714 research outputs found
Investigation of groundwater salinity using geophysical and geochemical approaches: Heuningnes catchment coastal aquifer. Western Cape Province, South Africa
Seawater intrusion is recognised as a major factor affecting the quality of groundwater in coastal aquifers around the world. To determine the occurrence and extent of saline groundwater intrusion prevailing in the Heuningnes Catchment, a 2D resistivity imaging survey was carried out in three sites within the catchment. Eleven resistivity profile lines were conducted using the multi-electrode ABEM SAS resistivity meter system, employing the Wenner Array to map the extent of saline groundwater. A 2D resistivity image was acquired for these resistivity profile lines and the data were processed using Res2DINV software to produce the 2D inverse resistivity models. The interpretation of the resistivity models revealed three resistivity zones. The first zone is characterised by low resistivity (1â3 Ω.m); this represents an aquifer filled with saline water. The second resistivity zone has values ranging from (3â30 Ω.m); this represents a brackish saturated formation. The third resistivity zone denotes a sandy layer/sandstone saturated with freshwater having high resistivity values above 30 Ω.m. Geochemical analysis results of water samples at 11 locations along the electrical resistivity profile lines also suggest high salinity, indicated by high levels of Clâ, Na+, Total Dissolved Solids, and Electrical Conductivity
Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei
We present an investigation of H-alpha emission line variations observed in
the massive Algol binary, RY Per. We give new radial velocity data for the
secondary based upon our optical spectra and for the primary based upon high
dispersion UV spectra. We present revised orbital elements and an estimate of
the primary's projected rotational velocity (which indicates that the primary
is rotating 7 times faster than synchronous). We use a Doppler tomography
algorithm to reconstruct the individual primary and secondary spectra in the
region of H-alpha, and we subtract the latter from each of our observations to
obtain profiles of the primary and its disk alone. Our H-alpha observations of
RY Per show that the mass gaining primary is surrounded by a persistent but
time variable accretion disk. The profile that is observed outside-of-eclipse
has weak, double-peaked emission flanking a deep central absorption, and we
find that these properties can be reproduced by a disk model that includes the
absorption of photospheric light by the band of the disk seen in projection
against the face of the star. We developed a new method to reconstruct the disk
surface density distribution from the ensemble of H-alpha profiles observed
around the orbit, and this method accounts for the effects of disk occultation
by the stellar components, the obscuration of the primary by the disk, and flux
contributions from optically thick disk elements. The resulting surface density
distribution is elongated along the axis joining the stars, in the same way as
seen in hydrodynamical simulations of gas flows that strike the mass gainer
near trailing edge of the star. This type of gas stream configuration is
optimal for the transfer of angular momentum, and we show that rapid rotation
is found in other Algols that have passed through a similar stage.Comment: 39 pages, 12 figures, ApJ in press, 2004 June 20 issu
X-Ray Binary Populations: The Luminosity Function of NGC1569
Using the population synthesis code StarTrack we construct the first
synthetic X-ray binary populations for direct comparison with the X-ray
luminosity function (XLF) of NGC 1569 observed with Chandra.
Our main goal is to examine whether it is possible to reproduce the XLF shape
with our models, given the current knowledge for the star-formation history of
this starburst galaxy. We thus produce hybrid models meant to represent the two
stellar populations: one old, metal-poor with continuous star-formation for 1.5
Gyr and another recent and metal-rich population. To examine the validity of
the models we compare XLFs calculated for varying ages of the populations and
varying relative weights for the star-formation rates in the two populations.
We find that, for typical binary evolution parameters, it is indeed possible
to quite closely match the observed XLF shape.
The robust match is achieved for an age of the young population and a ratio
of star formation rates in the two populations that are within factors of 1.5
and 2, respectively, of those inferred from HST observations of NGC 1569.
In view of this encouraging first step, we discuss the implications of our
X-ray binary models and their potential as tools to study binary populations in
galaxies.Comment: 5 pages, 2 figures, accepted to ApJ Let
How long can you hold the filler:Maintenance and retrieval
This study attempts to reveal the mechanisms behind the online formation of Wh-Filler-Gap Dependencies (WhFGD). Specifically, we aim to uncover the way in which maintenance and retrieval work in WhFGD processing, by paying special attention to the information that is retrieved when the gap is recognized. We use the agreement attraction phenomenon (Wagers, M. W., Lau, E. F., & Phillips, C. (2009). Agreement attraction in comprehension: Representations and processes. Journal of Memory and Language, 61(2), 206-237) as a probe. The first and second experiments examined the type of information that is maintained and how maintenance is motivated, investigating the retrieved information at the gap for reactivated fillers and definite NPs. The third experiment examined the role of the retrieval, comparing reactivated and active fillers. We contend that the information being accessed reflects the extent to which the filler is maintained, where the reader is able to access fine-grained information including category information as well as a representation of both the head and the modifier at the verb
N-band Imaging of Seyfert Nuclei and the MIR-X-ray correlation
We present new mid-infrared (N-band) images of a sample of eight nearby
Seyfert galaxies. In all of our targets, we detect a central unresolved source,
which in some cases has been identified for the first time. In particular, we
have detected the mid-infrared emission from the active nucleus of NGC 4945,
which previously remained undetected at any wavelength but hard X-rays. We also
detect circumnuclear extended emission in the Circinus galaxy along its major
axis, and find marginal evidence for extended circumnuclear emission in NGC
3281.
The high spatial resolution (1.7") of our data allows us to separate the flux
of the nuclear point sources from the extended circumnuclear starburst (if
present). We complement our sample with literature data for a number of
non-active starburst galaxies, and relate the nuclear N-band flux to published
hard (2-10 kev) X-ray fluxes. We find tight and well-separated correlations
between nuclear N-band flux and X-ray flux for both Seyfert and starburst
nuclei which span over 3 orders of magnitude in luminosity. We demonstrate that
these correlations can be used as a powerful classification tool for galactic
nuclei.
For example, we find strong evidence against NGC 1808 currently harbouring an
active Seyfert nucleus based on its position in the mid-infrared-X-ray diagram.
On the other hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.Comment: 31 pages, incl. 4 figures, uses AASTex. Replaced with accepted
version after minor modifications. To appear in Ap
Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in samples with
well-defined ages and initial chemical composition. Here we present a survey of
projected rotational velocities for a large sample of mainly B-type stars in
young clusters to study the time evolution of the rotational properties of
massive stars. The survey is based upon moderate resolution spectra made with
the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs,
and the target stars are members of 19 young open clusters with an age range of
approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387,
4471 and Mg II 4481 using model theoretical profiles to find projected
rotational velocities for a total of 496 OB stars. We find that there are fewer
slow rotators among the cluster B-type stars relative to nearby B stars in the
field. We present evidence consistent with the idea that the more massive B
stars (M > 9 solar masses) spin down during their main sequence phase. However,
we also find that the rotational velocity distribution appears to show an
increase in the numbers of rapid rotators among clusters with ages of 10 Myr
and higher. These rapid rotators appear to be distributed between the zero age
and terminal age main sequence locations in the Hertzsprung-Russell diagram,
and thus only a minority of them can be explained as the result of a spin up at
the terminal age main sequence due to core contraction. We suggest instead that
some of these rapid rotators may have been spun up through mass transfer in
close binary systems.Comment: 33 pages, 11 figures, accepted by Ap
Luminosity function of binary X-ray sources calculated using the Scenario Machine
Using the ``Scenario Machine'' we have carried out a population synthesis of
X-ray binaries for the purpose of modelling of X-ray luminosity functions
(XLFs) in different types of galaxies: star burst, spiral, and elliptical. This
computer code allows to calculate, by using Monte Carlo simulations, the
evolution of a large ensemble of binary systems, with proper accounting for the
spin evolution of magnetized neutron stars.
We show that the XLF has no universal type. It depends on the star formation
rate in the galaxy. Also it is of importance to take into account the evolution
of binary systems and life times of X-ray stages in theoretical models of such
functions. We have calculated cumulative and differential XLFs for the galaxy
with the constant star formation rate. Also we have calculated cumulative
luminosity functions for different intervals of time after the star formation
burst in the galaxy and curves depicting the evolution of the X-ray luminosity
after the star formation burst in the galaxy.Comment: 22 pages, 13 figure
The Nuclear Stellar Cluster in the Seyfert~1 Galaxy NGC 3227: High Angular Resolution NIR Imaging and Spectroscopy
NIR high angular resolution speckle imaging and imaging spectroscopy of the
nuclear region (10'' ~ 840pc) of the Seyfert1 galaxy NGC3227 are presented. A
nuclear stellar cluster is slightly resolved in the J and H band with
increasing contribution to the NIR continuum from the K to the J band. The
stellar absorption lines are extended compared to the neighboring continuum
suggesting a cluster size of ~ 70pc FWHM. Analysis of those lines suggests that
the stars are contributing about 65% (40%) of the total continuum emission in
the H (K) band in a 3.6'' aperture. Population synthesis in conjunction with
NIR spectral synthesis indicates an age of 25 to 50 Myr when red supergiants
contribute most to the NIR light. This is supported by published optical data
on the MgIb line and the CaII triplet. Although a higher age of ~ 0.5 Gyr where
AGB stars dominate the NIR light can not be excluded, the observed parameters
are at the limit of those expected for a cluster dominated by AGB stars.
However, in either case the resolved stellar cluster contributes only about ~
15 % of the total dynamical mass in the inner 300pc implying another much older
stellar population. Pure constant star formation over the last 10 Gyr can be
excluded. Therefore, at least two star formation/starburst events took place in
the nucleus of NGC3227. Since such sequences in the nuclear star formation
history are also observed in the nuclei of other galaxies a link between the
activity of the star formation and the AGN itself seems likely.Comment: accepted for publication in the Astrophysical Journal, 46 pages, 15
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