form a bis-aziridine derivative which acts as an alkylating agent
High Spectral and Spatial Resolution Observations of Shocked Molecular Hydrogen at the Galactic Center
The presence of OH (1720 MHz) masers, and the absence of counterparts at
1665/1667 MHz has proved to be a clear diagnostic of shocked molecular gas
associated with Galactic supernova remnants. This suggests that shocked
molecular gas should be associated with the OH (1720 MHz) masers that have been
detected in the circumnuclear disk (CND) and Sgr A East at the Galactic center.
In order to test this hypothesis, we observed the H 1--0 S(1) and Br
lines using NICMOS on the HST and UNSWIRF on the AAT, near the regions
where OH (1720 MHz) masers are detected in the CND and Sgr A East. We present
the distribution of H in the North and South lobes of the CND and in Sgr A
East. H emission accompanies almost all of the maser spots detected at the
Galactic center. In particular, we find a striking filamentary structure near
the Northwest of the CND and evidence that shocked molecular gas is associated
with the 70 \kms molecular cloud at the Galactic center. We argue that the
emission from the CND could arise in gas heated by the dissipation of the
random motion of clumps by collisions or the dissipation of turbulence in a
more homogeneous medium. In addition, highly red-shifted gas of up to 140 \kms\
close to the eastern edge of the Sgr A East shell is detected. These
observations combined with OH (1720 MHz) results suggest that the H gas is
shocked and accelerated by the expansion of Sgr A East into the 50 and the 70
\kms cloud and into the lobes of the CND.Comment: 31 pages plus 14 figures, ApJ (in press
Concurrent 43 and 86 GHz Very Long Baseline Polarimetry of 3C273
We present sub-milliarcsecond resolution total intensity and linear
polarization VLBI images of 3C273, using concurrent 43 and 86 GHz data taken
with the Very Long Baseline Array in May 2002. The structure seen in the
innermost jet suggest that we have fortuitously caught the jet in the act of
changing direction. The polarization images confirm that the core is
unpolarized (fractional polarization m < 1 %) at 86 GHz, but also show well
ordered magnetic fields (m ~ 15 %) in the inner jet, at a projected distance of
2.3 pc from the core. In this strongly polarized region, the rotation measure
changes across the jet by 4.2 x 10^{4} rad m^{-2} over an angular width of
about 0.3 milliarcseconds. If the lack of polarization in the core is also
attributed to a Faraday screen, then a rotation measure dispersion > 5.2 x
10^{4} rad m^{-2} must be present in or in front of that region. These are
among the highest rotation measures reported so far in the nucleus of any
active galaxy or quasar, and must occur outside (but probably close to) the
radio emitting region. The transverse rotation measure gradient is in the same
sense as that observed by Asada et al and by Zavala and Taylor at greater core
distances. The magnitude of the transverse gradient decreases rapidly with
distance down the jet, and appears to be variable.Comment: 4 pages, LaTeX, 3 postscript figures, submitted to Astrophysical
Journal Letter
Adjusting the frequency of mammography screening on the basis of genetic risk: Attitudes among women in the UK.
Purpose
To explore public attitudes towards modifying frequency of mammography screening based on genetic risk.
Methods
Home-based interviews were carried out with a population-based sample of 942 women aged 18ā74 years in the UK. Demographic characteristics and perceived breast cancer (BC) risk were examined as predictors of support for risk-stratified BC screening and of the acceptability of raised or lowered screening frequency based on genetic risk, using multivariate logistic regression.
Results
Over two-thirds of respondents (65.8%) supported the idea of varying screening frequency on the basis of genetic risk. The majority (85.4%) were willing to have more frequent breast screening if they were found to be at higher risk, but fewer (58.8%) were willing to have less frequent screening if at lower risk (t (956) = 15.6, p < 0.001). Ethnic minority status was associated with less acceptability of more frequent screening (OR = 0.40, 95% CI = 0.21ā0.74), but there were no other significant demographic correlates. Higher perceived risk of BC was associated with greater acceptability of more frequent screening (OR = 1.71, 95%CI = 1.27ā2.30).
Conclusion
Women were positive about adjusting the frequency of mammography screening in line with personal genetic risk, but it will be important to develop effective communication materials to minimise resistance to reducing screening frequency for those at lower genetic risk
Using a hypothetical scenario to assess public preferences for colorectal surveillance following screening-detected, intermediate-risk adenomas: annual home-based stool test vs. triennial colonoscopy
Background To assess public preferences for colorectal cancer (CRC) surveillance tests for intermediate-risk adenomas, using a hypothetical scenario. Methods Adults aged 45ā54 years without CRC were identified from three General Practices in England (two in Cumbria, one in London). A postal survey was carried out during a separate study on preferences for different first-line CRC screening modalities (non- or full-laxative computed tomographic colonography, flexible sigmoidoscopy, or colonoscopy). Individuals were allocated at random to receive a pack containing information on one first-line test, and a paragraph describing CRC surveillance recommendations for people who are diagnosed with intermediate-risk adenomas during screening. All participants received a description of two surveillance options: annual single-sample, home-based stool testing (consistent with Faecal Immunochemical Tests; FIT) or triennial colonoscopy. Invitees were asked to imagine they had been diagnosed with intermediate-risk adenomas, and then complete a questionnaire on their surveillance preferences. Results 22.1 % (686/3,100) questionnaires were returned. 491 (15.8 %) were eligible for analysis. The majority of participants stated a surveillance preference for the stool test over colonoscopy (60.8 % vs 31.0 %; no preference: 8.1 %; no surveillance: 0.2 %). Women were more likely to prefer the stool test than men (66.7 % vs. 53.6 %; pā=ā.011). The primary reason for preferring the stool test was that it would be done more frequently. The main reason to prefer colonoscopy was its superiority at finding polyps. Conclusions A majority of participants stated a preference for a surveillance test resembling FIT over colonoscopy. Future research should test whether this translates to greater adherence in a real surveillance setting
Shocked molecular hydrogen towards the Tornado nebula
We present near-infrared and millimetre-line observations of the Tornado
nebula (G357.7-0.1). We detected 2.12 micron_m H2 1-0 S(1) line emission
towards the suspected site of interaction with a molecular cloud revealed by
the presence of an OH(1720 MHz) maser. The distribution of the H2 emission is
well correlated with the nonthermal radio continuum emission from the Tornado,
and the velocity of the H2 emission spans over 100 km/s, which both imply that
the H2 emission is shock excited. We also detected millimetre-lines from 12CO
and 13CO transitions at the velocity of the maser, and mapped the distribution
of the molecular cloud in a 2 x 2 arcmin^2 region around the maser. The peak of
the molecular cloud aligns well with an indentation in the nebula's radio
continuum distribution, suggesting that the nebula's shock is being decelerated
at this location, which is consistent with the presence of the OH(1720 MHz)
maser and shocked H2 emission at that location.Comment: 10 pages, 8 figures, minor changes, accepted to MNRA
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