70 research outputs found
Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes
Less than 3% of the known exoplanets were directly imaged for two main
reasons. They are angularly very close to their parent star, which is several
magnitudes brighter. Direct imaging of exoplanets thus requires a dedicated
instrumentation with large telescopes and accurate wavefront control devices
for high-angular resolution and coronagraphs for attenuating the stellar light.
Coronagraphs are usually chromatic and they cannot perform high-contrast
imaging over a wide spectral bandwidth. That chromaticity will be critical for
future instruments. Enlarging the coronagraph spectral range is a challenge for
future exoplanet imaging instruments on both space-based and ground-based
telescopes. We propose the multi-stage four-quadrant phase mask that associates
several monochromatic four-quadrant phase mask coronagraphs in series.
Monochromatic device performance has already been demonstrated and the
manufacturing procedures are well-under control since their development for
previous instruments on VLT and JWST. The multi-stage implementation simplicity
is thus appealing. We present the instrument principle and we describe the
laboratory performance for large spectral bandwidths and for both pupil shapes
for space- (off-axis telescope) and ground-based (E-ELT) telescopes. The
multi-stage four-quadrant phase mask reduces the stellar flux over a wide
spectral range (30%) and it is a very good candidate to be associated with a
spectrometer for future exoplanet imaging instruments in ground- and
space-based observatories.Comment: 7 pages, 11 figures, 4 tables, accepted in A&
Near-UV to near-IR disk-averaged Earth's reflectance spectra
We report 320 to 1020nm disk-averaged Earth reflectance spectra obtained from
Moon's Earthshine observations with the EMMI spectrograph on the NTT at ESO La
Silla (Chile). The spectral signatures of Earth atmosphere and ground
vegetation are observed. A vegetation red-edge of up to 9% is observed on
Europe and Africa and ~2% upon Pacific Ocean. The spectra also show that Earth
is a blue planet when Rayleigh scattering dominates, or totally white when the
cloud cover is large.Comment: Proceeding of an oral prensentation at the UAI No200 Colloquiu
A test for the search for life on extrasolar planets: Looking for the terrestrial vegetation signature in the Earthshine spectrum
We report spectroscopic observations (400 to 800nm, R = approx 100) of
Earthshine in June, July and October 2001 from which normalised Earth albedo
spectra have been derived. The resulting spectra clearly show the blue colour
of the Earth due to Rayleigh diffusion in its atmosphere. They also show the
signatures of oxygen, ozone and water vapour. We tried to extract from these
spectra the signature of Earth vegetation. A variable signal (4 to 10 +/-3%)
around 700nm has been measured in the Earth albedo. It is interpreted as being
due to the vegetation red edge, expected to be between 2 to 10% of the Earth
albedo at 700nm, depending on models. We discuss the primary goal of the
present observations: their application to the detection of vegetation-like
biosignatures on extrasolar planets.Comment: 7 pages, 7 figures. A&A, accepted 6 May 200
The four-quadrant phase-mask coronagraph: white light laboratory results with an achromatic device
Achromatic coronagraphs are the subject of intensive research since they will be mandatory for many programs which aim at detecting and characterizing exoplanets. We report a laboratory experiment assessing the performance of the Four-Quadrant Phase-Mask coronagraph (FQPM) over a broadband wavelength range (R≈2). The achromatization of the FQPM is provided by achromatic halfwave plates (HWP). These phase shifters combine birefringent plates made of different materials with properly chosen thicknesses. The HWP thickness control is relaxed by two orders of magnitudes with respect to the classical (non-birefringent) dispersive plate approach. In our experiment we used a two stage stack of Quartz and MgF_2. This combination allows to cover a large spectral range in the visible (500-900nm) with a small phase error residual around π(≈0.12 rad rms). With this achromatization, we obtained an attenuation of 755 on the white light PSF peak. This solution is directly applicable to ground-based telescopes using high order adaptive optics such as the ESO's VLT-Planet Finder project and could easily be transposed in the mid-infrared domain for future space-based missions like DARWIN/TPF
Coronagraphic imaging of three weak-line T Tauri stars: evidence of planetary formation around PDS 70
Context.High angular resolution imaging of nearby pre-main sequence stars with ages between 1 and 30 Myr can give valuable information on planet formation mechanisms. This range of ages is thought to correspond to the dissipation of the optically thick dust disks surrounding young stars and to the end of the planet formation.
Aims.This paper presents new observations of three weak-line T Tauri Stars (WTTS) of intermediate ages ranging from 7 to 16 Myr. It aims at increasing the knowledge and sample of circumstellar disks around "old" WTTS.
Methods.We observed three stars with the VLT's NAOS-CONICA adaptive optics system in coronagraphic mode. The four-quadrant phase mask coronagraph was used to improve the dynamic range (by a factor of ~100) while preserving the high angular resolution (inner working angle of 0".15).
Results.One object of our sample (PDS 70), a K5 star, exhibits a brown dwarf companion and a disk in scattered light with a surface brightness power law of r^-2.8, extending from a distance of 14 to 140 AU (assuming a stellar distance of 140 pc) and an integrated luminosity of 16.7 mJy in the K_s-band. The mass of the companion can be estimated to be within a range between 27 and 50 Jupiter masses with an effective temperature of 2750 ± 100K. This object also shows a resolved outflow stretching up to ~550 AU.
Conclusions.This newly detected circumstellar disk shows strong similarities with the disk around TW Hya, and adds to the observed population of "old" TTS surrounded by circumstellar material. Moreover, three clues of planetary formation are brought to light by this study
Near-UV to near-IR disk-averaged Earth's spectra from Moon's Earthshine observations
We discuss a series of Earthshine spectra obtained with the NTT/EMMI
instrument between 320nm and 1020nm with a resolution of R~450 in the blue and
R~250 in the red. These ascending and descending Moon's Earthshine spectra
taken from Chile give disk-averaged spectra for two different Earth's phases.
The spectra show the ozone (Huggins and Chappuis bands), oxygen and water
vapour absorption bands, and also the stronger Rayleigh scattering in the blue.
Removing the known telluric absorptions reveals a spectral feature around 700nm
which is attributed to the vegetation stronger reflectivity in the near-IR,
so-called vegetation red-edge.Comment: 2 pages, 1 figure, proceedings from a poster at 'Semaine de
l'Astrophysique Francaise', 27th june-1st july 2005, Strasbour
Precise Wavefront Correction with an Unbalanced Nulling Interferometer for Exo-Planet Imaging Coronagraphs
Very high dynamical range coronagraphs targeting direct exo-planet detection
(10^9 - 10^10 contrast) at small angular separation (few lambda/D units)
usually require an input wavefront quality on the order of ten thousandths of
wavelength RMS. We propose a novel method based on a pre-optics setup that
behaves partly as a low-efficiency coronagraph, and partly as a
high-sensitivity wavefront aberration compensator (phase and amplitude). The
combination of the two effects results in a highly accurate corrected
wavefront. First, an (intensity-) unbalanced nulling interferometer (UNI)
performs a rejection of part of the wavefront electric field. Then the
recombined output wavefront has its input aberrations magnified. Because of the
unbalanced recombination scheme, aberrations can be free of phase singular
points (zeros) and can therefore be compensated by a downstream phase and
amplitude correction (PAC) adaptive optics system, using two deformable
mirrors. In the image plane, the central star's peak intensity and the noise
level of its speckled halo are reduced by the UNI-PAC combination: the
output-corrected wavefront aberrations can be interpreted as an improved
compensation of the initial (eventually already corrected) incident wavefront
aberrations. The important conclusion is that not all the elements in the
optical setup using UNI-PAC need to reach the lambda/10000 rms surface error
quality.Comment: Accepted for publication in A&
Morphology of the very inclined debris disk around HD 32297
Direct imaging of circumstellar disks at high angular resolution is mandatory
to provide morphological information that bring constraints on their
properties, in particular the spatial distribution of dust. New techniques
combining observing strategy and data processing now allow very high contrast
imaging with 8-m class ground-based telescopes (10^-4 to 10^-5 at ~1") and
complement space telescopes while improving angular resolution at near infrared
wavelengths. We carried out a program at the VLT with NACO to image known
debris disks with higher angular resolution in the near IR than ever before in
order to study morphological properties and ultimately to detect signpost of
planets. The observing method makes use of advanced techniques: Adaptive
Optics, Coronagraphy and Differential Imaging, a combination designed to
directly image exoplanets with the upcoming generation of "planet finders" like
GPI (Gemini Planet Imager) and SPHERE (Spectro-Polarimetric High contrast
Exoplanet REsearch). Applied to extended objects like circumstellar disks, the
method is still successful but produces significant biases in terms of
photometry and morphology. We developed a new model-matching procedure to
correct for these biases and hence to bring constraints on the morphology of
debris disks. From our program, we present new images of the disk around the
star HD 32297 obtained in the H (1.6mic) and Ks (2.2mic) bands with an
unprecedented angular resolution (~65 mas). The images show an inclined thin
disk detected at separations larger than 0.5-0.6". The modeling stage confirms
a very high inclination (i=88{\deg}) and the presence of an inner cavity inside
r_0~110AU. We also found that the spine (line of maximum intensity along the
midplane) of the disk is curved and we attributed this feature to a large
anisotropic scattering factor (g~0.5, valid for an non-edge on disk). Abridged
...Comment: 12 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Laboratory comparison of coronagraphic concepts under dynamical seeing and high-order adaptive optics correction
The exoplanetary science through direct imaging and spectroscopy will largely
expand with the forthcoming development of new instruments at the VLT (SPHERE),
Gemini (GPI), Subaru (HiCIAO), and Palomar (Project 1640) observatories. All
these ground-based adaptive optics instruments combine extremely high
performance adaptive optics (XAO) systems correcting for the atmospheric
turbulence with advanced starlight-cancellation techniques such as coronagraphy
to deliver contrast ratios of about 10-6 to 10-7. While the past fifteen years
have seen intensive research and the development of high-contrast coronagraph
concepts, very few concepts have been tested under dynamical seeing conditions
(either during sky observation or in a realistic laboratory environment). In
this paper, we discuss the results obtained with four different coronagraphs --
phase and amplitude types -- on the High-Order Testbench (HOT), the adaptive
optics facility developed at ESO. This facility emphasizes realistic conditions
encountered at a telescope (e.g., VLT), including a turbulence generator and a
high-order adaptive optics system. It enables to evaluate the performance of
high-contrast coronagraphs in the near-IR operating with an AO-corrected PSF of
90% Strehl ratio under 0.5 arcsec dynamical seeing.Comment: Accepted for publication in MNRA
Direct imaging of extra-solar planets in star forming regions: Lessons learned from a false positive around IM Lup
Most exoplanet imagers consist of ground-based adaptive optics coronagraphic
cameras which are currently limited in contrast, sensitivity and astrometric
precision, but advantageously observe in the near-IR (1- 5{\mu}m). Because of
these practical limitations, our current observational aim at detecting and
characterizing planets puts heavy constraints on target selection, observing
strategies, data reduction, and follow-up. Most surveys so far have thus
targeted young systems (1-100Myr) to catch the putative remnant thermal
radiation of giant planets, which peaks in the near-IR. They also favor systems
in the solar neighborhood (d<80pc), which eases angular resolution requirements
but also ensures a good knowledge of the distance and proper motion, which are
critical to secure the planet status, and enable subsequent characterization.
Because of their youth, it is very tempting to target the nearby star forming
regions, which are typically twice as far as the bulk of objects usually combed
for planets by direct imaging. Probing these interesting reservoirs sets
additional constraints that we review in this paper by presenting the planet
search that we initiated in 2008 around the disk-bearing T Tauri star IM Lup
(Lupus star forming region, 140-190pc). We show and discuss why age
determination, the choice of evolutionary model for the central star and the
planet, precise knowledge of the host star proper motion, relative or absolute
astrometric accuracy, and patience are the key ingredients for exoplanet
searches around more distant young stars. Unfortunately, most of the time,
precision and perseverance are not paying off: we discovered a candidate
companion around IM Lup in 2008, which we report here to be an unbound
background object. We nevertheless review in details the lessons learned from
our endeavor, and additionally present the best detection limits ever
calculated for IM Lup.Comment: 8 pages, 3 figures, 3 tables, accepted to A&
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