765 research outputs found
Two Component Heat Diffusion Observed in CMR Manganites
We investigate the low-temperature electron, lattice, and spin dynamics of
LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance
spectroscopy. Probing the high-spin d-d transition as a function of time delay
and probe energy, we compare the responses of the Mott insulator and the
double-exchange metal to the photoexcitation. Attempts have previously been
made to describe the sub-picosecond dynamics of CMR manganites in terms of a
phenomenological three temperature model describing the energy transfer between
the electron, lattice and spin subsystems followed by a comparatively slow
exponential decay back to the ground state. However, conflicting results have
been reported. Here we first show clear evidence of an additional component in
the long term relaxation due to film-to-substrate heat diffusion and then
develop a modified three temperature model that gives a consistent account for
this feature. We confirm our interpretation by using it to deduce the bandgap
in LMO. In addition we also model the non-thermal sub-picosecond dynamics,
giving a full account of all observed transient features both in the insulating
LMO and the metallic LCMO.Comment: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434
v2: Abstract correcte
Periodic variable stars in CoRoT field LRa02 observed with BEST II
The Berlin Exoplanet Search Telescope II (BEST II) is a small wide
field-of-view photometric survey telescope system located at the Observatorio
Cerro Armazones, Chile. The high duty cycle combined with excellent observing
conditions and millimagnitude photometric precision makes this instrument
suitable for ground based support observations for the CoRoT space mission.
Photometric data of the CoRoT LRa02 target field collected between November
2008 and March 2009 were analysed for stellar variability. The presented
results will help in the future analysis of the CoRoT data, particularly in
additional science programs related to variable stars. BEST II observes
selected CoRoT target fields ahead of the space mission. The photometric data
acquired are searched for stellar variability, periodic variable stars are
identified with time series analysis of the obtained stellar light curves. We
obtained the light curves of 104335 stars in the CoRoT LRa02 field over 41
nights. Variability was detected in light curves of 3726 stars of which 350
showed a regular period. These stars are, with the exception of 5 previously
known variable stars, new discoveries.Comment: The figures with light curves can be find in the A&A journal as
online onl
Characterization of CoRoT target fields with BEST: Identification of periodic variable stars in the IR01 field
We report on observations of the CoRoT IR01 field with the Berlin Exoplanet
Search Telescope (BEST). BEST is a small aperture telescope with a wide field
of view (FOV). It is dedicated to search for variable stars within the target
fields of the CoRoT space mission to aid in minimizing false-alarm rates and
identify potential targets for additional science. CoRoT's observational
programm started in February 2007 with the "initial run" field (IR01) observed
for about two months. BEST observed this field for 12 nights spread over three
months in winter 2006. From the total of 30426 stars observed in the IR01 field
3769 were marked as suspected variable stars and 54 from them showed clear
periodicity. From these 19 periodic stars are within the part of the CoRoT FOV
covered in our data set
Estimating precipitation on early Mars using a radiative-convective model of the atmosphere and comparison with inferred runoff from geomorphology
We compare estimates of atmospheric precipitation during the Martian
Noachian-Hesperian boundary 3.8 Gyr ago as calculated in a radiative-convective
column model of the atmosphere with runoff values estimated from a
geomorphological analysis of dendritic valley network discharge rates. In the
atmospheric model, we assume CO2-H2O-N2 atmospheres with surface pressures
varying from 20 mb to 3 bar with input solar luminosity reduced to 75% the
modern value.
Results from the valley network analysis are of the order of a few mm d-1
liquid water precipitation (1.5-10.6 mm d-1, with a median of 3.1 mm d-1).
Atmospheric model results are much lower, from about 0.001-1 mm d-1 of snowfall
(depending on CO2 partial pressure). Hence, the atmospheric model predicts a
significantly lower amount of precipitated water than estimated from the
geomorphological analysis. Furthermore, global mean surface temperatures are
below freezing, i.e. runoff is most likely not directly linked to
precipitation. Therefore, our results strongly favor a cold early Mars with
episodic snowmelt as a source for runoff.
Our approach is challenged by mostly unconstrained parameters, e.g.
greenhouse gas abundance, global meteorology (for example, clouds) and
planetary parameters such as obliquity- which affect the atmospheric result -
as as well as by inherent problems in estimating discharge and runoff on
ancient Mars, such as a lack of knowledge on infiltration and evaporation rates
and on flooding timescales, which affect the geomorphological data.
Nevertheless, our work represents a first step in combining and interpreting
quantitative tools applied in early Mars atmospheric and geomorphological
studies.Comment: accepted in Planetary and Space Science, 37 pages, 14 figures, 2
table
Theoretical Transmission Spectra During Extrasolar Giant Planet Transits
The recent transit observation of HD 209458 b - an extrasolar planet orbiting
a sun-like star - confirmed that it is a gas giant and determined that its
orbital inclination is 85 degrees. This inclination makes possible
investigations of the planet atmosphere. In this paper we discuss the planet
transmission spectra during a transit. The basic tenet of the method is that
the planet atmosphere absorption features will be superimposed on the stellar
flux as the stellar flux passes through the planet atmosphere above the limb.
The ratio of the planet's transparent atmosphere area to the star area is
small, approximately 10^{-3} to 10^{-4}; for this method to work very strong
planet spectral features are necessary. We use our models of close-in
extrasolar giant planets to estimate promising absorption signatures: the
alkali metal lines, in particular the Na I and K I resonance doublets, and the
He I - triplet line at 1083.0 nm. If successful, observations
will constrain the line-of-sight temperature, pressure, and density. The most
important point is that observations will constrain the cloud depth, which in
turn will distinguish between different atmosphere models. We also discuss the
potential of this method for EGPs at different orbital distances and orbiting
non-solar-type stars.Comment: revised to agree with accepted paper, ApJ, in press. 12 page
Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module
The atmospheres of small, potentially rocky exoplanets are expected to cover
a diverse range in composition and mass. Studying such objects therefore
requires flexible and wide-ranging modeling capabilities. We present in this
work the essential development steps that lead to our flexible radiative
transfer module, REDFOX, and validate REDFOX for the Solar system planets
Earth, Venus and Mars, as well as for steam atmospheres. REDFOX is a
k-distribution model using the correlated-k approach with random overlap method
for the calculation of opacities used in the -two-stream approximation
for radiative transfer. Opacity contributions from Rayleigh scattering, UV /
visible cross sections and continua can be added selectively. With the improved
capabilities of our new model, we calculate various atmospheric scenarios for
K2-18b, a super-Earth / sub-Neptune with 8 M orbiting in the
temperate zone around an M-star, with recently observed HO spectral
features in the infrared. We model Earth-like, Venus-like, as well as H-He
primary atmospheres of different Solar metallicity and show resulting climates
and spectral characteristics, compared to observed data. Our results suggest
that K2-18b has an H-He atmosphere with limited amounts of HO and
CH. Results do not support the possibility of K2-18b having a water
reservoir directly exposed to the atmosphere, which would reduce atmospheric
scale heights, hence too the amplitudes of spectral features inconsistent with
the observations. We also performed tests for H-He atmospheres up to 50
times Solar metallicity, all compatible with the observations.Comment: 28 pages, 13 figures, accepted for publication in Ap
Variability survey in the CoRoT SRa01 field: Implications of eclipsing binary distribution on cluster formation in NGC 2264
Time-series photometry of the CoRoT field SRa01 was carried out with the
Berlin Exoplanet Search Telescope II (BEST II) in 2008/2009. A total of 1,161
variable stars were detected, of which 241 were previously known and 920 are
newly found. Several new, variable young stellar objects have been discovered.
The study of the spatial distribution of eclipsing binaries revealed the higher
relative frequency of Algols toward the center of the young open cluster NGC
2264. In general Algol frequency obeys an isotropic distribution of their
angular momentum vectors, except inside the cluster, where a specific
orientation of the inclinations is the case. We suggest that we see the orbital
plane of the binaries almost edge-on.Comment: 18 pages, 8 figures, accepted for publication in Ap
The Berlin Exoplanet Search Telescope II. Catalog of Variable Stars. I. Characterization of Three Southern Target Fields
A photometric survey of three Southern target fields with BEST II yielded the
detection of 2,406 previously unknown variable stars and an additional 617
stars with suspected variability. This study presents a catalog including their
coordinates, magnitudes, light curves, ephemerides, amplitudes, and type of
variability. In addition, the variability of 17 known objects is confirmed,
thus validating the results. The catalog contains a number of known and new
variables that are of interest for further astrophysical investigations, in
order to, e.g., search for additional bodies in eclipsing binary systems, or to
test stellar interior models.
Altogether, 209,070 stars were monitored with BEST II during a total of 128
nights in 2009/2010. The overall variability fraction of 1.2-1.5% in these
target fields is well comparable to similar ground-based photometric surveys.
Within the main magnitude range of , we identify
0.67(3)% of all stars to be eclipsing binaries, which indicates a completeness
of about one third for this particular type in comparison to space surveys.Comment: accepted to A
NGTS-4b: A sub-Neptune transiting in the desert
We report the discovery of NGTS-4b, a sub-Neptune-sized planet transiting a 13th magnitude K-dwarf in a 1.34 d orbit. NGTS-4b has a mass M = 20.6 ± 3.0 M⊕ and radius R = 3.18 ± 0.26 R⊕, which places it well within the so-called ‘Neptunian Desert’. The mean density of the planet (3.45 ± 0.95 g cm−3) is consistent with a composition of 100  per cent H2O or a rocky core with a volatile envelope. NGTS-4b is likely to suffer significant mass loss due to relatively strong EUV/X-ray irradiation. Its survival in the Neptunian desert may be due to an unusually high-core mass, or it may have avoided the most intense X-ray irradiation by migrating after the initial activity of its host star had subsided. With a transit depth of 0.13 ± 0.02 per cent, NGTS-4b represents the shallowest transiting system ever discovered from the ground, and is the smallest planet discovered in a wide-field ground-based photometric survey
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