15,427 research outputs found
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
The role of translational invariance in non linear gauge theories of gravity
The internal structure of the tetrads in a Poincar\'e non linear gauge theory
of gravity is considered. Minkowskian coordinates becomes dynamical degrees of
freedom playing the role of Goldstone bosons of the translations. A critical
length allowing a covariant expansion similar to the weak field approach is
deduced, the zeroth order metric being maximally symmetric (Minkowskian in some
cases).Comment: 17 pages, LaTe
Testing the Relation Between the Local and Cosmic Star Formation Histories
Recently, there has been great progress toward observationally determining
the mean star formation history of the universe. When accurately known, the
cosmic star formation rate could provide much information about Galactic
evolution, if the Milky Way's star formation rate is representative of the
average cosmic star formation history. A simple hypothesis is that our local
star formation rate is proportional to the cosmic mean. In addition, to specify
a star formation history, one must also adopt an initial mass function (IMF);
typically it is assumed that the IMF is a smooth function which is constant in
time. We show how to test directly the compatibility of all these assumptions,
by making use of the local (solar neighborhood) star formation record encoded
in the present-day stellar mass function. Present data suggests that at least
one of the following is false: (1) the local IMF is constant in time; (2) the
local IMF is a smooth (unimodal) function; and/or (3) star formation in the
Galactic disk was representative of the cosmic mean. We briefly discuss how to
determine which of these assumptions fail, and improvements in observations
which will sharpen this test.Comment: 14 pages in LaTeX (uses aaspp4.sty). 5 postscript figures. To appear
in the Astrophysical Journa
Perspectiva biomimética do ninho de andorinha-dos-beirais
A terra é um material natural, ecológico, reciclável e abundante e, como tal, foi sempre e volta agora a
ser considerado como sendo um dos materiais/soluções de construção de eleição, com enorme
potencial. Na construção tradicional portuguesa encontram-se várias soluções construtivas
recorrendo ao uso da terra crua, com uma importante expressão. Este património construído
necessita de ser conservado. Neste contexto, o presente trabalho tem como principal objectivo dar
um contributo na temática das soluções de melhoramento do comportamento e reforço de elementos
construtivos em terra a partir do estudo biomimético de um ninho de andorinha-dos-beirais. De modo
a identificar a eventual existência de algum fenómeno de aglutinação ocorrido durante o processo de
construção de um ninho de andorinha-dos-beirais, procedeu-se a um trabalho experimental de
caracterização a partir de amostras recolhidas em diferentes ninhos encontrados na zona de Vila
Real. Para a identificação da composição química inorgânica e mineralógica das amostras recolhidas
realizaram-se análises em Scanning Electron Microscopy / Energy Dispersive Spectroscopy
(SEM/EDS) e por difracção de raios-X. Para a identificação da composição orgânica das amostras
recolhidas foi feita a quantificação colorimétrica do teor de proteínas pelo método de biureto, e do teor
de polissacarídeos/açucares pelo método dos açúcares totais. Os resultados experimentais relativos
às amostras retiradas dos ninhos quando comparados com resultados similares relativos a amostras
de argila recolhidas na mesma zona de origem dos ninhos, são muito semelhantes, destacando-se
apenas a presença de polissacarídeos/açucares nas amostras de ninho.
Pretende-se que a partir dos resultados da caracterização do material constituinte dos ninhos se
possam retirar sugestões para o desenvolvimento de soluções de estabilização e de melhoramento
de elementos construtivos existentes de terra crua
Possible Stellar Metallicity Enhancements from the Accretion of Planets
A number of recently discovered extrasolar planet candidates have
surprisingly small orbits, which may indicate that considerable orbital
migration takes place in protoplanetary systems. A natural consequence of
orbital migration is for a series of planets to be accreted, destroyed, and
then thoroughly mixed into the convective envelope of the central star. We
study the ramifications of planet accretion for the final main sequence
metallicity of the star. If maximum disk lifetimes are on the order of 10 Myr,
stars with masses near 1 solar mass are predicted to have virtually no
metallicity enhancement. On the other hand, early F and late A type stars with
masses of 1.5--2.0 solar masses can experience significant metallicity
enhancements due to their considerably smaller convection zones during the
first 10 Myr of pre-main-sequence evolution. We show that the metallicities of
an aggregate of unevolved F stars are consistent with an average star accreting
about 2 Jupiter-mass planets from a protoplanetary disk having a 10 Myr
dispersal time.Comment: 14 pages, AAS LaTeX, 3 figures, accepted to ApJ Letter
A Schmidt-Kennicutt law for star formation in the Milky Way disk
We use a new method to trace backwards the star formation history of the
Milky Way disk, using a sample of M dwarfs in the solar neighbourhood which is
representative for the entire solar circle. M stars are used because they show
H_alpha emission until a particular age which is a well calibrated function of
their absolute magnitudes. This allows us to reconstruct the rate at which disk
stars have been born over about half the disk's lifetime. Our star formation
rate agrees well with those obtained by using other, independent, methods and
seems to rule out a constant star formation rate.
The principal result of this study is to show that a relation of the
Schmidt-Kennicut type (which relates the star formation rate to the
interstellar gas content of galaxy disks) has pertained in the Milky Way disk
during the last 5 Gyr. The star formation rate we derive from the M dwarfs and
the interstellar gas content of the disk can be inferred as a function of time
from a model of the chemical enrichment of the disk, which is well constrained
by the observations indicating that the metallicity of the Galactic disk has
remained nearly constant over the timescales involved. We demonstrate that the
star formation rate and gas surface densities over the last 5 Gyrs can be
accurately described by a Schmidt-Kennicutt law with an index of Gamma = 1.45
(+0.22,-0.09). This is, within statistical uncertainties, the same value found
for other galaxies.Comment: 7 pages, 5 figures, accepted by Astron.
Effective action in DSR1 quantum field theory
We present the one-loop effective action of a quantum scalar field with DSR1
space-time symmetry as a sum over field modes. The effective action has real
and imaginary parts and manifest charge conjugation asymmetry, which provides
an alternative theoretical setting to the study of the particle-antiparticle
asymmetry in nature.Comment: 8 page
Improving the Knowledge on Seismogenic Sources in the Lower Tagus Valley for Seismic Hazard Purposes
The Lower Tagus Valley, that includes the metropolitan area of Lisbon, has been struck by several earthquakes which produced significant material damage and loss of lives. Their exact location remains unknown. Our goal is to shed some light into the seismogenic sources in the area using seismic reflection and geological data. In areas with no seismic coverage, potential-field data interpretation was carried out. Seismicity was overlaid to the potential seismogenic structures and high-resolution data was acquired in order to confirm which structures have been active into the Quaternary. Three major fault-zones affecting the Neogene were identified: V. F. Xira, Samora-Alcochete and Pinhal Novo. For the first fault, strong evidences suggest it is active. The other two fault-zones and other structures previously unknown can be correlated with several epicentres. Empirical relationships between maximum moment magnitude and fault area indicate that MW > 6.5 earthquakes can be expected for the larger structures
A new improved optimization of perturbation theory: applications to the oscillator energy levels and Bose-Einstein critical temperature
Improving perturbation theory via a variational optimization has generally
produced in higher orders an embarrassingly large set of solutions, most of
them unphysical (complex). We introduce an extension of the optimized
perturbation method which leads to a drastic reduction of the number of
acceptable solutions. The properties of this new method are studied and it is
then applied to the calculation of relevant quantities in different
models, such as the anharmonic oscillator energy levels and the critical
Bose-Einstein Condensation temperature shift recently investigated
by various authors. Our present estimates of , incorporating the
most recently available six and seven loop perturbative information, are in
excellent agreement with all the available lattice numerical simulations. This
represents a very substantial improvement over previous treatments.Comment: 9 pages, no figures. v2: minor wording changes in title/abstract, to
appear in Phys.Rev.
Interpersonal functioning in obsessive-compulsive personality disorder
The core symptoms of obsessive-compulsive personality disorder (OCPD) often lead to interpersonal difficulties. However, little research has explored interpersonal functioning in OCPD. This study examined interpersonal problems, interpersonal sensitivities, empathy, and systemizing, the drive to analyze and derive underlying rules for systems, in a sample of 25 OCPD individuals, 25 individuals with comorbid OCPD and obsessive-compulsive disorder (OCD), and 25 healthy controls. We found that OCPD individuals reported hostile-dominant interpersonal problems and sensitivities with warm-dominant behavior by others, whereas OCPD+OCD individuals reported submissive interpersonal problems and sensitivities with warm-submissive behavior by others. Individuals with OCPD, with and without OCD, reported less empathic perspective taking relative to healthy controls. Finally, we found that OCPD males reported a higher drive to analyze and derive rules for systems than OCPD females. Overall, results suggest that there are interpersonal deficits associated with OCPD and the clinical implications of these deficits are discussed
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