2,398 research outputs found
Uniqueness of the Fock quantization of a free scalar field on with time dependent mass
We analyze the quantum description of a free scalar field on the circle in
the presence of an explicitly time dependent potential, also interpretable as a
time dependent mass. Classically, the field satisfies a linear wave equation of
the form . We prove that the representation of the
canonical commutation relations corresponding to the particular case of a
massless free field () provides a unitary implementation of the dynamics
for sufficiently general mass terms, . Furthermore, this representation
is uniquely specified, among the class of representations determined by
-invariant complex structures, as the only one allowing a unitary
dynamics. These conclusions can be extended in fact to fields on the two-sphere
possessing axial symmetry. This generalizes a uniqueness result previously
obtained in the context of the quantum field description of the Gowdy
cosmologies, in the case of linear polarization and for any of the possible
topologies of the spatial sections.Comment: 13 pages, typos corrected, version accepted for publication in
Physical Review
Asymptotics of Regulated Field Commutators for Einstein-Rosen Waves
We discuss the asymptotic behavior of regulated field commutators for
linearly polarized, cylindrically symmetric gravitational waves and the
mathematical techniques needed for this analysis. We concentrate our attention
on the effects brought about by the introduction of a physical cut-off in the
study of the microcausality of the model and describe how the different
physically relevant regimes are affected by its presence. Specifically we
discuss how genuine quantum gravity effects can be disentangled from those
originating in the introduction of a regulator.Comment: 9 figures, 19 pages in DIN A4 format. Accepted for publication in
Journal of Mathematical Physic
New and updated stellar parameters for 71 evolved planet hosts. On the metallicity - giant planet connection
It is still being debated whether the well-known metallicity - giant planet
correlation for dwarf stars is also valid for giant stars. For this reason,
having precise metallicities is very important. Different methods can provide
different results that lead to discrepancies in the analysis of planet hosts.
To study the impact of different analyses on the metallicity scale for evolved
stars, we compare different iron line lists to use in the atmospheric parameter
derivation of evolved stars. Therefore, we use a sample of 71 evolved stars
with planets. With these new homogeneous parameters, we revisit the metallicity
- giant planet connection for evolved stars. A spectroscopic analysis based on
Kurucz models in local thermodynamic equilibrium (LTE) was performed through
the MOOG code to derive the atmospheric parameters. Two different iron line
list sets were used, one built for cool FGK stars in general, and the other for
giant FGK stars. Masses were calculated through isochrone fitting, using the
Padova models. Kolmogorov-Smirnov tests (K-S tests) were then performed on the
metallicity distributions of various different samples of evolved stars and red
giants. All parameters compare well using a line list set, designed
specifically for cool and solar-like stars to provide more accurate
temperatures. All parameters derived with this line list set are preferred and
are thus adopted for future analysis. We find that evolved planet hosts are
more metal-poor than dwarf stars with giant planets. However, a bias in giant
stellar samples that are searched for planets is present. Because of a colour
cut-off, metal-rich low-gravity stars are left out of the samples, making it
hard to compare dwarf stars with giant stars. Furthermore, no metallicity
enhancement is found for red giants with planets (\,dex) with
respect to red giants without planets.Comment: 22 pages, 10 figures, 12 tables, accepted to A&
New and updated stellar parameters for 90 transit hosts. The effect of the surface gravity
Context. Precise stellar parameters are crucial in exoplanet research for
correctly determining of the planetary parameters. For stars hosting a
transiting planet, determining of the planetary mass and radius depends on the
stellar mass and radius, which in turn depend on the atmospheric stellar
parameters. Different methods can provide different results, which leads to
different planet characteristics.}%Spectroscopic surface gravities have shown
to be poorly constrained, but the photometry of the transiting planet can
provide an independent measurement of the surface gravity.
Aims. In this paper, we use a uniform method to spectroscopically derive
stellar atmospheric parameters, chemical abundances, stellar masses, and
stellar radii for a sample of 90 transit hosts. Surface gravities are also
derived photometrically using the stellar density as derived from the light
curve. We study the effect of using these different surface gravities on the
determination of the chemical abundances and the stellar mass and radius.
Methods. A spectroscopic analysis based on Kurucz models in LTE was performed
through the MOOG code to derive the atmospheric parameters and the chemical
abundances. The photometric surface gravity was determined through isochrone
fitting and the use of the stellar density, directly determined from the light
curve. Stellar masses and radii are determined through calibration formulae.
Results. Spectroscopic and photometric surface gravities differ, but this has
very little effect on the precise determination of the stellar mass in our
spectroscopic analysis. The stellar radius, and hence the planetary radius, is
most affected by the surface gravity discrepancies. For the chemical
abundances, the difference is, as expected, only noticable for the abundances
derived from analyzing of lines of ionized species.Comment: 12 pages, 6 figures, 5 tables, accepted to A&
Uniqueness of the Fock representation of the Gowdy and models
After a suitable gauge fixing, the local gravitational degrees of freedom of
the Gowdy and cosmologies are encoded in an axisymmetric
field on the sphere . Recently, it has been shown that a standard field
parametrization of these reduced models admits no Fock quantization with a
unitary dynamics. This lack of unitarity is surpassed by a convenient
redefinition of the field and the choice of an adequate complex structure. The
result is a Fock quantization where both the dynamics and the SO(3)-symmetries
of the field equations are unitarily implemented. The present work proves that
this Fock representation is in fact unique inasmuch as, up to equivalence,
there exists no other possible choice of SO(3)-invariant complex structure
leading to a unitary implementation of the time evolution.Comment: 10 pages, minor changes, version accepted for publication in
Classical and Quantum Gravit
NIR spectroscopy of the Sun and HD20010 - Compiling a new linelist in the NIR
Context: Effective temperature, surface gravity, and metallicity are basic
spectroscopic stellar parameters necessary to characterize a star or a
planetary system. Reliable atmospheric parameters for FGK stars have been
obtained mostly from methods that relay on high resolution and high
signal-to-noise optical spectroscopy. The advent of a new generation of high
resolution near-IR spectrographs opens the possibility of using classic
spectroscopic methods with high resolution and high signal-to-noise in the NIR
spectral window. Aims: We aim to compile a new iron line list in the NIR from a
solar spectrum to derive precise stellar atmospheric parameters, comparable to
the ones already obtained from high resolution optical spectra. The spectral
range covers 10 000 {\AA} to 25 000 {\AA}, which is equivalent to the Y, J, H,
and K bands. Methods: Our spectroscopic analysis is based on the iron
excitation and ionization balance done in LTE. We use a high resolution and
high signal-to-noise ratio spectrum of the Sun from the Kitt Peak telescope as
a starting point to compile the iron line list. The oscillator strengths (log
gf) of the iron lines were calibrated for the Sun. The abundance analysis was
done using the MOOG code after measuring equivalent widths of 357 solar iron
lines. Results: We successfully derived stellar atmospheric parameters for the
Sun. Furthermore, we analysed HD20010, a F8IV star, from which we derived
stellar atmospheric parameters using the same line list as for the Sun. The
spectrum was obtained from the CRIRES- POP database. The results are compatible
with the ones found in the literature, confirming the reliability of our line
list. However, due to the quality of the data we obtain large errors.Comment: 9 pages and 9 figure
WKB-type Approximation to Noncommutative Quantum Cosmology
In this work, we develop and apply the WKB approximation to several examples
of noncommutative quantum cosmology, obtaining the time evolution of the
noncommutative universe, this is done starting from a noncommutative quantum
formulation of cosmology where the noncommutativity is introduced by a
deformation on the minisuperspace variables. This procedure gives a
straightforward algorithm to incorporate noncommutativity to cosmology and
inflation.Comment: Revtex4, 6 pages, no figure
Chemical abundances of 1111 FGK stars from the HARPS GTO planet search program II: Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd and Eu
To understand the formation and evolution of the different stellar
populations within our Galaxy it is essential to combine detailed kinematical
and chemical information for large samples of stars. We derive chemical
abundances of Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd and Eu for a large sample of more
than 1000 FGK dwarf stars with high-resolution (\,115000) and
high-quality spectra from the HARPS-GTO program. The abundances are derived by
a standard Local Thermodinamyc Equilibrium (LTE) analysis using measured
Equivalent Widths (EWs) injected to the code MOOG and a grid of Kurucz ATLAS9
atmospheres. We find that thick disk stars are chemically disjunct for Zn and
Eu and also show on average higher Zr but lower Ba and Y when compared to the
thin disk stars. We also discovered that the previously identified
high- metal-rich population is also enhanced in Cu, Zn, Nd and Eu with
respect to the thin disk but presents Ba and Y abundances lower on average,
following the trend of thick disk stars towards higher metallities and further
supporting the different chemical composition of this population. The ratio of
heavy-s to light-s elements of thin disk stars presents the expected behaviour
(increasing towards lower metallicities) and can be explained by a major
contribution of low-mass AGB stars for s-process production at disk
metallicities. However, the opposite trend found for thick disk stars suggests
that intermediate-mass AGB stars played an important role in the enrichment of
the gas from where these stars formed. Previous works in the literature also
point to a possible primary production of light-s elements at low metallicities
to explain this trend. Finally, we also find an enhancement of light-s elements
in the thin disk at super solar metallicities which could be caused by the
contribution of metal-rich AGB stars. (short version)Comment: 20 pages, 19 figures, accepted by A&
Soil radon pulses related to the initial phase of volcanic euruptions
Soil radon behaviour related to the initial phase of volcanic eruptions is analysed from reported values related to the explosivity of four American stratovolcanoes: El Chichon (1982) and Popocatepetl (1994) in Mexico, Poas
(1987-1990) in Costa Rica and Cerro Negro (1995) in Nicaragua. The measurements in the field were performed with solid-state nuclear track detectors and electrets.
The ratio between the magnitudes of the radon in soil peaks generated when the eruptive period started and the average radon values corresponding to quiescence periods indicate a dependence on the Volcanic Eruptive Index for each one of the eruptive periods
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