2,583 research outputs found
UHE nuclei propagation and the interpretation of the ankle in the cosmic-ray spectrum
We consider the stochastic propagation of high-energy protons and nuclei in
the cosmological microwave and infrared backgrounds, using revised photonuclear
cross-sections and following primary and secondary nuclei in the full 2D
nuclear chart. We confirm earlier results showing that the high-energy data can
be fit with a pure proton extragalactic cosmic ray (EGCR) component if the
source spectrum is \propto E^{-2.6}. In this case the ankle in the CR spectrum
may be interpreted as a pair-production dip associated with the propagation. We
show that when heavier nuclei are included in the source with a composition
similar to that of Galactic cosmic-rays (GCRs), the pair-production dip is not
present unless the proton fraction is higher than 85%. In the mixed composition
case, the ankle recovers the past interpretation as the transition from GCRs to
EGCRs and the highest energy data can be explained by a harder source spectrum
\propto E^{-2.2} - E^{-2.3}, reminiscent of relativistic shock acceleration
predictions, and in good agreement with the GCR data at low-energy and holistic
scenarios.Comment: 4 pages, 4 figures, submitted to A&A Letters (minor changes, two
figures replaced, two references added
Broad-band photometric colors and effective temperature calibrations for late-type giants. II. Z<0.02
(Abridged) We investigate the effects of metallicity on the broad-band
photometric colors of late-type giants, and make a comparison of synthetic
colors with observed photometric properties of late-type giants over a wide
range of effective temperatures (T_eff=3500-4800 K) and gravities (log
g=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on the
synthetic photometric colors is generally small at effective temperatures above
\~3800 K, but the effects grow larger at lower T_eff, due to the changing
efficiency of molecule formation which reduces molecular opacities at lower
[M/H]. To make a detailed comparison of the synthetic and observed photometric
colors of late type giants in the T_eff--color and color--color planes, we
derive a set of new T_eff--log g--color relations based on synthetic
photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. While differences
between the new T_eff--color relations and those available from the literature
are typically well within ~100 K, effective temperatures predicted by the
scales based on synthetic colors tend to be slightly higher than those
resulting from the T_eff--color relations based on observations, with the
offsets up to ~100 K. This is clearly seen both at [M/H]=-1.0 and -2.0,
especially in the T_eff--(B-V) and T_eff--(V-K) planes. The consistency between
T_eff--log g--color scales based on synthetic colors calculated with different
stellar atmosphere codes is very good, with typical differences being well
within \Delta T_eff~70 K at [M/H]=-1.0 and \Delta T_eff~40 K at [M/H]=-2.0.Comment: 20 pages, 11 figures, A&A accepte
Metallicity of M dwarfs III. Planet-metallicity and planet-stellar mass correlations of the HARPS GTO M dwarf sample
Aims. The aim of this work is the study of the planet-metallicity and the
planet-stellar mass correlations for M dwarfs from the HARPS GTO M dwarf
subsample
Methods. We use a new method that takes advantage of the HARPS
high-resolution spectra to increase the precision of metallicity, using
previous photometric calibrations of [Fe/H] and effective temperature as
starting values.
Results. In this work we use our new calibration (rms = 0.08 dex) to study
the planet-metallicity relation of our sample. The well-known correlation for
Giant planet FGKM hosts with metallicity is present. Regarding Neptunians and
smaller hosts no correlation is found but there is a hint that an
anti-correlation with [Fe/H] may exist. We combined our sample with the
California Planet Survey late-K and M-type dwarf sample to increase our
statistics but found no new trends. We fitted a power law to the frequency
histogram of the Jovian hosts for our sample and for the combined sample, f_p =
C10^\alpha[Fe/H], using two different approaches: a direct bin fitting and a
bayesian fitting procedure. We obtained a value for C between 0.02 and 0.04 and
for \alpha between 1.26 and 2.94.
Regarding stellar mass, an hypothetical correlation with planets was
discovered, but was found to be the result of a detection bias.Comment: Accepted for publication in A&A. 18 pages, 11 Figures, 12 Table
Metallicity of M dwarfs IV. A high-precision [Fe/H] and Teff technique from high-resolution optical spectra for M dwarfs
Aims. In this work we develop a technique to obtain high precision
determinations of both metallicity and effective temperature of M dwarfs in the
optical.
Methods. A new method is presented that makes use of the information of 4104
lines in the 530-690 nm spectral region. It consists in the measurement of
pseudo equivalent widths and their correlation with established scales of
[Fe/H] and .
Results. Our technique achieves a of 0.080.01 for [Fe/H],
9113 K for , and is valid in the (-0.85, 0.26 dex), (2800, 4100
K), and (M0.0, M5.0) intervals for [Fe/H], and spectral type
respectively. We also calculated the RMSE which estimates uncertainties
of the order of 0.12 dex for the metallicity and of 293 K for the effective
temperature. The technique has an activity limit and should only be used for
stars with . Our method is available
online at \url{http://www.astro.up.pt/resources/mcal}.Comment: Accepted in Astronomy and Astrophysics. Updated one important
reference in the introduction. Some typos correcte
Ultra High Energy Cosmic Rays: The disappointing model
We develop a model for explaining the data of Pierre Auger Observatory
(Auger) for Ultra High Energy Cosmic Rays (UHECR), in particular, the mass
composition being steadily heavier with increasing energy from 3 EeV to 35 EeV.
The model is based on the proton-dominated composition in the energy range (1 -
3) EeV observed in both Auger and HiRes experiments. Assuming extragalactic
origin of this component, we argue that it must disappear at higher energies
due to a low maximum energy of acceleration, E_p^{\max} \sim (4 - 10) EeV.
Under an assumption of rigidity acceleration mechanism, the maximum
acceleration energy for a nucleus with the charge number Z is ZE_p^{\max}, and
the highest energy in the spectrum, reached by Iron, does not exceed (100 -
200) EeV. The growth of atomic weight with energy, observed in Auger, is
provided by the rigidity mechanism of acceleration, since at each energy
E=ZE_p^{\max} the contribution of nuclei with Z' < Z vanishes. The described
model has disappointing consequences for future observations in UHECR: Since
average energies per nucleon for all nuclei are less than (2 - 4) EeV, (i) pion
photo-production on CMB photons in extragalactic space is absent; (ii) GZK
cutoff in the spectrum does not exist; (iii) cosmogenic neutrinos produced on
CMBR are absent; (iv) fluxes of cosmogenic neutrinos produced on infrared -
optical background radiation are too low for registration by existing detectors
and projects. Due to nuclei deflection in galactic magnetic fields, the
correlation with nearby sources is absent even at highest energies.Comment: Essentially revised version as published in Astropart. Physics 10
pages, 6 figure
Signatures of the extragalactic cosmic-ray source composition from spectrum and shower depth measurements
We discuss the differences induced by the assumed composition of extragalactic sources on the predicted UHECR spectrum and the energy evolution of the cosmic-ray shower . We show that different assumptions for the source power evolution do not modify our earlier finding that in the case of a mixed composition the ankle can be interpreted as the end of the transition from galactic to extragalactic cosmic rays. We show that the features associated with this transition in each cosmic-ray source model are essentially independent of the assumed hadronic model. In the mixed composition cases, a signature of the interactions of nuclei with the photon backgrounds is also expected above eV. The comparisons with Stereo HiRes and Fly's Eye data favour an extragalactic mixed composition and the corresponding interpretation of the ankle. Confrontation of model predictions with future data at the highest energies will allow a better determination of the transition features and of the cosmic-ray source composition, independently of hadronic models. We also emphasize that in the pure proton case, a combined analysis of the spectrum and composition below the ankle could lead to constraints on the source power evolution with redshift
On the transition from Galactic to extragalactic cosmic-rays: spectral and composition features from two opposite scenarios
We study the phenomenology of cosmic-rays (CRs) at the galactic/extragalactic
transition, focusing on two opposite models for the composition of the
extragalactic (EG) component. Model A assumes a mixed source composition, with
nuclear abundances similar to that of the low-energy CRs, while model B assumes
that the EG sources accelerate only protons. We study the limits within which
both scenarios can reproduce the high-energy CR data, from the point of view of
the energy spectrum, each with its own interpretation of the ankle (as the
GCR/EGCR transition for model A; as a pair-production dip for model B) and its
own source spectrum (in E?2.3 model A; in E?2.6 or E?2.7 for model B). We then
focus on the energy evolution of the high-energy CR composition and compare our
predictions for two main composition-related observables (Xmax and hlnAi) with
the available data, to conclude that model A is currently favoured.
Uncertainties are discussed and distinctive features of the two models are
identified, which should allow one to distinguish between the models in the
near future when more precise measurements are available with higher-statistics
experiments.Comment: 26 pages 9 figures. Calculations updated using a recent IRB density
and evolution (see text). Minor quantitative changes, conclusions unchange
Signature of a three-dimensional photonic band gap observed on silicon inverse woodpile photonic crystals
We have studied the reflectivity of CMOS-compatible three-dimensional silicon
inverse woodpile photonic crystals at near-infrared frequencies.
Polarization-resolved reflectivity spectra were obtained from two orthogonal
crystal surfaces corresponding to 1.88 pi sr solid angle. The spectra reveal
broad peaks with high reflectivity up to 67 % that are independent of the
spatial position on the crystals. The spectrally overlapping reflectivity peaks
for all directions and polarizations form the signature of a broad photonic
band gap with a relative bandwidth up to 16 %. This signature is supported with
stopgaps in plane wave bandstructure calculations and with the frequency region
of the expected band gap.Comment: 9 pages, 5 figure
Photometric colors of late-type giants: theory versus observations
To assess the current status in the theoretical modeling of the spectral
properties of late-type giants, we provide a comparison of synthetic
photometric colors of late-type giants (calculated with PHOENIX, MARCS and
ATLAS model atmospheres) with observations, at [M/H]=0.0 and -2.0. Overall,
there is a good agreement between observed and synthetic colors, and synthetic
colors and published Teff-color relations, both at [M/H]=0.0 and -2.0.
Deviations from the observed trends in Teff-color planes are generally within
\pm 150K (or less) in the effective temperature range Teff=3500-4800K.
Synthetic colors calculated with different stellar atmosphere models typically
agree to ~100K, within a large range of effective temperatures and gravities.
Some discrepancies are seen in the Teff-(B-V) plane below Teff~3800K at
[M/H]=0.0, due to difficulties in reproducing the 'turn-off' to the bluer
colors which is seen in the observed data at Teff~3600K. Note that at
[M/H]=-2.0 effective temperatures given by the scale of Alonso et al. (1999)
are generally lower than those resulting from other Teff-color relations based
both on observed and synthetic colors.Comment: 2 pages, 1 figure. Proceedings of the IAU Symposium 232 "The
Scientific Requirements for Extremely Large Telescopes", eds. P. Whitelock,
B. Leibundgut, and M. Dennefel
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