15,796 research outputs found
Evidence for anthropogenic <sup>14</sup>C-enrichment in estuarine waters adjacent to the North Sea
[1] The isotopic composition (d13C and D14C) of high
molecular weight dissolved organic carbon (HMW DOC)
was studied in the Tyne and Tweed estuaries, NE England.
Despite significant removal of terrigenous HMW DOC in
the low salinity regions (S < 15), D14C remained modern
with little variation around 115%. This lack of apparent age
discrimination was attributed to either non-oxidative
removal or the absence of a significant proportion of old
refractory C in the HMW DOC pool. At S < 15, we
observed seaward increases in d13C and D14C. With no
documented local 14C inputs, we attributed non-bomb
related 14C-enrichment at S < 15 to a possible ālingering
effectā of distal anthropogenic sources in near-coastal North
Sea HMW DOC. Given the global distribution of potential
sources, we propose that anthropogenic 14C should be
considered in assigning ages of DOC pools in near-coastal
waters and suggest its possible use as a tracer for DOC
transformations
Evidence for deceleration in the radio jets of GRS1915+105?
There is currently a clear discrepancy in the proper motions measured on
different angular scales in the approaching radio jets of the black hole X-ray
binary GRS1915+105. Lower velocities were measured with the Very Large Array
(VLA) prior to 1996 than were subsequently found from higher-resolution
observations made with the Very Long Baseline Array and the Multi-Element Radio
Linked Interferometer Network. We initiated an observing campaign to use all
three arrays to attempt to track the motion of the jet knots from the 2006
February outburst of the source, giving us unprecedented simultaneous coverage
of all angular scales, from milliarcsecond scales out to arcsecond scales. The
derived proper motion, which was dominated by the VLA measurements, was found
to be 17.0 mas per day, demonstrating that there has been no significant
permanent change in the properties of the jets since 1994. We find no
conclusive evidence for deceleration of the jet knots, unless this occurs
within 70 mas of the core. We discuss possible causes for the varying proper
motions recorded in the literature.Comment: Accepted for publication in MNRAS. 13 pages, 10 figure
A Robust Numerical Method for Integration of Point-Vortex Trajectories in Two Dimensions
The venerable 2D point-vortex model plays an important role as a simplified
version of many disparate physical systems, including superfluids,
Bose-Einstein condensates, certain plasma configurations, and inviscid
turbulence. This system is also a veritable mathematical playground, touching
upon many different disciplines from topology to dynamic systems theory.
Point-vortex dynamics are described by a relatively simple system of nonlinear
ODEs which can easily be integrated numerically using an appropriate adaptive
time stepping method. As the separation between a pair of vortices relative to
all other inter-vortex length scales decreases, however, the computational time
required diverges. Accuracy is usually the most discouraging casualty when
trying to account for such vortex motion, though the varying energy of this
ostensibly Hamiltonian system is a potentially more serious problem. We solve
these problems by a series of coordinate transformations: We first transform to
action-angle coordinates, which, to lowest order, treat the close pair as a
single vortex amongst all others with an internal degree of freedom. We next,
and most importantly, apply Lie transform perturbation theory to remove the
higher-order correction terms in succession. The overall transformation
drastically increases the numerical efficiency and ensures that the total
energy remains constant to high accuracy.Comment: 21 pages, 4 figure
Spectral evidence for a powerful compact jet from XTE J1118+480
We present observations of the X-ray transient XTE J1118+480 during its
Low/Hard X-ray state outburst in 2000, at radio and sub-millimetre wavelengths
with the VLA, Ryle Telescope, MERLIN and JCMT. The high-resolution MERLIN
observations reveal all the radio emission (at 5 GHz) to come from a compact
core with physical dimensions smaller than 65*d(kpc) AU. The combined radio
data reveal a persistent and inverted radio spectrum, with spectral index
\~+0.5. The source is also detected at 350 GHz, on an extrapolation of the
radio spectrum. Flat or inverted radio spectra are now known to be typical of
the Low/Hard X-ray state, and are believed to arise in synchrotron emission
from a partially self-absorbed jet. Comparison of the radio and sub-millimetre
data with reported near-infrared observations suggest that the synchrotron
emission from the jet extends to the near-infrared, or possibly even optical
regimes. In this case the ratio of jet power to total X-ray luminosity is
likely to be P_J/L_X >> 0.01, depending on the radiative efficiency and
relativistic Doppler factor of the jet. Based on these arguments we conclude
that during the period of our observations XTE J1118+480 was producing a
powerful outflow which extracted a large fraction of the total accretion power.Comment: Accepted for publication as a Letter in MNRA
On the Availability of European Mean Sea Level Data
Over the past two years a major effort has been made to bring up-to-date the European Mean Sea Level (MSL) data set with the result that significant updates have been obtained from all European coastlines, except one, which are known to operate tide gauges. Several important historical time series, not hitherto included in the Permanent Service for Mean Sea Level (PSMSL) data bank, have also been acquired. The resulting total European MSL data set, which is available for analysis by any interested research worker, will be employed subsequently in extensive climatological and geological investigations of European MSL interannual variability and long-term trends. This report presents a review of the quantity and quality of available European MSL data with particular emphasis on the recently-obtained information
The effect of bedrest on various parameters of physiological function. part xii- the effect of bedrest on bone mass and calcium balance
Bed rest effect on various parameters of physiological functions - bone mass and calcium balanc
MERLIN observations of relativistic ejections from GRS 1915+105
We present high resolution MERLIN radio images of multiple relativistic
ejections from GRS 1915+105 in 1997 October / November. The observations were
made at a time of complex radio behaviour, corresponding to multiple
optically-thin outbursts and several days of rapid radio flux oscillations. The
radio imaging resolved four major ejection events from the system. As
previously reported from earlier VLA observations of the source, we observe
apparent superluminal motions resulting from intrinsically relativistic motions
of the ejecta. However, our measured proper motions are significantly greater
than those observed on larger angular scales with the VLA. Under the assumption
of an intrinsically symmetric ejection, we can place an upper limit on the
distance to GRS 1915+105 of 11.2 +/- 0.8 kpc. Solutions for the velocities
unambiguously require a higher intrinsic speed by about 0.1c than that derived
from the earlier VLA observations, whilst the angle to the line-of-sight is not
found to be significantly different. At a distance of 11 kpc, we obtain
solutions of v = 0.98 (-0.05,+0.02)c and theta = 66 +/- 2 degrees. The jet also
appears to be curved on a scale which corresponds to a period of around 7 days.
We observe significant evolution of the linear polarisation of the approaching
component, with large rotations in position angle and a general decrease in
fractional polarisation. The power input into the formation of the jet is very
large, >10^38 erg/s at 11 kpc for a pair plasma. If the plasma contains a cold
proton for each electron, then the mass outflow rate, >10^18 g/sec is
comparable to inflow rates previously derived from X-ray spectral fits.Comment: 14 pages, 7 figures. Accepted for publication in MNRA
Models of the distribution of 210Pb in a section across the North Equatorial Atlantic Ocean
The deficiency of 210Pb relative to 226Ra in the world\u27s deep ocean is well documented, and the overall residence time of 210Pb bas been calculated to be about 15-100 years. It has been assumed, generally, that the removal mechanism is one of in situ adsorption on settling particles, but Bacon et al. (1976) suggested that a boundary scavenging process with diffusive and advective fluxes of 210Pb from the interior ocean may be a significant factor...
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