High-contrast imaging and spectroscopy provide unique constraints for exoplanet formation models as well as for planetary atmosphere models. Instrumentation techniques in this field have greatly ...improved over the last two decades, with the development of stellar coronagraphy, in parallel with specific methods of wavefront sensing and control. Next generation space- and ground-based telescopes will enable the characterization of cold solar-system-like planets for the first time and maybe even in situ detection of bio-markers. However, the growth of primary mirror diameters, necessary for these detections, comes with an increase of their complexity (segmentation, secondary mirror features). These discontinuities in the aperture can greatly limit the performance of coronagraphic instruments. In this context, we introduced a new technique, Active Correction of Aperture Discontinuities-Optimized Stroke Minimization (ACAD-OSM), to correct for the diffractive effects of aperture discontinuities in the final image plane of a coronagraph, using deformable mirrors. In this paper, we present several tools that can be used to optimize the performance of this technique for its application to future large missions. In particular, we analyzed the influence of the deformable setup (size and separating distance) and found that there is an optimal point for this setup, optimizing the performance of the instrument in contrast and throughput while minimizing the strokes applied to the deformable mirrors. These results will help us design future coronagraphic instruments to obtain the best performance.
We present a new method for numerical propagation through Lyot-style coronagraphs using finite occulting masks. Standard methods for coronagraphic simulations involve Fast Fourier Transforms (FFT) of ...very large arrays, and computing power is an issue for the design and tolerancing of coronagraphs on segmented Extremely Large Telescopes (ELT) in order to handle both the speed and memory requirements. Our method combines a semi-analytical approach with non-FFT based Fourier transform algorithms. It enables both fast and memory-efficient computations without introducing any additional approximations. Typical speed improvements based on computation costs are of about ten to fifty for propagations from pupil to Lyot plane, with thirty to sixty times less memory needed. Our method makes it possible to perform numerical coronagraphic studies even in the case of ELTs using a contemporary commercial laptop computer, or any standard commercial workstation computer.
We present an analysis of the orbital motion of the four substellar objects orbiting HR 8799. Our study relies on the published astrometric history of this system augmented with an epoch obtained ...with the Project 1640 coronagraph with an integral field spectrograph (IFS) installed at the Palomar Hale telescope. We first focus on the intricacies associated with astrometric estimation using the combination of an extreme adaptive optics system (PALM-3000), a coronagraph, and an IFS. We introduce two new algorithms. The first one retrieves the stellar focal plane position when the star is occulted by a coronagraphic stop. The second one yields precise astrometric and spectrophotometric estimates of faint point sources even when they are initially buried in the speckle noise. The second part of our paper is devoted to studying orbital motion in this system. In order to complement the orbital architectures discussed in the literature, we determine an ensemble of likely Keplerian orbits for HR 8799bcde, using a Bayesian analysis with maximally vague priors regarding the overall configuration of the system. Although the astrometric history is currently too scarce to formally rule out coplanarity, HR 8799d appears to be misaligned with respect to the most likely planes of HR 8799bce orbits. This misalignment is sufficient to question the strictly coplanar assumption made by various authors when identifying a Laplace resonance as a potential architecture. Finally, we establish a high likelihood that HR 8799de have dynamical masses below 13 M sub(jup), using a loose dynamical survival argument based on geometric close encounters. We illustrate how future dynamical analyses will further constrain dynamical masses in the entire system.
Future searches for bio-markers on habitable exoplanets will rely on telescope instruments that achieve extremely high contrast at small planet-to-star angular separations. Coronagraphy is a ...promising starlight suppression technique, providing excellent contrast and throughput for off-axis sources on clear apertures. However, the complexity of space- and ground-based telescope apertures goes on increasing over time, owing to the combination of primary mirror segmentation, the secondary mirror, and its support structures. These discontinuities in the telescope aperture limit the coronagraph performance. In this paper, we present ACAD-OSM, a novel active method to correct for the diffractive effects of aperture discontinuities in the final image plane of a coronagraph. Active methods use one or several deformable mirrors that are controlled with an interaction matrix to correct for the aberrations in the pupil. However, they are often limited by the amount of aberrations introduced by aperture discontinuities. This algorithm relies on the recalibration of the interaction matrix during the correction process to overcome this limitation. We first describe the ACAD-OSM technique and compare it to the previous active methods for the correction of aperture discontinuities. We then show its performance in terms of contrast and off-axis throughput for static aperture discontinuities (segmentation, struts) and for some aberrations evolving over the life of the instrument (residual phase aberrations, artifacts in the aperture, misalignments in the coronagraph design). This technique can now obtain the Earth-like planet detection threshold of contrast on any given aperture over at least a 10% spectral bandwidth, with several coronagraph designs.
In this Letter we study the utility and limitations of ground-based coronagraphy with adaptive optics (AO). In very high AO correction regimes, residual speckles are pinned on the diffraction rings ...of the Airy pattern. We show that this effect is due to small errors in the complex wave in the focal plane, amplified by the coherent part of the wave. The statistics of these speckles are fairly well described by a modified Rician distribution. The variance of the speckles, at high flux and at photon-counting levels, finds simple expressions. The total variance can be partitioned into two contributions: one that can be suppressed by a coronagraph and one that cannot. Different regimes can be identified. These results enable us to analyze when a coronagraph can defeat the noise variance, and they provide a criterion for the effectiveness of such instruments.
Context. Specific high-contrast imaging instruments are mandatory to characterize circumstellar disks and exoplanets around nearby stars. Coronagraphs are commonly used in these facilities to reject ...the diffracted light of an observed star and enable direct imaging and spectroscopy of its circumstellar environment. One important property of the coronagraph is to be able to work in broadband light. Aims. Among several proposed coronagraphs, the dual-zone phase mask coronagraph is a promising solution for starlight rejection in broadband light. In this paper, we perform the first validation of this concept in laboratory. Methods. First, we consider the principle of the dual-zone phase mask coronagraph. Then, we describe the high-contrast imaging THD testbed, the manufacturing of the components, and the quality control procedures. Finally, we study the sensitivity of our coronagraph to low-order aberrations (inner working angle and defocus) and estimate its contrast performance. Our experimental broadband light results are compared with numerical simulations to check agreement with the performance predictions. Results. With the manufactured prototype and using a dark hole technique based on the self-coherent camera, we obtain contrast levels down to 2 × 10-8 between 5 and 17λ0/D in monochromatic light (640 nm). We also reach contrast levels of 4 × 10-8 between 7 and 17λ0/D in broadband (λ0 = 675 nm, Δλ = 250 and Δλ/λ0 = 40%), which demonstrates the excellent chromatic performance of the dual-zone phase mask coronagraph. Conclusions. The performance reached by the dual-zone phase mask coronagraph is promising for future high-contrast imaging instruments that aim to detect and spectrally characterize old or light gaseous planets.
ABSTRACT
Planetesimals – asteroids and comets – are the building blocks of planets in protoplanetary discs and the source of dust, ice, and gas in debris discs. Along with planets they comprise the ...left-over material after star formation that constitutes a planetary system. Planets influence the dynamics of planetesimals, sculpting the orbits of debris belts to produce asymmetries or gaps. We can constrain the architecture of planetary systems, and infer the presence of unseen planetary companions, by high spatial resolution imaging of debris discs. HD 16743 is a relatively young F-type star that hosts a bright edge-on debris disc. Based on far-infrared Herschel observations its disc was thought to be stirred by a planetary companion. Here, we present the first spatially resolved observations at near-infrared and millimetre wavelengths with HST and ALMA, revealing the disc to be highly inclined at $87{_{.}^{\circ}} 3~^{+1{_{.}^{\circ}} 9}_{-2{_{.}^{\circ}} 5}$ with a radial extent of 157.7$^{+2.6}_{-1.5}$ au and an full width at half maximum of 79.4$^{+8.1}_{-7.8}$ au (ΔR/R = 0.5). The vertical scale height of the disc is 0.13 ± 0.02, significantly greater than typically assumed unstirred value of 0.05, and could be indicative of stirring of the dust-producing planetesimals within the disc by bodies at least a few times the mass of Pluto up to 18.3 M⊕ in the single object limit.
We obtained spectra in the wavelength range lambda = 995-1769 nm of all four known planets orbiting the star HR 8799. Using the suite of instrumentation known as Project 1640 on the Palomar 5 m Hale ...Telescope, we acquired data at two epochs. This allowed for multiple imaging detections of the companions and multiple extractions of low-resolution (R ~ 35) spectra. Data reduction employed two different methods of speckle suppression and spectrum extraction, both yielding results that agree. The spectra do not directly correspond to those of any known objects, although similarities with L and T dwarfs are present, as well as some characteristics similar to planets such as Saturn. We tentatively identify the presence of CH sub(4) along with NH sub(3) and/or C sub(2)H sub(2), and possibly CO sub(2) or HCN in varying amounts in each component of the system. Other studies suggested red colors for these faint companions, and our data confirm those observations. Cloudy models, based on previous photometric observations, may provide the best explanation for the new data presented here. Notable in our data is that these presumably co-eval objects of similar luminosity have significantly different spectra; the diversity of planets may be greater than previously thought. The techniques and methods employed in this paper represent a new capability to observe and rapidly characterize exoplanetary systems in a routine manner over a broad range of planet masses and separations. These are the first simultaneous spectroscopic observations of multiple planets in a planetary system other than our own.
Context. Characterization of directly imaged exoplanets is one of the most eagerly anticipated science functions of the James Webb Space Telescope. MIRI, the mid-IR instrument, has the capability to ...provide unique spatially resolved photometric data points in a spectral range never before achieved for such objects.
Aims. We aim to present the very first on-sky contrast measurements of the MIRI coronagraphs. In addition to a classical Lyot coronagraph at the longest wavelength, this observing mode implements the concept of the four-quadrant phase mask for the very first time in a space telescope.
Methods. We observed single stars together with a series of reference stars to measure raw contrasts as they are delivered on the detector, as well as reference-subtracted contrasts.
Results. The MIRI coronagraphs achieve raw contrasts better than 10−3 at the smallest angular separations (within 1″) and about 10−5 farther out (beyond 5 ~ 6″). Subtracting the residual diffracted light left behind the coronagraph has the potential to bring the final contrast down to the background- and detector-limited noise floor at most angular separations (a few times 10−5 at less than 1″).
Conclusions. The MIRI coronagraphs behave as expected from simulations. In particular, the raw contrasts for all four coronagraphs are fully consistent with the diffractive model. Contrasts obtained by subtracting reference stars also meet expectations and are fully demonstrated for two four-quadrant phase masks (F1065C and F1140C). The worst contrast, measured at F1550C, is very likely due to a variation in the phase aberrations at the primary mirror during the observations, and not an issue with the coronagraph itself. We did not perform reference star subtraction with the Lyot mask at F2300C, but we anticipate that it would bring the contrast down to the noise floor.
Abstract
We present new near-infrared Gemini Planet Imager (GPI) spectroscopy of HD 206893 B, a substellar companion orbiting within the debris disk of its F5V star. The
J
,
H
,
K1
, and
K2
spectra ...from GPI demonstrate the extraordinarily red colors of the object, confirming it as the reddest substellar object observed to date. The significant flux increase throughout the infrared presents a challenging atmosphere to model with existing grids. Best-fit values vary from 1200 to 1800 K for effective temperature and from 3.0 to 5.0 for log(
g
), depending on which individual wavelength band is fit and which model suite is applied. The extreme redness of the companion can be partially reconciled by invoking a high-altitude layer of submicron dust particles, similar to dereddening approaches applied to the peculiar red field L dwarf population. However, reconciling the HD 206893 B spectra with even those of the reddest low-gravity L dwarf spectra still requires the contribution of additional atmospheric dust, potentially due to the debris disk environment in which the companion resides. Orbit fitting from 4 yr of astrometric monitoring is consistent with a ∼30 yr period, an orbital inclination of 147°, and a semimajor axis of 10 au, well within the estimated disk inner radius of ∼50 au. As one of a very few substellar companions imaged interior to a circumstellar disk, the properties of this system offer important dynamical constraints on companion–disk interaction and provide a benchmark for substellar and planetary atmospheric study.
PDF
