13,990 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
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
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
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
Schwinger's Method for the Massive Casimir Effect
We apply to the massive scalar field a method recently proposed by Schwinger
to calculate the Casimir effect. The method is applied with two different
regularization schemes: the Schwinger original one by means of Poisson formula
and another one by means of analytical continuation.Comment: plain TeX, 6 pages, DFTUZ-93-2
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.
Exact 1/N and Optimized Perturbative Evaluation of mu_c for Homogeneous Interacting Bose Gases
In the framework of the O(N) three-dimensional effective scalar field model
for homogeneous dilute weakly interacting Bose gases we use the 1/N expansion
to evaluate, within the large N limit, the parameter r_c which is directly
related to the critical chemical potential mu_c. This quantity enters the
order-a^2 n^{2/3} coefficient contributing to the critical temperature shift
Delta T_c where a represents the s-wave scattering length and n represents the
density. Compared to the recent precise numerical lattice simulation results,
our calculation suggests that the large N approximation performs rather well
even for the physical case N=2. We then calculate the same quantity but using
different forms of the optimized perturbative (variational) method, showing
that these produce excellent results both for the finite N and large-N cases.Comment: 12 pages, 2 figures. We have performed a refined and extended
numerical analysis to take into account the very recent results of Ref. [15
O solo de várzea nas margens do rio Negro - Amazonas - Brasil.
Este trabalho teve como objetivo amostrar, caracterizar e estimar a área com predominância de um solo tipicamente de várzea (eutrófico) nas margens de um rio de água preta (rio Negro)
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