Direct Detection Of Extrasolar Planets Research Articles

Rotational-Shearing-Interferometer Response for a Star-Planet System without Star Cancellation

The Rotational Shearing Interforometer has been proposed for direct detection of extra-solar planets. This interferometer cancels the star radiation using destructive interference. However, the resulting signal is too small (few photons/s for each m2). We propose a novel method to enhance the signal magnitude by means of the star–planet interference when the star radiation is not cancelled. We use interferograms computationally simulated to confirm the viability of the technique.

Kernel nullers for an arbitrary number of apertures

Context.The use of interferometric nulling for the direct detection of extrasolar planets is in part limited by the extreme sensitivity of the instrumental response to tiny optical path differences between apertures. The recently proposed kernel-nuller architecture attempts to alleviate this effect with an all-in-one combiner design that enables the production of observables inherently robust to residual optical path differences (≪λ).Aims.To date, a unique kernel-nuller design has been proposed ad hoc for a four-beam combiner. We examine the properties of this original design and generalize them for an arbitrary number of apertures.Methods.We introduce a convenient graphical representation of the complex combiner matrices that model the kernel nuller and highlight the symmetry properties that enable the formation of kernel nulls. The analytical description of the nulled outputs we provide demonstrates the properties of a kernel nuller.Results.Our description helps outline a systematic way to build a kernel nuller for an arbitrary number of apertures. The designs for three- and six-input combiners are presented along with the original four-input concept. The combiner grows in complexity with the square of the number of apertures. While one can mitigate this complexity by multiplexing nullers working independently over a smaller number of sub-apertures, an all-in-one kernel nuller recombining a large number of apertures appears as the most efficient way to characterize a high-contrast complex astrophysical scene.Conclusions.Kernel nullers can be designed for an arbitrary number of apertures that produce observable quantities robust to residual perturbations. The designs we recommend are lossless and take full advantage of all the available interferometric baselines. They are complete, result in as many kernel nulls as the theoretically expected number of closure-phases, and are optimized to require the smallest possible number of outputs.

Focal Plane Phase Masks for PIAA: Design and Manufacturing

The Phase Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) is a coronagraph architecture for the direct detection of extrasolar planets, which can achieve close to the theoretical performance limit of any direct detection system. The primary components of a PIAACMC system are the Phase Induced Amplitude Apodization (PIAA) optics and the complex phase-shifting focal plane mask. PIAA optics have been produced and demonstrated with high coronagraph performance. In this paper, we describe the design process for the phase-shifting focal plane mask, and strategies for smoothing the mask profile. We describe the mask manufacturing process and show manufacturing results. Errors in the fabricated mask profile degrade the system performance, but we can recover performance by refining the manufacturing process and implementing wavefront control.

Mid-IR characterization of substellar companions with CanariCam

The direct detection of extrasolar planets is the only technique that leads to the most complete characterization of their physical properties. Here, we report on mid-IR observations of five directly imaged substellar companions (DF Tau B, FU Tau B, κ And b, GJ 758 B, and GJ 504 b) obtained with the Si-2 filter (8.7 µ m) in the CanariCam instrument mounted on the 10-m Gran Telescopio de Canarias (GTC). We show the detection of FU Tau B, and we set upper limits of 11.5-12.0 mag in the 8.7 µ m band for the other substellar targets. These data are useful to constrain their effective temperature and the presence of mid-IR flux excesses that may be due to surrounding disks.

Unbalanced nulling interferometer and precise wavefront control

A nulling interferometer was proposed to achieve direct detection of extra-solar planets (exoplanets) by suppressing light from the central star using a pair of telescopes. Recently, the stellar coronagraph method has shown rapid progress, which uses an extended concept of the nulling interferometer within single telescope optics. A dynamic range larger than 1 × 109 for the detection of Earth-like exoplanets can be attained by reducing diffraction patterns using the nulling coronagraph, and by suppressing speckle noise using an adaptive optics with an accuracy of λ/10000 rms. An unbalanced nulling interferometer (UNI), which is used as fore-optics, improves the wavefront sensing sensitivity and compensation level of the adaptive optics by a factor of 10. Consequently, the dynamic range of the coronagraph can also be improved by two orders of magnitude. The UNI is composed of a modified coronagraph or a traditional interferometer and magnifies the aberrations of incoming wavefronts.

ACHROMATIC EIGHT-OCTANT PHASE-MASK CORONAGRAPH USING PHOTONIC CRYSTAL

We designed and manufactured a photonic-crystal mask which can be used for an achromatic eight-octant phase-mask (EOPM) coronagraph for direct detection of extrasolar planets. Laboratory experiments of the EOPM coronagraph were carried out using two laser light sources as a simulated star (wavelengths of 532 and 633 nm). As a result, we attained high extinction of the simulated starlight in both the wavelengths. Halo intensity levels of about 10−6 and 10−7 were achieved at an angular distance of 3 and 13λ/D, respectively. We also discuss several issues, such as an effect of phase aberration on the coronagraphic performance, a transmittance of the proposed EOPM, and a novel two-channel coronagraphic configuration to improve system throughput.

Polarization interferometric nulling coronagraph for high-contrast imaging

We propose a novel, high-contrast imager called a polarization interferometric nulling coronagraph (PINC) for direct detection of extrasolar planets. The PINC uses achromatic half-wave plates (HWPs) installed in a fully symmetric beam combiner based on polarizing beam splitters. Jones calculus suggests that a stellar halo suppression level of 10(-10) can be achieved at 5 lambda/D for a broad wavelength range from 1.6 to 2.2 microm by using Fresnel-rhomb HWPs made of BK7. Laboratory experiments on the PINC used two laser light sources (wavelengths of lambda=532 and 671 nm), and we obtained a halo suppression level of approximately 10(-6) at 5 lambda/D for both wavelengths.

An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT

Wepresent theresultsof asurveyof 45young(P250Myr), close(P50pc) starswiththeSimultaneous Differential Imager (SDI) implemented at the VLT and the MMT for the direct detection of extrasolar planets. As part of the survey, we observed 54 objects, consisting of 45 close, young stars; two more distant ( 2 � which behaved consistently like a real object. From our survey null result,we can rule out (with 93% confidence) a model planet population where N(a) / constant out to a distance of 45 AU.

Segment aberration effects on contrast

High-contrast imaging, particularly the direct detection of extrasolar planets, is a major science driver for the next generation of telescopes. This science requires the suppression of scattered starlight at extremely high levels and that telescopes be correctly designed today to meet these stringent requirements in the future. The challenge increases in systems with complicated aperture geometries such as obscured, segmented telescopes. Such systems can also require intensive modeling and simulation efforts in order to understand the trade-offs between different optical parameters. The feasibility and development of a contrast prediction tool for use in the design and systems engineering of these telescopes is described. The performance of a particular starlight suppression system on a large segmented telescope is described analytically. These analytical results and the results of a contrast predictor are then compared with the results of a full wave-optics simulation.

Adaptive optics for direct detection of extrasolar planets: the Gemini Planet Imager

The direct detection of photons emitted or reflected by extrasolar planets, spatially resolved from their parent star, is a major frontier in the study of other solar systems. Direct detection will provide statistical information on planets in 5–50 AU orbits, inaccessible to current Doppler searches, and allow spectral characterization of radius, temperature, surface gravity, and perhaps composition. Achieving this will require new, dedicated, high-contrast instruments. One such system under construction is the Gemini Planet Imager (GPI). This combines a high-order/high-speed adaptive optics system to control wavefront errors from the Earth's atmosphere, an advanced coronagraph to block diffraction, ultrasmooth optics, a precision infrared interferometer to measure and correct systematic errors, and a integral field spectrograph/polarimeter to image and characterize target planetary systems. We predict that GPI will be able to detect planets with brightness less than 10 −7 of their parent star, sufficient to observe warm self-luminous planets around a large population of targets. To cite this article: B. Macintosh et al., C. R. Physique 8 (2007).

On-sky observations with an achromatic hybrid phase knife coronagraph in the visible

Context. The four-quadrant phase mask stellar coronagraph, introduced by D. Rouan et al., is capable of achieving very high dynamical range imaging and was studied in the context of the direct detection of extra-solar planets. Achromatic four-quadrant phase mask is currently being developed for broadband IR applications.Aims. We report on laboratory and on-sky tests of a prototype coronagraph in the visible. This prototype, the achromatic hybrid phase knife coronagraph , was derived from the four-quadrant phase mask principle.Methods. The instrumental setup implementing the coronagraph itself was designed to record the pre- and post-coronagraphic images simultaneously so that an efficient real-time image selection procedure can be performed. We describe the coronagraph and the associated tools that enable robust and repeatable observations. We present an algorithm of image selection that has been tested against the real on-sky data of the binary star HD 80081 (* 38 Lyn).Results. Although the observing conditions were poor, the efficiency of the proposed method is proven. From this experiment, we derive procedures that can apply to future focal instruments associating adaptive optics and coronagraphy, targeting high dynamic range imaging in astronomy, such as detecting extra-solar planets.

Feasibility of the Four‐Quadrant Phase Mask in the Mid‐Infrared on the James Webb Space Telescope

ABSTRACTThe James Webb Space Telescope (formerly the Next Generation Space Telescope) will be capable of unprecedented science, owing to its large diameter, its low background, and its dedicated IR instruments. One of these instruments is a mid‐IR facility called MIRI (Mid‐IR Instrument), which is being studied through a collaboration of US and European teams. Our group at Observatoire de Meudon has developed a coronagraphic device for the MIRI camera. Taking advantage of the reduced star‐planet contrast in this spectral range, the prime goal of this coronagraph is the direct detection of extrasolar planets. Differential imaging in several well‐chosen spectral windows is foreseen. In this paper, we evaluate the manufacturing process of the coronagraphic masks and compare the intrinsic performance, as measured in the laboratory, with the scientific requirements.

Ground-based direct detection of close-in extra-solar planets with nulling and high order adaptive optics

Ground-based direct detection of extra-solar planets is very challenging due to high planet to star brightness contrasts. For giant close-in planets, such as have been discovered by the radial velocity method, closer than 0.1 AU, the reflected light is predicted to be fairly high yielding a contrast ratio ranging from 10 -4 to 10 -5 at near infra-red wavelengths. In this paper, we investigate direct detection of reflected light from such planets using nulling interferometry, and high-order adaptive optics in conjunction with large double aperture ground-based telescopes. In this configuration, at least 10 -3 suppression of the entire stellar Airy pattern with small loss of planet flux as close as 0.03 arcsec is achievable. Distinguishing residual starlight from the planet signal is achieved by using the center of gravity shift method or multicolor differential imaging. Using these assumptions, we derive exposure times from a few minutes to several hours for direct detection of many of the known extra-solar planets with several short-baseline double aperture telescopes such as the Large Binocular Telescope (LBT), the Very Large Telescope (VLT) and the Keck Telescope.

High-order adaptive optics requirements for direct detection of extrasolar planets: Application to the SPHERE instrument

The detection of extrasolar planets implies an extremely high-contrast, long-exposure imaging capability at near infrared and probably visible wavelengths. We present here the core of any Planet Finder instrument, that is, the extreme adaptive optics (XAO) subsystem. The level of AO correction directly impacts the exposure time required for planet detection. In addition, the capacity of the AO system to calibrate all the instrument static defects ultimately limits detectivity. Hence, the extreme AO system has to adjust for the perturbations induced by the atmospheric turbulence, as well as for the internal aberrations of the instrument itself. We propose a feasibility study for an extreme AO system in the frame of the SPHERE (Spectro-Polarimetry High-contrast Exoplanet Research) instrument, which is currently under design and should equip one of the four VLT 8-m telescopes in 2010.

Direct detection of extrasolar planets

Direct detection of extrasolar planets

Tip‐Tilt Error in Lyot Coronagraphs

The direct detection of extrasolar planets by imaging means is limited by the large flux of light from the host star being scattered into the region of interest by a variety of processes, including diffraction. Coronagraphs are devices that suppress the undesirable scattering of light caused by diffraction. In a coronagraph the sensitivity limit for high dynamic range is limited by the propagation of errors introduced by the imperfect optical system to the final image. In this paper we develop theory and simulations to understand how such errors propagate in a coronagraph. We describe the response of classical and band-limited Lyot coronagraphs to small and large errors in the placement of the central star, and identify ways of making such coronagraphs more robust to small guiding errors. We also uncover features of the decentered point-spread function that can lead to the spurious detection of companions, especially with aggressive, high dynamic range coronagraphs dedicated to companion searches aimed at finding extrasolar terrestrial or Jovian planets.

Direct Detection of Thermal Emission from Extra-Solar Planets

We present a new test for the direct detection of extra-solar planets that relies on the presence of a strong molecular absorption band in the thermal emission from the planetary atmosphere. This signature will be sought by the method of differential astrometry, in which the astrometric centroid positions are compared in and out of the molecular band.

Two‐Mirror Apodization for High‐Contrast Imaging

Direct detection of extrasolar planets will require imaging systems capable of unprecedented contrast. Apodized pupils provide an attractive way to achieve such contrast but they are difficult, perhaps impossible, to manufacture to the required tolerance and they absorb about 90% of the light in order to create the apodization, which of course lengthens the exposure times needed for planet detection. A recently proposed alternative is to use two mirrors to accomplish the apodization. With such a system, no light is lost. In this paper, we provide a careful mathematical analysis, using one dimensional mirrors, of the on-axis and off-axis performance of such a two-mirror apodization system. There appear to be advantages and disadvantages to this approach. In addition to not losing any light, we show that the nonuniformity of the apodization implies an extra magnification of off-axis sources and thereby makes it possible to build a real system with about half the aperture that one would otherwise require or, equivalently, resolve planets at about half the angular separation as one can achieve with standard apodization. More specifically, ignoring pointing error and stellar disk size, a planet at $1.7 \lambda/D$ ought to be at the edge of detectability. However, we show that the non-zero size of a stellar disk pushes the threshold for high-contrast so that a planet must be at least $2.5 \lambda/D$ from its star to be detectable. The off-axis analysis of two-dimensional mirrors is left for future study.

Strategy for extrasolar planets observations with color-differential phase with the VLTI focal instrument AMBER

This paper describes our strategy and presents the specific additional calibration tool added to the near-infrared focal instrument AMBER of the Very Large Telescope Interferometer, to achieve the direct detection of extrasolar planets with color-differential interferometry. The measurement of the angular orbits, masses and spectra of hot giant extra solar planets can be achieved for Jupiter size planets up to 0.2 a.u. away from a 10 pc distant star, if it is limited only by photon, detector and background noises. However, this requires an extremely good control and calibration of the atmospheric and instrumental stability.

Fully symmetric nulling beam combiners

A simple method of nulling broadband light is presented. A mirror-symmetric pair of right-angle periscopes is first used to introduce a geometric field flip between two incident light beams, after which the light is combined by means of one of a number of constructive two-beam interferometers. A reciprocal pair of beam-splitter passages provides for complete symmetry. Such an approach greatly eases beam-splitter design requirements and should find use both in initial ground-based nulling experiments and ultimately in space-borne interferometers targeted at direct extrasolar planet detection.