27 research outputs found
A new algorithm for point spread function subtraction in high-contrast imaging: a demonstration with angular differential imaging
Direct imaging of exoplanets is limited by bright quasi-static speckles in
the point spread function (PSF) of the central star. This limitation can be
reduced by subtraction of reference PSF images. We have developed an algorithm
to construct an optimized reference PSF image from a set of reference images.
This image is built as a linear combination of the reference images available
and the coefficients of the combination are optimized inside multiple
subsections of the image independently to minimize the residual noise within
each subsection. The algorithm developed can be used with many high-contrast
imaging observing strategies relying on PSF subtraction, such as angular
differential imaging (ADI), roll subtraction, spectral differential imaging,
reference star observations, etc. The performance of the algorithm is
demonstrated for ADI data. It is shown that for this type of data the new
algorithm provides a gain in sensitivity by up to a factor 3 at small
separation over the algorithm used in Marois et al. (2006).Comment: 7 pages, 11 figures, to appear in May 10, 2007 issue of Ap
Confidence Level and Sensitivity Limits in High Contrast Imaging
In long adaptive optics corrected exposures, exoplanet detections are
currently limited by speckle noise originating from the telescope and
instrument optics, and it is expected that such noise will also limit future
high-contrast imaging instruments for both ground and space-based telescopes.
Previous theoretical analysis have shown that the time intensity variations of
a single speckle follows a modified Rician. It is first demonstrated here that
for a circular pupil this temporal intensity distribution also represents the
speckle spatial intensity distribution at a fix separation from the point
spread function center; this fact is demonstrated using numerical simulations
for coronagraphic and non-coronagraphic data. The real statistical distribution
of the noise needs to be taken into account explicitly when selecting a
detection threshold appropriate for some desired confidence level. In this
paper, a technique is described to obtain the pixel intensity distribution of
an image and its corresponding confidence level as a function of the detection
threshold. Using numerical simulations, it is shown that in the presence of
speckles noise, a detection threshold up to three times higher is required to
obtain a confidence level equivalent to that at 5sigma for Gaussian noise. The
technique is then tested using TRIDENT CFHT and angular differential imaging
NIRI Gemini adaptive optics data. It is found that the angular differential
imaging technique produces quasi-Gaussian residuals, a remarkable result
compared to classical adaptive optic imaging. A power-law is finally derived to
predict the 1-3*10^-7 confidence level detection threshold when averaging a
partially correlated non-Gaussian noise.Comment: 29 pages, 13 figures, accepted to Ap
IRAS04325+2402C: A very low mass object with an edge-on disk
IRAS04325+2402C is a low luminosity object located near a protostar in
Taurus. We present new spatially-resolved mm observations, near-infrared
spectroscopy, and Spitzer photometry that improve the constraints on the nature
of this source. The object is clearly detected in our 1.3 mm interferometry
map, allowing us to estimate the mass in a localized disk+envelope around it to
be in the range of 0.001 to 0.01Ms. Thus IRAS04325C is unlikely to accrete
significantly more mass. The near-infrared spectrum cannot be explained with an
extincted photosphere alone, but is consistent with a 0.03-0.1Ms central source
plus moderate veiling, seen in scattered light, confirming the edge-on nature
of the disk. Based on K-band flux and spectral slope we conclude that a central
object mass >~0.1Ms is unlikely. Our comparison of the full spectral energy
distribution, including new Spitzer photometry, with radiative transfer models
confirms the high inclination of the disk (>~80deg), the very low mass of the
central source, and the small amount of circumstellar material. IRAS04325C is
one of the lowest mass objects with a resolved edge-on disk known to date,
possibly a young brown dwarf, and a likely wide companion to a more massive
star. With these combined properties, it represents a unique case to study the
formation and early evolution of very low mass objects.Comment: 5 pages, 3 figures, Astrophysical Journal Letters, in pres
Accurate Astrometry and Photometry of Saturated and Coronagraphic Point Spread Functions
Accurate astrometry and photometry of saturated and coronagraphic point
spread functions (PSFs) are fundamental to both ground- and space-based high
contrast imaging projects. For ground-based adaptive optics imaging,
differential atmospheric refraction and flexure introduce a small drift of the
PSF with time, and seeing and sky transmission variations modify the PSF flux
distribution. For space-based imaging, vibrations, thermal fluctuations and
pointing jitters can modify the PSF core position and flux. These effects need
to be corrected to properly combine the images and obtain optimal
signal-to-noise ratios, accurate relative astrometry and photometry of detected
objects as well as precise detection limits. Usually, one can easily correct
for these effects by using the PSF core, but this is impossible when high
dynamic range observing techniques are used, like coronagrahy with a
non-transmissive occulting mask, or if the stellar PSF core is saturated. We
present a new technique that can solve these issues by using off-axis satellite
PSFs produced by a periodic amplitude or phase mask conjugated to a pupil
plane. It will be shown that these satellite PSFs track precisely the PSF
position, its Strehl ratio and its intensity and can thus be used to register
and to flux normalize the PSF. A laboratory experiment is also presented to
validate the theory. This approach can be easily implemented in existing
adaptive optics instruments and should be considered for future extreme
adaptive optics coronagraph instruments and in high-contrast imaging space
observatories.Comment: 25 pages, 6 figures, accepted for publication in Ap
The International Deep Planet Survey II: The frequency of directly imaged giant exoplanets with stellar mass
Radial velocity and transit methods are effective for the study of short
orbital period exoplanets but they hardly probe objects at large separations
for which direct imaging can be used. We carried out the international deep
planet survey of 292 young nearby stars to search for giant exoplanets and
determine their frequency. We developed a pipeline for a uniform processing of
all the data that we have recorded with NIRC2/Keck II, NIRI/Gemini North,
NICI/Gemini South, and NACO/VLT for 14 years. The pipeline first applies
cosmetic corrections and then reduces the speckle intensity to enhance the
contrast in the images. The main result of the international deep planet survey
is the discovery of the HR 8799 exoplanets. We also detected 59 visual multiple
systems including 16 new binary stars and 2 new triple stellar systems, as well
as 2,279 point-like sources. We used Monte Carlo simulations and the Bayesian
theorem to determine that 1.05[+2.80-0.70]% of stars harbor at least one giant
planet between 0.5 and 14M_J and between 20 and 300 AU. This result is obtained
assuming uniform distributions of planet masses and semi-major axes. If we
consider power law distributions as measured for close-in planets instead, the
derived frequency is 2.30[+5.95-1.55]%, recalling the strong impact of
assumptions on Monte Carlo output distributions. We also find no evidence that
the derived frequency depends on the mass of the hosting star, whereas it does
for close-in planets. The international deep planet survey provides a database
of confirmed background sources that may be useful for other exoplanet direct
imaging surveys. It also puts new constraints on the number of stars with at
least one giant planet reducing by a factor of two the frequencies derived by
almost all previous works.Comment: 83 pages, 13 figures, 15 Tables, accepted in A&
Four Aspects of Autoimmunity and How to Regain Tolerance to Self From an Autoimmune Disease Utilizing the Modified Vaccination Technique
Improving the speckle noise attenuation of simultaneous spectral differential imaging with a focal plane holographic diffuser
Direct exoplanet detection is limited by speckle noise in the point spread
function (PSF) of the central star. This noise can be reduced by subtracting
PSF images obtained simultaneously in adjacent narrow spectral bands using a
multi-channel camera (MCC), but only to a limit imposed by differential optical
aberrations in the MCC. To alleviate this problem, we suggest the introduction
of a holographic diffuser at the focal plane of the MCC to convert the PSF
image into an incoherent illumination scene that is then re-imaged with the
MCC. The re-imaging is equivalent to a convolution of the scene with the PSF of
each spectral channel of the camera. Optical aberrations in the MCC affect only
the convolution kernel of each channel and not the PSF globally, resulting in
better correlated images. We report laboratory measurements with a dual channel
prototype (1.575 micron and 1.625 micron) to validate this approach. A speckle
noise suppression factor of 12-14 was achieved, an improvement by a factor ~5
over that obtained without the holographic diffuser. Simulations of realistic
exoplanet populations for three representative target samples show that the
increase in speckle noise attenuation achieved in the laboratory would roughly
double the number of planets that could be detected with current adaptive
optics systems on 8-m telescopes.Comment: 9 pages, 8 figure, to be published in ApJ June 20, 200
Constraints on the architecture of the HD 95086 planetary system with the Gemini Planet Imager
We present astrometric monitoring of the young exoplanet HD 95086 b obtained
with the Gemini Planet Imager between 2013 and 2016. A small but significant
position angle change is detected at constant separation; the orbital motion is
confirmed with literature measurements. Efficient Monte Carlo techniques place
preliminary constraints on the orbital parameters of HD 95086 b. With 68%
confidence, a semimajor axis of 61.7^{+20.7}_{-8.4} au and an inclination of
153.0^{+9.7}_{-13.5} deg are favored, with eccentricity less than 0.21. Under
the assumption of a co-planar planet-disk system, the periastron of HD 95086 b
is beyond 51 au with 68% confidence. Therefore HD 95086 b cannot carve the
entire gap inferred from the measured infrared excess in the SED of HD 95086.
We use our sensitivity to additional planets to discuss specific scenarios
presented in the literature to explain the geometry of the debris belts. We
suggest that either two planets on moderately eccentric orbits or three to four
planets with inhomogeneous masses and orbital properties are possible. The
sensitivity to additional planetary companions within the observations
presented in this study can be used to help further constrain future dynamical
simulations of the planet-disk system.Comment: Accepted for publication in ApJ