2,254 research outputs found
Evidence for Inflation in an Axion Landscape
We discuss inflation models within supersymmetry and supergravity frameworks
with a landscape of chiral superfields and one shift symmetry which is
broken by non-perturbative symmetry breaking terms in the superpotential. We
label the pseudo scalar component of the chiral fields axions and their real
parts saxions. Thus in the models only one combination of axions will be a
pseudo-Nambu-Goldstone-boson which will act as the inflaton. The proposed
models constitute consistent inflation for the following reasons: The inflation
potential arises dynamically with stabilized saxions, the axion decay constant
can lie in the sub-Planckian region, and consistency with the Planck data is
achieved. The axion landscape consisting of axion pairs is assumed with the
axions in each pair having opposite charges. A fast roll--slow roll splitting
mechanism for the axion potential is proposed which is realized with a special
choice of the axion basis. In this basis the coupled equations split into
equations which enter in the fast roll and there is one unique linear
combination of the fields which controls the slow roll and thus the power
spectrum of curvature and tensor perturbations. It is shown that a significant
part of the parameter space exists where inflation is successful, i.e., , the spectral index of curvature perturbations, and
the ratio of the power spectrum of tensor perturbations and curvature
perturbations, lie in the experimentally allowed regions given by the Planck
experiment. Further, it is shown that the model allows for a significant region
of the parameter space where the effective axion decay constant can lie in the
sub-Planckian domain.Comment: 31 pages, 10 figures. Accepted for publication in JHE
The evolution of luminosity, colour and the mass-to-luminosity ratio of Galactic open clusters: comparison of discrete vs. continuous IMF models
(abridged) We found in previous studies that standard Simple Stellar
Population (SSP) models are unable to describe or explain the colours of
Galactic open clusters both in the visible and in the NIR spectral range. (...)
We construct a numerical SSP-model, with an underlying Salpeter IMF, valid
within an upper and lower stellar mass range, and with total masses
typical of open clusters. We assume that the mass
loss from a cluster is provided by mass loss from evolved stars and by the
dynamical evaporation of low-mass members due to two-body relaxation. The data
for the latter process were scaled to the models from high-resolution N-body
calculations. We also investigate how a change of the -limit influences
magnitudes and colours of clusters of a given mass and derive a necessary
condition for a luminosity and colour flash. The discreteness of the IMF leads
to bursts in magnitude and colour of model clusters at moments when red
supergiants or giants appear and then die. The amplitude of the burst depends
on the cluster mass and on the spectral range; it is strongly increased in the
NIR compared to optical passbands. In the discrete case, variations of the
parameter are able to substantially change the magnitude-age and
-age relations. For the colours, the lowering of considerably
amplifies the discreteness effect. The influence of dynamical mass loss on
colour and magnitude is weak, although it provides a change of the slopes of
the considered relations, improving their agreement with observations. For the
Galactic open clusters we determined luminosity and tidal mass independent of
each other. The derived mass-to-luminosity ratio shows, on average, an increase
with cluster age in the optical, but gradually declines with age in the NIR.
The observed flash statistics can be used to constrain in open clusters.Comment: 15 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Calibration of radii and masses of open clusters with a simulation
Context: A recent new approach to apply a simple dynamical mass estimate of
tidally limited star clusters is based on the identification of the tidal
radius in a King profile with the dynamical Jacobi radius. The application to
an unbiased open cluster catalogue yields significantly higher cluster masses
compared to the classical methods. Aims: We quantify the bias in the mass
determination as function of projection direction and cluster age by analysing
a simulated star cluster. Methods: We use direct -body simulations of a star
cluster including stellar evolution in an analytic Milky Way potential and
apply a best fit to the projected number density of cluster stars. Results: We
obtain significantly overestimated star cluster masses which depend strongly on
the viewing direction. The overestimation is typically in the range of 10-50
percent and reaches a factor of 3.5 for young clusters. Mass segregation
reduces the derived limiting radii systematically.Comment: 9 pages, 10+1 figures, accepted by Astronomy and Astrophysic
Using near infra-red spectroscopy for characterization of transiting exoplanets
We propose a method for observing transiting exoplanets with near-infrared
high-resolution spectrometers. We aim to create a robust data analysis method
for recovering atmospheric transmission spectra from transiting exoplanets over
a wide wavelength range in the near infrared. By using an inverse method
approach, combined with stellar models and telluric transmission spectra, the
method recovers the transiting exoplanet's atmospheric transmittance at high
precision over a wide wavelength range. We describe our method and have tested
it by simulating observations. This method is capable of recovering
transmission spectra of high enough accuracy to identify absorption features
from molecules such as O2, CH4, CO2, and H2O. This accuracy is achievable for
Jupiter-size exoplanetsat S/N that can be reached for 8m class telescopes using
high-resolution spectrometers (R>20 000) during a single transit, and for
Earth-size planets and super-Earths transiting late K or M dwarf stars at S/N
reachable during observations of less than 10 transits. We also analyse
potential error sources to show the robustness of the method. Detection and
characterization of atmospheres of both Jupiter-size planets and smaller rocky
planets looks promising using this set-up.Comment: 14 pages, 14 figures, accepted to A&
No magnetic field variation with pulsation phase in the roAp star gamma Equulei
We present an analysis of 210 high-resolution time-resolved
spectropolarimetric observations of the roAp star gamma Equ obtained over three
nights in August and September 2003. Radial velocity variations due to p-mode
non-radial pulsations are clearly detected in the lines of rare-earth elements,
in particular Pr III, Nd II and Nd III. In contrast, we find absolutely no
evidence for the variation of the mean longitudinal magnetic field over the
pulsation period in gamma Equ at the level of 110-240 G which was recently
reported by Leone & Kurtz (2003). Our investigation of the variability of
circularly polarized profiles of 13 Nd III lines demonstrates that, at the 3
sigma confidence level, no magnetic field variation with an amplitude above
40-60 G was present in gamma Equ during our monitoring of this star.Comment: accepted for publication by Astronomy & Astrophysic
Least squares deconvolution of the stellar intensity and polarization spectra
Least squares deconvolution (LSD) is a powerful method of extracting
high-precision average line profiles from the stellar intensity and
polarization spectra. Despite its common usage, the LSD method is poorly
documented and has never been tested using realistic synthetic spectra. In this
study we revisit the key assumptions of the LSD technique, clarify its
numerical implementation, discuss possible improvements and give
recommendations how to make LSD results understandable and reproducible. We
also address the problem of interpretation of the moments and shapes of the LSD
profiles in terms of physical parameters. We have developed an improved,
multiprofile version of LSD and have extended the deconvolution procedure to
linear polarization analysis taking into account anomalous Zeeman splitting of
spectral lines. This code is applied to the theoretical Stokes parameter
spectra. We test various methods of interpreting the mean profiles,
investigating how coarse approximations of the multiline technique translate
into errors of the derived parameters. We find that, generally, the Stokes
parameter LSD profiles do not behave as a real spectral line with respect to
the variation of magnetic field and elemental abundance. This problem is
especially prominent for the Stokes I variation with abundance and Stokes Q
variation with magnetic field. At the same time, the Stokes V LSD spectra
closely resemble profile of a properly chosen synthetic line for the magnetic
field strength up to 1 kG. We conclude that the usual method of interpreting
the LSD profiles by assuming that they are equivalent to a real spectral line
gives satisfactory results only in a limited parameter range and thus should be
applied with caution. A more trustworthy approach is to abandon the single-line
approximation of the average profiles and apply LSD consistently to
observations and synthetic spectra.Comment: Accepted for publication in Astronomy & Astrophysics; 15 pages, 12
figures; second version includes minor language correction
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