298 research outputs found
Astrometric detection of exoplanets from the ground
Astrometry is a powerful technique to study the populations of extrasolar
planets around nearby stars. It gives access to a unique parameter space and is
therefore required for obtaining a comprehensive picture of the properties,
abundances, and architectures of exoplanetary systems. In this review, we
discuss the scientific potential, present the available techniques and
instruments, and highlight a few results of astrometric planet searches, with
an emphasis on observations from the ground. In particular, we discuss
astrometric observations with the Very Large Telescope (VLT) Interferometer and
a programme employing optical imaging with a VLT camera, both aimed at the
astrometric detection of exoplanets. Finally, we set these efforts into the
context of Gaia, ESA's astrometry mission scheduled for launch in 2013, and
present an outlook on the future of astrometric exoplanet detection from the
ground.Comment: 9 pages, 3 figures. Invited contribution to the SPIE conference
"Techniques and Instrumentation for Detection of Exoplanets VI" held in San
Diego, CA, August 25-29, 201
High dynamic range imaging by pupil single-mode filtering and remapping
Because of atmospheric turbulence, obtaining high angular resolution images
with a high dynamic range is difficult even in the near infrared domain of
wavelengths. We propose a novel technique to overcome this issue. The
fundamental idea is to apply techniques developed for long baseline
interferometry to the case of a single-aperture telescope. The pupil of the
telescope is broken down into coherent sub-apertures each feeding a single-mode
fiber. A remapping of the exit pupil allows interfering all sub-apertures
non-redundantly. A diffraction-limited image with very high dynamic range is
reconstructed from the fringe pattern analysis with aperture synthesis
techniques, free of speckle noise. The performances of the technique are
demonstrated with simulations in the visible range with an 8 meter telescope.
Raw dynamic ranges of 1: can be obtained in only a few tens of seconds of
integration time for bright objects.Comment: 5 pages, 3 figures. accepted for publication in MNRA
Science with the Keck Interferometer ASTRA Program
The ASTrometric and phase-Referenced Astronomy (ASTRA) project will provide
phase referencing and astrometric observations at the Keck Interferometer,
leading to enhanced sensitivity and the ability to monitor orbits at an
accuracy level of 30-100 microarcseconds. Here we discuss recent scientific
results from ASTRA, and describe new scientific programs that will begin in
2010-2011. We begin with results from the "self phase referencing" (SPR) mode
of ASTRA, which uses continuum light to correct atmospheric phase variations
and produce a phase-stabilized channel for spectroscopy. We have observed a
number of protoplanetary disks using SPR and a grism providing a spectral
dispersion of ~2000. In our data we spatially resolve emission from dust as
well as gas. Hydrogen line emission is spectrally resolved, allowing
differential phase measurements across the emission line that constrain the
relative centroids of different velocity components at the 10 microarcsecond
level. In the upcoming year, we will begin dual-field phase referencing (DFPR)
measurements of the Galactic Center and a number of exoplanet systems. These
observations will, in part, serve as precursors to astrometric monitoring of
stellar orbits in the Galactic Center and stellar wobbles of exoplanet host
stars. We describe the design of several scientific investigations capitalizing
on the upcoming phase-referencing and astrometric capabilities of ASTRA.Comment: Published in the proceedings of the SPIE 2010 conference on "Optical
and Infrared Interferometry II
Spatially and Spectrally Resolved Hydrogen Gas within 0.1 AU of T Tauri and Herbig Ae/Be Stars
We present near-infrared observations of T Tauri and Herbig Ae/Be stars with
a spatial resolution of a few milli-arcseconds and a spectral resolution of
~2000. Our observations spatially resolve gas and dust in the inner regions of
protoplanetary disks, and spectrally resolve broad-linewidth emission from the
Brackett gamma transition of hydrogen gas. We use the technique of
spectro-astrometry to determine centroids of different velocity components of
this gaseous emission at a precision orders of magnitude better than the
angular resolution. In all sources, we find the gaseous emission to be more
compact than or distributed on similar spatial scales to the dust emission. We
attempt to fit the data with models including both dust and Brackett
gamma-emitting gas, and we consider both disk and infall/outflow morphologies
for the gaseous matter. In most cases where we can distinguish between these
two models, the data show a preference for infall/outflow models. In all cases,
our data appear consistent with the presence of some gas at stellocentric radii
of ~0.01 AU. Our findings support the hypothesis that Brackett gamma emission
generally traces magnetospherically driven accretion and/or outflows in young
star/disk systems.Comment: 48 pages, including 17 figures. Accepted for publication by Ap
Reaching micro-arcsecond astrometry with long baseline optical interferometry; application to the GRAVITY instrument
A basic principle of long baseline interferometry is that an optical path
difference (OPD) directly translates into an astrometric measurement. In the
simplest case, the OPD is equal to the scalar product between the vector
linking the two telescopes and the normalized vector pointing toward the star.
However, a too simple interpretation of this scalar product leads to seemingly
conflicting results, called here "the baseline paradox". For micro-arcsecond
accuracy astrometry, we have to model in full the metrology measurement. It
involves a complex system subject to many optical effects: from pure baseline
errors to static, quasi-static and high order optical aberrations. The goal of
this paper is to present the strategy used by the "General Relativity Analysis
via VLT InTerferometrY" instrument (GRAVITY) to minimize the biases introduced
by these defects. It is possible to give an analytical formula on how the
baselines and tip-tilt errors affect the astrometric measurement. This formula
depends on the limit-points of three type of baselines: the wide-angle
baseline, the narrow-angle baseline, and the imaging baseline. We also,
numerically, include non-common path higher-order aberrations, whose amplitude
were measured during technical time at the Very Large Telescope Interferometer.
We end by simulating the influence of high-order common-path aberrations due to
atmospheric residuals calculated from a Monte-Carlo simulation tool for
Adaptive optics systems. The result of this work is an error budget of the
biases caused by the multiple optical imperfections, including optical
dispersion. We show that the beam stabilization through both focal and pupil
tracking is crucial to the GRAVITY system. Assuming the instrument pupil is
stabilized at a 4 cm level on M1, and a field tracking below 0.2, we
show that GRAVITY will be able to reach its objective of 10as accuracy.Comment: 14 pages. Accepted by A&
Disentangling Confused Stars at the Galactic Center with Long Baseline Infrared Interferometry
We present simulations of Keck Interferometer ASTRA and VLTI GRAVITY
observations of mock star fields in orbit within ~50 milliarcseconds of Sgr A*.
Dual-field phase referencing techniques, as implemented on ASTRA and planned
for GRAVITY, will provide the sensitivity to observe Sgr A* with infrared
interferometers. Our results show an improvement in the confusion noise limit
over current astrometric surveys, opening a window to study stellar sources in
the region. Since the Keck Interferometer has only a single baseline, the
improvement in the confusion limit depends on source position angles. The
GRAVITY instrument will yield a more compact and symmetric PSF, providing an
improvement in confusion noise which will not depend as strongly on position
angle. Our Keck results show the ability to characterize the star field as
containing zero, few, or many bright stellar sources. We are also able to
detect and track a source down to mK~18 through the least confused regions of
our field of view at a precision of ~200 microarcseconds along the baseline
direction. This level of precision improves with source brightness. Our GRAVITY
results show the potential to detect and track multiple sources in the field.
GRAVITY will perform ~10 microarcsecond astrometry on a mK=16.3 source and ~200
microarcsecond astrometry on a mK=18.8 source in six hours of monitoring a
crowded field. Monitoring the orbits of several stars will provide the ability
to distinguish between multiple post-Newtonian orbital effects, including those
due to an extended mass distribution around Sgr A* and to low-order General
Relativistic effects. Early characterizations of the field by ASTRA including
the possibility of a precise source detection, could provide valuable
information for future GRAVITY implementation and observation.Comment: Accepted for publication in Ap
The interferometric baselines and GRAVITY astrometric error budget
GRAVITY is a new generation beam combination instrument for the VLTI. Its
goal is to achieve microarsecond astrometric accuracy between objects separated
by a few arcsec. This accuracy on astrometric measurements is the most
important challenge of the instrument, and careful error budget have been
paramount during the technical design of the instrument. In this poster, we
will focus on baselines induced errors, which is part of a larger error budget.Comment: SPIE Meeting 2014 -- Montrea
First Faint Dual-field Off-axis Observations in Optical Long Baseline Interferometry
Ground-based long baseline interferometers have long been limited in sensitivity in part by the short integration periods imposed by atmospheric turbulence. The first observation fainter than this limit was performed on 2011 January 22 when the Keck Interferometer observed a K = 11.5 target, about 1 mag fainter than its K = 10.3 atmospherically imposed limit; the currently demonstrated limit is K = 12.5. These observations were made possible by the Dual-Field Phase-Referencing (DFPR) instrument, part of the NSF-funded ASTrometry and phase-Referenced Astronomy project; integration times longer than the turbulence time scale are made possible by its ability to simultaneously measure the real-time effects of the atmosphere on a nearby bright guide star and correct for it on the faint target. We present the implementation of DFPR on the Keck Interferometer. Then, we detail its on-sky performance focusing on the accuracy of the turbulence correction and the resulting fringe contrast stability
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