169 research outputs found
Pioneer anomaly: What can we learn from LISA?
The Doppler tracking data from two deep-space spacecraft, Pioneer 10 and 11,
show an anomalous blueshift, which has been dubbed the "Pioneer anomaly". The
effect is most commonly interpreted as a real deceleration of the spacecraft -
an interpretation that faces serious challenges from planetary ephemerides. The
Pioneer anomaly could as well indicate an unknown effect on the radio signal
itself. Several authors have made suggestions how such a blueshift could be
related to cosmology. We consider this interpretation of the Pioneer anomaly
and study the impact of an anomalous blueshift on the Laser Interferometer
Space Antenna (LISA), a planned joint ESA-NASA mission aiming at the detection
of gravitational waves. The relative frequency shift (proportional to the light
travel time) for the LISA arm length is estimated to 10E-16, which is much
bigger than the expected amplitude of gravitational waves. The anomalous
blueshift enters the LISA signal in two ways, as a small term folded with the
gravitational wave signal, and as larger term at low frequencies. A detail
analysis shows that both contributions remain undetectable and do not impair
the gravitational-wave detection. This suggests that the Pioneer anomaly will
have to be tested in the outer Solar system regardless if the effect is caused
by an anomalous blueshift or by a real force.Comment: 19 pages, 4 figures. Talk given by D. Defrere at the conference
"Lasers, Clocks, and Drag-Free", ZARM, Bremen, Germany, 30 May - 1 June 200
Detection of exozodiacal dust: a step toward Earth-like planet characterization with infrared interferometry
The existence of other habitable worlds and the possible development of life elsewhere in the Universe have been among mankind's fundamental questions for thousands of years. These interrogations about our origins and place in the Universe are today at the dawn of being answered in scientific terms. The key year was 1995 with the discovery of the first extrasolar planet orbiting around a solar-type star. About 400 extrasolar planets are known today and the possibility to identify habitable worlds and even life among them largely contributes to the growing interest about their nature and properties. However, characterizing planetary systems is a very difficult task due to both the huge contrast and the small angular separation between the host stars and their environment. New techniques have emerged during the past decades with the purpose of tackling these fantastic observational challenges. In that context, infrared interferometry is a very promising technique, since it provides the required angular resolution to separate the emission of the star from that of its environment. This dissertation is devoted to the characterization of extrasolar planetary systems using the high angular resolution and dynamic range capabilities of infrared interferometric techniques. The first part of the present work is devoted to the detection with current interferometric facilities of warm dust within the first few astronomical units of massive debris discs around nearby stars. In order to extend the imaging of planetary systems to fainter discs and to extrasolar planets, we investigate in a second step the performance of future space-based nulling interferometers and make a comparison with ground-based projects. Finally, the third part of this work is dedicated to the impact of exozodiacal discs on the performance of future life-searching space missions, the goal being to characterize extrasolar planets with sizes down to that of the Earth
Mid-Infrared Imaging of Exo-Earths: Impact of Exozodiacal Disk Structures
The characterization of Earth-like extrasolar planets in the mid-infrared is a significant observational challenge that could be tackled by future space-based interferometers. The presence of large amounts of exozodiacal dust around nearby main sequence stars represents however a potential hurdle to obtain mid-infrared spectra of Earth-like planets. Whereas the disk brightness only affects the integration time, the emission of resonant dust structures mixes with the planet signal at the output of the interferometer and could jeopardize the spectroscopic analysis of an Earth-like planet. Fortunately, the high angular resolution provided by space-based interferometry is sufficient to spatially distinguish most of the extended exozodiacal emission from the planetary signal and only the dust located near the planet significantly contributes to the noise level. Considering modeled resonant structures created by Earth-like planets, we address in this talk the role of exozodiacal dust in two different cases: the characterization of Super-Earth planets with single space-based Bracewell interferometers (e.g., the FKSI mission) and the characterization of Earth-like planets with 4-telescope space-based nulling interferometers (e.g., the TPF-I and Darwin projects). In each case, we derive constraints on the disk parameters that can be tolerated without jeopardizing the detection of Earth-like planet
BelSAR : the first Belgian airborne campaign for L-band, full polarimetric bistatic and interferometric SAR acquisitions over an agricultural site in Belgium
The BelSAR airborne campaign was set up to prepare for future bistatic and interferometric SAR missions. The objective was to acquire L-band, full-polarimetric signatures by two airplanes, flying in various bistatic and interferometric geometries. The scientific scope was focused on agriculture and soil humidity, and the region of interest was part of the Belgian BELAIR HESBANIA test site, located close to Gembloux. Five campaigns took place between May and September 2018. Ground devices were deployed and ground measurements were obtained simultaneously to the imaging passes. The results will be exploited in a scientific project to be initiated in 2019.BelSA
Impact of {\eta}earth on the capabilities of affordable space missions to detect biosignatures on extrasolar planets
We present an analytic model to estimate the capabilities of space missions
dedicated to the search for biosignatures in the atmosphere of rocky planets
located in the habitable zone of nearby stars. Relations between performance
and mission parameters such as mirror diameter, distance to targets, and radius
of planets, are obtained. Two types of instruments are considered: coronagraphs
observing in the visible, and nulling interferometers in the thermal infrared.
Missions considered are: single-pupil coronagraphs with a 2.4 m primary mirror,
and formation flying interferometers with 4 x 0.75 m collecting mirrors. The
numbers of accessible planets are calculated as a function of {\eta}earth. When
Kepler gives its final estimation for {\eta}earth, the model will permit a
precise assessment of the potential of each instrument. Based on current
estimations, {\eta}earth = 10% around FGK stars and 50% around M stars, the
coronagraph could study in spectroscopy only ~1.5 relevant planets, and the
interferometer ~14.0. These numbers are obtained under the major hypothesis
that the exozodiacal light around the target stars is low enough for each
instrument. In both cases, a prior detection of planets is assumed and a target
list established. For the long-term future, building both types of
spectroscopic instruments, and using them on the same targets, will be the
optimal solution because they provide complementary information. But as a first
affordable space mission, the interferometer looks the more promising in term
of biosignature harvest.Comment: Accepted by Ap
Unveiling new stellar companions from the PIONIER exozodi survey
The main goal of the EXOZODI survey is to detect and characterize
circumstellar dust and to propose the first statistical study of exozodiacal
disks in the near-infrared using telescopes in both hemispheres. For this
purpose, Ertel et al. have conducted in 2012 a survey of nearby main sequence
stars with VLTI/PIONIER to search for the presence of circumstellar dust. This
survey, carried out during 12 nights, comprises about 100 stars. For each star,
we obtained typically three OBs and we searched for circumstellar emission
based on the measurement of squared visibilities at short baselines. A drop in
the measured visibilities with respect to the expected photospheric visibility
indicates the presence of resolved emission around the target star. It is
however generally not possible to conclude on the morphology of the detected
emission based solely on the squared visibilities. Here, we focus on closure
phases to search for faint companions around the whole sample. Indeed, to
derive robust statistics on the occurrence rate of bright exozodiacal disks, we
need to discriminate between companions and disks. For this reason, the main
goal of this paper is to discriminate between circumstellar disks (which show
no closure phase provided that they are point-symmetric) and faint companions
(point-like sources, creating non-zero closure phases). We also aim to reveal
new companions that do not necessarily produce a significant signature in the
squared visibilities, as the signature of the companion may show up more
prominently in the closure phases. In this process, we reveal four new stellar
companions with contrasts ranging from 2% to 95% (i.e., up to equal flux
binaries). We also tentatively detect faint companions around one other target
that will require follow-up observations to be confirmed or infirmed. We
discuss the implications of these discoveries on the results of the exozodi
survey.Comment: To appear in SPIE proceedings vol. 914
Searching for faint companions with VLTI/PIONIER. II. 92 main sequence stars from the Exozodi survey
The Exozodi survey aims to determine the occurrence rate of bright
exozodiacal discs around nearby main sequence stars using infrared
interferometry. Although the Exozodi survey targets have been carefully
selected to avoid the presence of binary stars, the results of this survey can
still be biased by the presence of unidentified stellar companions. Using the
PIONIER data set collected within the Exozodi survey, we aim to search for the
signature of point-like companions around the Exozodi target stars. We use both
the closure phases and squared visibilities collected by PIONIER to search for
companions within the ~100 mas interferometric field of view. The presence of a
companion is assessed by computing the goodness of fit to the data for a series
of binary models with various separations and contrasts. Five stellar
companions are resolved for the first time around five A-type stars: HD 4150,
HD 16555, HD 29388, HD 202730, and HD 224392 (although the companion to HD
16555 was independently resolved by speckle interferometry while we were
carrying out the survey). In the most likely case of main sequence companions,
their spectral types range from A5V to K4V. Three of these stars were already
suspected to be binaries from Hipparcos astrometric measurements, although no
information was available on the companions themselves so far. In addition to
debiasing the statistics of the Exozodi survey, these results can also be used
to revise the fraction of visual binaries among A-type stars, suggesting that
an extra ~13% A-type stars are visual binaries in addition to the ones detected
in previous direct imaging surveys. We estimate that about half the population
of nearby A-type stars could be resolved as visual binaries using a combination
of state-of-the-art interferometry and single-aperture imaging, and we suggest
that a significant fraction of these binaries remains undetected to date.Comment: Accepted for publication in A&
Vortex Image Processing (VIP) package for high-contrast direct imaging
VIP is a Python instrument-agnostic toolbox featuring a flexible framework for reproducible and robust data reduction. VIP currently supports three high-contrast imaging observational techniques: angular, reference-star and multi-spectral differential imaging. The code can be downloaded from our git repository on Github: http://github.com/vortex-exoplanet/VI
Delay Compensation for Real Time Disturbance Estimation at Extremely Large Telescopes
In ground-based astronomy, aberrations due to structural vibrations, such as piston, limit the achievable resolution and cannot be corrected using adaptive optics (AO) for large telescopes. We present a model-free strategy to estimate and compensate piston aberrations due to the vibrations of optical components using accelerometer disturbance feed forward, eventually allowing the use of fainter guide stars both for the fringe detector and in the AO loop. Because the correction performance is very sensitive to signal delays, we present a strategy to add a delay compensation to the developed disturbance estimator, which can, in principle, be applied to many other applications outside of astronomy that lack observer performance due to a measurement delay or need a prediction to compensate for input delays. The ability to estimate vibration disturbances in the critical frequency range of 8-60 Hz is demonstrated with on sky data from the Large Binocular Telescope (LBT) Interferometer, an interferometer at the LBT. The experimental results are promising, indicating the ability to suppress differential piston induced by telescope vibrations by a factor of about 3 (rms), which is significantly better than any currently commissioned system
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