260 research outputs found
Trace element incorporation in silicate melts and glasses at high pressure
Trace elements are highly fractionated during large-scale melting associated with
planetary differentiation events. The resulting partition coefficients are used
to constrain a range of geological processes and are known to be influenced
by pressure, temperature, and compositional changes in crystalline structures.
Although recent studies have shown that melt compositional changes affect the
partitioning of trace elements, the degree to which these ratios are influenced
by alterations in the melt structure, especially with increasing pressure, is
poorly constrained due to the difficulty of collecting structural information on
bonding environments in situ. A basic understanding of how these elements
are incorporated in silicate melts is critical to interpreting early planetary
differentiation and crust forming events. This thesis presents results from both
x-ray diffraction and absorption techniques on trace element (Y, Zr, Lu and Nd)
incorporation in silicate melt structures. The structure of two rare Earth element
doped model end member silicate liquids, a highly polymerised haplogranite (Si-
Al-Na-K-O) and a less polymerised anorthite-diopside (Si-Al-Mg-Ca-O), have
been studied. The results are the first to identify trace rare Earth element (REE)
incorporation in silicate melts at high pressure using x-ray diffraction techniques.
The local melt structure around Y and Zr in a highly polymerised haplogranite
has been studied using x-ray absorption spectroscopy up to 8GPa and 1650 K.
Both elements appear to adopt 8-fold coordination within the melt structure
with no variation over the pressure range studied. This was also found for the Lu
bonding environment in the same composition where the coordination number of
Lu-O was found to be 8, with a bond distance rLu-O = 2:36A in the haplogranite
melt. At low pressures, < 5GPa, the bonding environment of Lu-O was found
to be dependent on composition with coordination decreasing to CNLu-O = 6
and rLu-O = 2:29A in the anorthite-diopside melt. This compositional variance
in coordination number at low pressure is consistent with observations made for
Y-O in glasses at ambient conditions and is coincident with a dramatic increase
in the partition coefficients previously observed for rare Earth elements (REE)
with increasing melt polymerisation. However, an abrupt change in both Lu-O
coordination and bond distance is observed at 5GPa in the anorthite-diopside
melt, with CNLu-O increasing from 6 to 8-fold and rLu-O from 2.29 to 2.39A.
This occurs over a similar pressure range where a reduction in the reported heavy
REE partition coefficients is observed.
X-ray diffraction experiments up to 60GPa and 2000K have also been performed
on the incorporation of the larger light REE, Nd, in basaltic-like melts.
The results presented show that incorporation within the anorthite-diopside
composition is dependent on the size of the REE. Nd-O initially shows the
same 6-fold coordination as Lu-O at ambient conditions, although the change
to 8-fold coordination appears to occur at considerably lower pressure between
1-2GPa. Coordination change in both cases can be attributed to collapse of
the silicate network and an increase in the average number of available 'crystal
like' sites in the liquid, with ionic radius of the REE controlling at which
pressure the preference for these sites in the melt occurs. Published mineral-melt
partition coefficients for Nd, with major mineral phases such as garnet, show
very little variation with pressure, in contrast to Lu. The difference in structural
incorporation of Lu and Nd in the melts presented in this thesis could explain
this partitioning behaviour.
Overall this thesis highlights that important structural changes of the trace
element bonding environment in silicate melts occur with both compositional
variation and pressure. Melt structural changes with pressure cannot be
neglected in predictive models of trace element behaviour, and using a single
melt term to normalise the effects of melt on trace element partitioning will not
accurately predict partitioning behaviour at depth during magma formation or
differentiation
Ring Star Formation Rates in Barred and Nonbarred Galaxies
Nonbarred ringed galaxies are relatively normal galaxies showing bright rings
of star formation in spite of lacking a strong bar. This morphology is
interesting because it is generally accepted that a typical ring forms when
material collects near a resonance, set up by the pattern speed of a bar or
bar-like perturbation. Our goal in this paper is to examine whether the ring
star formation properties are related to the non-axisymmetric gravity potential
in general. For this purpose, we obtained H{\alpha} emission line images and
calculated the line fluxes and star formation rates (SFRs) for 16 nonbarred SA
galaxies and four weakly barred SAB galaxies with rings. For comparison, we
combine our observations with a re-analysis of previously published data on
five SA, seven SAB, and 15 SB galaxies with rings, three of which are
duplicates from our sample. With these data, we examine what role a bar may
play in the star formation process in rings. Compared to barred ringed
galaxies, we find that the inner ring SFRs and H{\alpha}+[N ii] equivalent
widths in nonbarred ringed galaxies show a similar range and trend with
absolute blue magnitude, revised Hubble type, and other parameters. On the
whole, the star formation properties of inner rings, excluding the distribution
of H ii regions, are independent of the ring shapes and the bar strength in our
small samples. We confirm that the deprojected axis ratios of inner rings
correlate with maximum relative gravitational force Q_g; however, if we
consider all rings, a better correlation is found when local bar forcing at the
radius of the ring, Q_r, is used. Individual cases are described and other
correlations are discussed. By studying the physical properties of these
galaxies, we hope to gain a better understanding of their placement in the
scheme of the Hubble sequence and how they formed rings without the driving
force of a bar.Comment: 55 pages; 21 figures and 9 tables. Article has been accepted for
publication in the Astronomical Journa
Numerical study of jets produced by conical wire arrays on the Magpie pulsed power generator
The aim of this work is to model the jets produced by conical wire arrays on
the MAGPIE generator, and to design and test new setups to strengthen the link
between laboratory and astrophysical jets. We performed the modelling with
direct three-dimensional magneto-hydro-dynamic numerical simulations using the
code GORGON. We applied our code to the typical MAGPIE setup and we
successfully reproduced the experiments. We found that a minimum resolution of
approximately 100 is required to retrieve the unstable character of the jet. We
investigated the effect of changing the number of wires and found that arrays
with less wires produce more unstable jets, and that this effect has magnetic
origin. Finally, we studied the behaviour of the conical array together with a
conical shield on top of it to reduce the presence of unwanted low density
plasma flows. The resulting jet is shorter and less dense.Comment: Accepted for publication in Astrophysics & Space Science. HEDLA 2010
conference procedings. Final pubblication will be available on Springe
A strategy for the characterization of minute chromosome rearrangements using multiple color fluorescence in situ hybridization with chromosome-specific DNA libraries and YAC clones
The identification of marker chromosomes in clinical and tumor cytogenetics by chromosome banding analysis can create problems. In this study, we present a strategy to define minute chromosomal rearrangements by multicolor fluorescence in situ hybridization (FISH) with whole chromosome painting probes derived from chromosome-specific DNA libraries and Alu-polymerase chain reaction (PCR) products of various region-specific yeast artificial chromosome (YAC) clones. To demonstrate the usefulness of this strategy for the characterization of chromosome rearrangements unidentifiable by banding techniques, an 8p+ marker chromosome with two extra bands present in the karyotype of a child with multiple anomalies, malformations, and severe mental retardation was investigated. A series of seven-color FISH experiments with sets of fluorochrome-labeled DNA library probes from flow-sorted chromosomes demonstrated that the additional segment on 8p+ was derived from chromosome 6. For a more detailed characterization of the marker chromosome, three-color FISH experiments with library probes specific to chromosomes 6 and 8 were performed in combination with newly established telomeric and subtelomeric YAC clones from 6q25, 6p23, and 8p23. These experiments demonstrated a trisomy 6pter6p22 and a monosomy 8pter8p23 in the patient. The present limitations for a broad application of this strategy and its possible improvements are discusse
Near-IR Atlas of S0-Sa galaxies (NIRS0S)
An atlas of Ks-band images of 206 early-type galaxies is presented, including
160 S0-S0/a galaxies, 12 ellipticals, and 33 Sa galaxies. A majority of the
Atlas galaxies belong to a magnitude-limited (mB<12.5 mag) sample of 185 NIRS0S
(Near-IR S0 galaxy Survey) galaxies. To assure that mis-classified S0s are not
omitted, 25 ellipticals from RC3 classified as S0s in the Carnegie Atlas were
included in the sample. The images are 2-3 mag deeper than 2MASS images. Both
visual and photometric classifications are made. Special attention is paid to
the classification of lenses, coded in a systematic manner. A new lens-type,
called a 'barlens', is introduced. Also, boxy/peanut/x-shaped structures are
identified in many barred galaxies, even-though the galaxies are not seen in
edge-on view, indicating that vertical thickening is not enough to explain
them. Multiple lenses appear in 25% of the Atlas galaxies, which is a challenge
to the hierarchical evolutionary picture of galaxies. Such models need to
explain how the lenses were formed and survived in multiple merger events that
galaxies may have suffered during their lifetimes. Following the early
suggestion by van den Bergh, candidates of S0c galaxies are shown, which
galaxies are expected to be former Sc-type spirals stripped out of gas.Comment: 67 pages (include 16 figures and 6 tables). Accepted to MNRAS 2011
June 1
BOW SHOCK FRAGMENTATION DRIVEN BY A THERMAL INSTABILITY IN LABORATORY ASTROPHYSICS EXPERIMENTS
The role of radiative cooling during the evolution of a bow shock was studied
in laboratory-astrophysics experiments that are scalable to bow shocks present
in jets from young stellar objects. The laboratory bow shock is formed during
the collision of two counter-streaming, supersonic plasma jets produced by an
opposing pair of radial foil Z-pinches driven by the current pulse from the
MAGPIE pulsed-power generator. The jets have different flow velocities in the
laboratory frame and the experiments are driven over many times the
characteristic cooling time-scale. The initially smooth bow shock rapidly
develops small-scale non-uniformities over temporal and spatial scales that are
consistent with a thermal instability triggered by strong radiative cooling in
the shock. The growth of these perturbations eventually results in a global
fragmentation of the bow shock front. The formation of a thermal instability is
supported by analysis of the plasma cooling function calculated for the
experimental conditions with the radiative packages ABAKO/RAPCAL.Comment: 9 pages, 5 figures, Accepted for publication in The Astrophysical
Journal on 5th November 201
Probing the X-Ray Binary Populations of the Ring Galaxy NGC 1291
We present Chandra studies of the X-ray binary (XRB) populations in the bulge
and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources
in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available
Chandra observations totaling an effective exposure of 179 ks. We report
photometric properties of these sources in a point-source catalog. There are
~40% of the bulge sources and ~25% of the ring sources showing >3\sigma
long-term variability in their X-ray count rate. The X-ray colors suggest that
a significant fraction of the bulge (~75%) and ring (~65%) sources are likely
low-mass X-ray binaries (LMXBs). The spectra of the nuclear source indicate
that it is a low-luminosity AGN with moderate obscuration; spectral variability
is observed between individual observations. We construct 0.3-8.0 keV X-ray
luminosity functions (XLFs) for the bulge and ring XRB populations, taking into
account the detection incompleteness and background AGN contamination. We reach
90% completeness limits of ~1.5\times10^{37} and ~2.2\times10^{37} erg/s for
the bulge and ring populations, respectively. Both XLFs can be fit with a
broken power-law model, and the shapes are consistent with those expected for
populations dominated by LMXBs. We perform detailed population synthesis
modeling of the XRB populations in NGC 1291, which suggests that the observed
combined XLF is dominated by an old LMXB population. We compare the bulge and
ring XRB populations, and argue that the ring XRBs are associated with a
younger stellar population than the bulge sources, based on the relative
overdensity of X-ray sources in the ring, the generally harder X-ray color of
the ring sources, the overabundance of luminous sources in the combined XLF,
and the flatter shape of the ring XLF.Comment: 15 pages, 11 figures. Accepted for publication in Ap
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