285 research outputs found
Photometric stability analysis of the Exoplanet Characterisation Observatory
Photometric stability is a key requirement for time-resolved spectroscopic
observations of transiting extrasolar planets. In the context of the Exoplanet
Characterisation Observatory (EChO) mission design, we here present and
investigate means of translating spacecraft pointing instabilities as well as
temperature fluctuation of its optical chain into an overall error budget of
the exoplanetary spectrum to be retrieved. Given the instrument specifications
as of date, we investigate the magnitudes of these photometric instabilities in
the context of simulated observations of the exoplanet HD189733b secondary
eclipse.Comment: submitted to MNRA
EChOSim: The Exoplanet Characterisation Observatory software simulator
EChOSim is the end-to-end time-domain simulator of the Exoplanet
Characterisation Observatory (EChO) space mission. EChOSim has been developed
to assess the capability EChO has to detect and characterize the atmospheres of
transiting exoplanets, and through this revolutionize the knowledge we have of
the Milky Way and of our place in the Galaxy. Here we discuss the details of
the EChOSim implementation and describe the models used to represent the
instrument and to simulate the detection. Software simulators have assumed a
central role in the design of new instrumentation and in assessing the level of
systematics affecting the measurements of existing experiments. Thanks to its
high modularity, EChOSim can simulate basic aspects of several existing and
proposed spectrometers for exoplanet transits, including instruments on the
Hubble Space Telescope and Spitzer, or ground-based and balloon borne
experiments. A discussion of different uses of EChOSim is given, including
examples of simulations performed to assess the EChO mission
Carglumic acid: an additional therapy in the treatment of organic acidurias with hyperammonemia?
<p>Abstract</p> <p>Background</p> <p>Hyperammonemia in patients with methylmalonic aciduria (MMA) and propionic aciduria (PA) is caused by accumulation of propionyl-CoA which decreases the synthesis of N-acetyl-glutamate, the natural activator of carbamyl phosphate synthetase 1. A treatment approach with carglumic acid, the structural analogue of N-acetyl-glutamate, has been proposed to decrease high ammonia levels encountered in MMA and PA crises.</p> <p>Case presentation</p> <p>We described two patients (one with MMA and one with PA) with hyperammonemia at diagnosis. Carglumic acid, when associated with standard treatment of organic acidurias, may be helpful in normalizing the ammonia level.</p> <p>Conclusion</p> <p>Even though the usual treatment which decreases toxic metabolites remains the standard, carglumic acid could be helpful in lowering plasma ammonia levels over 400 micromol/L more rapidly.</p
Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band
Our objective is to prove that integrated optics (IO) is not only a good
concept for astronomical interferometry but also a working technique with high
performance. We used the commissioning data obtained with the dedicated K-band
integrated optics two-telescope beam combiner which now replaces the fiber
coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of
this IO device and compare its properties to other single mode beam combiner
like the previously used MONA fiber coupler. The IO combiner provides a high
optical throughput, a contrast of 89% with a night-to-night stability of a few
percent. Even if a dispersive phase is present, we show that it does not bias
the measured Fourier visibility estimate. An upper limit of 0.005 for the
cross-talk between linear polarization states has been measured. We take
advantage of the intrinsic contrast stability to test a new astronomical
prodecure for calibrating diameters of simple stars by simultaneously fitting
the instrumental contrast and the apparent stellar diameters. This method
reaches an accuracy with diameter errors of the order of previous ones but
without the need of an already known calibrator. These results are an important
step of integrated optics and paves the road to incoming imaging interferometer
projects
Tests of stellar model atmospheres by optical interferometry: VLTI/VINCI limb-darkening measurements of the M4 giant psi phe
We present K-band interferometric measurements of the limb-darkened intensity
profile of the M4 giant star psi Phoenicis obtained with VLTI/VINCI.
High-precision squared visibility amplitudes in the second lobe of the
visibility function were obtained employing two 8.2 m UTs. This succeeded one
month after light from UTs was first combined for interferometric fringes. In
addition, we sampled the visibility function at small spatial frequencies using
the 40cm test siderostats. Our measurement constrains the diameter of the star
as well as its CLV. We construct a spherical hydrostatic PHOENIX model
atmosphere based on spectrophotometric data from the literature and confront
its CLV prediction with our interferometric measurement. We compare as well CLV
predictions by plane-parallel hydrostatic PHOENIX, ATLAS9, and ATLAS12 models.
We find that the Rosseland angular diameter as predicted by comparison of the
spherical PHOENIX model with spectrophotometry is in good agreement with our
interferometric diameter measurement. The shape of our measured visibility
function in the second lobe is consistent with all considered PHOENIX and ATLAS
model predictions, and significantly different from UD and FDD models. We
derive high-precision fundamental parameters for psi Phe, namely a Rosseland
angular diameter of 8.13 +/- 0.2 mas, with the Hipparcos parallax corresponding
to a Rosseland linear radius R of 86 +/- 3 Rsun and an effective temperature of
3550 +/- 50 K, with R corresponding to a luminosity of log (L/Lsun)=3.02 +/-
0.06. Together with evolutionary models, these values are consistent with a
mass of 1.3 +/- 0.2 Msun, and a surface gravity of log g = 0.68 +/- 0.11.Comment: 13 pages, 6 figures, accepted for publication in A&
The angular diameter and distance of the Cepheid Zeta Geminorum
Cepheids are the primary distance indicators for extragalactic astronomy and
therefore are of very high astrophysical interest. Unfortunately, they are rare
stars, situated very far from Earth.Though they are supergiants, their typical
angular diameter is only a few milliarcseconds, making them very challenging
targets even for long-baseline interferometers. We report observations that
were obtained in the K prime band (2-2.3 microns), on the Cepheid Zeta
Geminorum with the FLUOR beam combiner, installed at the IOTA interferometer.
The mean uniform disk angular diameter was measured to be 1.64 +0.14 -0.16 mas.
Pulsational variations are not detected at a significant statistical level, but
future observations with longer baselines should allow a much better estimation
of their amplitude. The distance to Zeta Gem is evaluated using Baade-Wesselink
diameter determinations, giving a distance of 502 +/- 88 pc.Comment: 8 pages, 3 figure
Nulling interferometry: performance comparison between Antarctica and other ground-based sites
Detecting the presence of circumstellar dust around nearby solar-type main
sequence stars is an important pre-requisite for the design of future
life-finding space missions such as ESA's Darwin or NASA's Terrestrial Planet
Finder (TPF). The high Antarctic plateau may provide appropriate conditions to
perform such a survey from the ground. We investigate the performance of a
nulling interferometer optimised for the detection of exozodiacal discs at Dome
C, on the high Antarctic plateau, and compare it to the expected performance of
similar instruments at temperate sites. Based on the currently available
measurements of the turbulence characteristics at Dome C, we adapt the GENIEsim
software (Absil et al. 2006, A&A 448) to simulate the performance of a nulling
interferometer on the high Antarctic plateau. To feed a realistic instrumental
configuration into the simulator, we propose a conceptual design for ALADDIN,
the Antarctic L-band Astrophysics Discovery Demonstrator for Interferometric
Nulling. We assume that this instrument can be placed above the 30-m high
boundary layer, where most of the atmospheric turbulence originates. We show
that an optimised nulling interferometer operating on a pair of 1-m class
telescopes located 30 m above the ground could achieve a better sensitivity
than a similar instrument working with two 8-m class telescopes at a temperate
site such as Cerro Paranal. The detection of circumstellar discs about 20 times
as dense as our local zodiacal cloud seems within reach for typical Darwin/TPF
targets in a integration time of a few hours. Moreover, the exceptional
turbulence conditions significantly relax the requirements on real-time control
loops, which has favourable consequences on the feasibility of the nulling
instrument.Comment: 10 pages, accepted for publication in A&
The radius and effective temperature of the binary Ap star beta CrB from CHARA/FLUOR and VLT/NACO observations
The prospects for using asteroseismology of rapidly oscillating Ap (roAp)
stars are hampered by the large uncertainty in fundamental stellar parameters.
Results in the literature for the effective temperature (Teff) often span a
range of 1000 K. Our goal is to reduce systematic errors and improve the Teff
calibration of Ap stars based on new interferometric measurements. We obtained
long-baseline interferometric observations of beta CrB using the CHARA/FLUOR
instrument. To disentangle the flux contributions of the two components of this
binary star, we obtained VLT/NACO adaptive optics images. We determined limb
darkened angular diameters of 0.699+-0.017 mas for beta CrB A (from
interferometry) and 0.415+-0.017 mas for beta CrB B (from surface brightness-
color relations), corresponding to radii of 2.63+-0.09 Rsun (3.4 percent
uncertainty) and 1.56+-0.07 Rsun (4.5 percent). The combined bolometric flux of
the A and B components was determined from satellite UV data, spectrophotometry
in the visible and broadband data in the infrared. The flux from the B
component constitutes 16+-4 percent of the total flux and was determined by
fitting an ATLAS9 model atmosphere to the broad-band NACO J and K magnitudes.
Combining the flux of the A component with its measured angular diameter, we
determine the effective temperature Teff(A) = 7980+-180 K (2.3 percent). Our
new interferometric and imaging data enable a nearly model-independent
determination of the effective temperature of beta CrB A. Including our recent
study of alpha Cir, we now have direct Teff measurements of two of the
brightest roAp stars, providing a strong benchmark for an improved calibration
of the Teff scale for Ap stars. This will support the use of potentially strong
constraints imposed by asteroseismic studies of roAp stars.Comment: 7 pages, accepted by A&
Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models
We present K'-band observations of five Mira stars with the IOTA
interferometer. The interferograms were obtained with the FLUOR fiber optics
beam combiner, which provides high-accuracy visibility measurements in spite of
time-variable atmospheric conditions. For the M-type Miras X Oph, R Aql, RU
Her, R Ser, and the C-type Mira V CrB we derived the uniform-disk diameters
11.7mas, 10.9mas, 8.4mas, 8.1mas, and 7.9mas (+/- 0.3mas), respectively.
Simultaneous photometric observations yielded the bolometric fluxes. The
derived angular Rosseland radii and the bolometric fluxes allowed the
determination of effective temperatures. For instance, the effective
temperature of R Aql was determined to be 2970 +/- 110 K. A linear Rosseland
radius for R Aql of (250 +100/-60) Rsun was derived from the angular Rosseland
radius of 5.5mas +/- 0.2mas and the HIPPARCOS parallax of 4.73mas +/- 1.19mas.
The observations were compared with theoretical Mira star models of Bessel et
al. (1996) and Hofmann et al. (1998). The effective temperatures of the M-type
Miras and the linear radius of R Aql indicate fundamental mode pulsation.Comment: 12 pages, 4 postscript figure
Asteroseismology and interferometry of the red giant star epsilon Oph
The GIII red giant star epsilon Oph has been found to exhibit several modes
of oscillation by the MOST mission. We interpret the observed frequencies of
oscillation in terms of theoretical radial p-mode frequencies of stellar
models. Evolutionary models of this star, in both shell H-burning and core
He-burning phases of evolution, are constructed using as constraints a
combination of measurements from classical ground-based observations (for
luminosity, temperature, and chemical composition) and seismic observations
from MOST. Radial frequencies of models in either evolutionary phase can
reproduce the observed frequency spectrum of epsilon Oph almost equally well.
The best-fit models indicate a mass in the range of 1.85 +/- 0.05 Msun with
radius of 10.55 +/- 0.15 Rsun. We also obtain an independent estimate of the
radius of epsilon Oph using high accuracy interferometric observations in the
infrared K' band, using the CHARA/FLUOR instrument. The measured limb darkened
disk angular diameter of epsilon Oph is 2.961 +/- 0.007 mas. Together with the
Hipparcos parallax, this translates into a photospheric radius of 10.39 +/-
0.07 Rsun. The radius obtained from the asteroseismic analysis matches the
interferometric value quite closely even though the radius was not constrained
during the modelling.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic
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