4,022 research outputs found
A Reappraisal of the Solar Photospheric C/O Ratio
Accurate determination of photospheric solar abundances requires detailed
modeling of the solar granulation and accounting for departures from local
thermodynamical equilibrium (LTE). We argue that the forbidden C I line at 8727
A is largely immune to departures from LTE, and can be realistically modeled
using LTE radiative transfer in a time-dependent three-dimensional simulation
of solar surface convection. We analyze the [C I] line in the solar flux
spectrum to derive the abundance log epsilon(C)= 8.39 +/- 0.04 dex. Combining
this result with our parallel analysis of the [O I] 6300 A line, we find
C/O=0.50 +/- 0.07, in agreement with the ratios measured in the solar corona
from gamma-ray spectroscopy and solar energetic particles.Comment: 5 pages, 2 figures, to appear in ApJL July 1
Seeing, Wind and Outer Scale Effects on Image Quality at the Magellan Telescopes
We present an analysis of the science image quality obtained on the twin 6.5
metre Magellan telescopes over a 1.5 year period, using images of ~10^5 stars.
We find that the telescopes generally obtain significantly better image quality
than the DIMM-measured seeing. This is qualitatively consistent with
expectations for large telescopes, where the wavefront outer scale of the
turbulence spectrum plays a significant role. However, the dominant effect is
found to be wind speed with Magellan outperforming the DIMMs most markedly when
the wind is strongest. Excluding data taken during strong wind conditions (>10
m/s), we find that the Magellan telescopes still significantly outperform the
DIMM seeing, and we estimate the site to have L_0 ~ 25 m on average. We also
report on the first detection of a negative bias in DIMM data. This is found to
occur, as predicted, when the DIMM is affected by certain optical aberrations
and the turbulence profile is dominated by the upper layers of the atmosphere.Comment: Accepted for publication in PASP. 10 pages, 12 figures
Socioeconomic segregation of students
School segregation may be questioning the equity of the Spanish educational system. In this research, we analyse the school segregation by socioeconomic status using the Hutchens Index. Specifically, we decompose the effect of attending public vs semiprivate schools in terms of segregation. The analyses is based on data collecting information from the population of Andalusian students attending 5th grade. We found significant differences in the level of segregation between both schools types. Particularly, the results show that students from high socioeconomic and cultural status are more segregated in public schools, while students from a low socioeconomic and cultural status are more segregated in semiprivate schools. These differences provide evidence of the inequity of the education policy, which should rethink funding mechanisms as a means to fight against inequality.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech.
The data used in this research has been provided by the Consejería de Educación de la Junta de Andalucía. This work has been partly supported by the Ministerio de Economía y Competitividad under Research Project ECO2017-88883-R; Consejería de Innovación, Ciencia y Empresa de la Junta de Andalucía under PAI group SEJ-532. Claudia Prieto Latorre acknowledges the scholarship FPU2017 00432 of the Ministry of Education, Culture and Sports [Ministerio de Educación, Cultura y Deporte] and the training received from the University of Malaga PhD Program in Economics and Business [Programa de Doctorado en Economía y Empresa de la Universidad de Malaga]
Moduli Stabilization and Stability in Type II/F-theory flux compactifications
In this thesis we study String Theory compactifications to four dimensions
focusing on the moduli stabilization process and the associated vacua structure
in various frameworks, from Type IIA to F-theory, interpreting the results in
the context of the Swampland Program. More specifically, we generalize the
bilinear formalism of the scalar potential to include the contributions of
geometric fluxes, which we use to perform a systematic search of vacua. We also
consider the 10d uplift of AdS4 vacua arising from the 4d massive Type IIA
effective theory with only RR and NSNS fluxes. Using the language of
SU(3)xSU(3) structures and performing an expansion around the smearing
approximation in powers of the string coupling, we study the stability of the
SUSY solution and its non-SUSY partner. We contrast the results with the Weak
Gravity Conjecture and the AdS instability conjecture in toroidal orbifold
examples and find that some non-SUSY cases are in tension with the predictions
of those conjectures. From the F-theory perspective, we study moduli
stabilization in the complex structure sector of elliptically fibered
Calabi-Yau 4-folds in the Large Complex Structure limit. Using homological
mirror symmetry, we replicate the analysis for the Type IIA case and give a
bilinear expression for the scalar potential, allowing for a detailed study of
the vacua structure. We find two distinct families of flux configurations
compatible with the tadpole constraints that enable full moduli stabilization.
We thoroughly examine the most generic one in the Type IIB limit, where the
superpotential is also quadratic and polynomial corrections can be considered
at all orders. Finally, we show that at this level of approximation
supersymmetric SUSY vacua always contain flat directions. We conclude with a
summary of the results and some comments about open questions and future lines
of research.Comment: Ph.D. Thesis (Advisor: Fernando Marchesano), 377 page
Elemental Abundance Survey of The Galactic Thick Disk
[Abridged abstract] We have performed an abundance analysis for 176 F- and G-
dwarfs of the Galactic thick disk component. Using accurate radial velocities
combined with astrometry, kinematics (U, V, and W) and Galactic
orbital parameters were computed. We estimate the probability for a star to
belong to the thin disk, the thick disk or the halo.
Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn, Y, Ba, Ce, Nd, and Eu have been obtained. The abundances for thick disk
stars are compared with those for thin disk members from Reddy et al. (2003).
The ratios of -elements (O, Mg, Si, Ca and Ti) to iron for thick disk
disk stars show a clear enhancement compared to thin disk members in the range
[Fe/H] . There are also other elements -- Al, Sc, V, Co, and
possibly Zn -- which show enhanced ratios to iron in the thick disk relative to
the thin disk. The abundances of Na, Cr, Mn, Ni, and Cu (relative to Fe) are
very similar for thin and thick disk stars. The dispersion in abundance ratios
[X/Fe] at given [Fe/H] for thick disk stars is consistent with the expected
scatter due to measurement errors, suggesting a lack of `cosmic' scatter. The
observed compositions of the thin and thick disks seem to be consistent with
models of galaxy formation by hierarchical clustering in a CDM
universe. In particular, the distinct abundance patterns observed in the thin
and thick disks, and the chemical homogeneity of the thick disk at different
galactocentric distances favor a scenario in which the majority of thick-disk
stars were formed {\it in situ}, from gas rich merging blocks.Comment: 57 pages (text: 27 pages in MNRAS format + 27 figures) Accepted for
publication in MNRA
Analysis and design of close operations at Phobos
This Thesis presents the analysis of a descent maneuver to the Martian
moon Phobos, treated mathematically as a Low-Thrust Optimal Control
Problem. In order to increase the accuracy of the problem, the gravity
acceleration at each of the trajectory nodes is obtained by applying the
Polyhedron method, discussed in Chapter 2.
The following document is structured by discussing rst the general aspects
of the problem analyzed, focusing then completely on the Optimal
Control Problem solved.
Firstly, the scienti c aims that a mission to Phobos presents are discussed,
together with the uncertainties that are still present when studying a mission
to this Martian moon. Then, a theoretical approach to the Polyhedron
method is discussed, as well as the reasons by which this method is
chosen ahead of others.
From that point on, a closer approach to the problem is made; rst,
by making a general theoretical discussion about the numerical methods
present in the GPOPS program that could be used to solve the problem
stated, reasoning then the election made and the aspects that have
prevailed for this decision to be taken. Subsequently, a detailed characterization
of both the dynamical and path constraints implemented is made,
as well as a brief description of the propulsive parameters chosen and the
reference frame selected, in the same fashion as other articles [1] that treat
a descent maneuver too.
Finally, the analysis is centered exclusively on the solving process followed.
The computational work carried out is explained rst, detailing
the modi cations made in order to overcome the computational resources'
problem that arose. The results for the optimization are then presented,
making a thorough description of the physical phenomena occurring in
the spacecraft dynamics. As concluding remarks, a summary of the results obtained is made, together
with their limitations; a roadmap for future work is added too,
setting the steps that should be taken in the future for making an even
more realistic study, which could be used as a rst pragmatic approach to
propose a real mission to Phobos.Ingeniería Aeroespacia
STIP VI - TARGET 9. RADIATION DAMAGE ASSESSMENT BASED ON A MCNPX SIMULATION
The SINQ Target Irradiation Program (STIP) has been developed at Paul Scherrer Institute
(PSI) during the last twenty years. It is aimed to analyze the radiation damage induced by
spallation reactions in different structural materials. It is also providing relevant data for
developing future spallation sources as well as advanced spallation targets.
STIP VI was conducted in SINQ Target 9 during 2011/2012 receiving a total proton charge of
13.16 Ah. The Monte Carlo N-Particle eXtended code (MCNPX) is used for implementing the
model and performing the simulation to obtain the proton and neutron fluences as well as the
energy deposition in the different points of interest. Afterwards, this information is used for
calculating the displacement per atom (dpa) plus the Helium and Hydrogen production in
atomic parts per million (appm). The results will contribute to understand the irradiation effect
in different structural materials. It will also be useful for comparing it with the real radiation
damage measurements of the irradiated materials after its cool down period.
The target mainly consists of the AlMg3 inverted semi-sphere Beam Entrance Window
(BEW) and the cylindrical Rod Container Box (RCB) which keeps inside 306 rods fitting 36
rows. There is also the Safety Hull Tube (SHT), made of AlMg3, which consists of a double
cylinder walls surrounding the RCB and linked to the inverted BEW at the bottom. The
source definition is implemented in the MCNPX input file from the gamma mapping
performed at the irradiated Target 9. The material specification of the specimen rods is
implemented by taking its average composition, assuming a uniform distribution.
The maximum proton flux obtained at the inverted BEW is 1.75x1014 p/(cm2
·s·mA). The
maximum proton fluence in the target is obtained in Row 0 becoming 8.33x1025 p/m2
. The
maximum neutron fluence is obtained in Row 8 with a value of 1.99x1026 n/m2
, which in flux
units stands for 4.19x1014 n/(cm2
·s·mA). Regarding the SHT, the maximum proton flux
escaping from the target is obtained above Row 36 with a value of 5.99x1011 p/(cm2
·s·mA)
and the maximum neutron flux is escaping around the middle part of the target with a value
of 1.39x1014 n/(cm2
·s·mA). The maximum energy deposition in Zircaloy-2 cladding tubes is
obtained in the center rod placed in Row 2 with a value of 400.32 W/(cm3
·mA).
Page 2
The irradiation parameters have been calculated by multiplying the proton and neutron
fluence with its corresponding material cross section data. The results obtained at the center
of the inverted BEW are 8.85 dpa, 2447 appm He and 4854 appm H. For the SHT, the
maximum values of the irradiation parameters are obtained around the middle part of the
target with the following values: 3.14 dpa, 17.63 appm He and 71.64 appm H.
Finally, the maximum displacement per atom in the specimen rods is obtained for Zircaloy-2
in Rod 1, placed in Row 2, with a value of 72.07 dpa and the maximum Helium & Hydrogen
concentration are obtained for Tantalum in Rod A, placed in Row 1, with the following values:
3629 appm He and 17418 appm H
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