High-contrast Adaptive Optics Research Articles

Direct Exoplanet Detection using Convolutional Image Reconstruction (ConStruct): A New Algorithm for Post-processing High-contrast Images

We present a novel machine-learning approach for detecting faint point sources in high-contrast adaptive optics (AO) imaging data sets. The most widely used algorithms for primary subtraction aim to decouple bright stellar speckle noise from planetary signatures by subtracting an approximation of the temporally evolving stellar noise from each frame in an imaging sequence. Our approach aims to improve the stellar noise approximation and increase the planet detection sensitivity by leveraging deep learning in a novel direct imaging post-processing algorithm. We show that a convolutional autoencoder neural network, trained on an extensive reference library of real imaging sequences, accurately reconstructs the stellar speckle noise at the location of a potential planet signal. This tool is used in a post-processing algorithm we call Direct Exoplanet Detection with Convolutional Image Reconstruction, or ConStruct. The reliability and sensitivity of ConStruct are assessed using real Keck/NIRC2 angular differential imaging data sets. Of the 30 unique point sources we examine, ConStruct yields a higher signal-to-noise ratio than traditional principal component analysis-based processing for 67% of the cases and improves the relative contrast by up to a factor of 2.6. This work demonstrates the value and potential of deep learning to take advantage of a diverse reference library of point-spread function realizations to improve direct imaging post-processing. ConStruct and its future improvements may be particularly useful as tools for post-processing high-contrast images from JWST and extreme AO instruments, both for the current generation and those being designed for the upcoming 30 m class telescopes.

Behind the mask: can HARMONI@ELT detect biosignatures in the reflected light of Proxima b?

ABSTRACT Proxima b is a rocky exoplanet in the habitable zone of the nearest star system and a key test case in the search for extraterrestrial life. Here, we investigate the characterization of a potential Earth-like atmosphere around Proxima b in reflected light via molecule mapping, combining high-resolution spectroscopy (HRS) and high contrast imaging, using the first-generation integral field spectrograph, High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI), on the 39-m Extremely Large Telescope (ELT). We simulate comprehensive observations of Proxima b at an assumed 45° inclination using HARMONI’s High Contrast Adaptive Optics mode, with spatial resolution $\sim 8 \,\rm mas$ ($3.88 \,\rm mas$ spaxel−1) and spectral resolving power R ≃ 17 000 between 1.538–$1.678\,\mathrm{\mu }\mathrm{m}$, containing the spectral features of water, carbon dioxide, and methane. Tellurics, stellar features, and additional noise sources are included, and removed using established molecule mapping techniques. We find that HARMONI’s current focal plane mask (FPM) is too large and obscures the orbit of Proxima b and thus explore smaller and offset FPMs to yield a detection. An $\rm {S/N}=5$ detection of Proxima b’s reflected light, suitable for atmospheric characterization, is possible with such modifications, requiring a minimum of 20 h, but ideally at least 30 h of integration time. We highlight that such detections do not scale with the photon noise, hence suitably detailed simulations of future instruments for the ELTs are needed to fully understand their ability to perform HRS observations of exoplanet atmospheres. Alterations to the HARMONI FPM design are feasible at this stage, but must be considered in context of other science cases.

Lab tests of segment/petal phasing with a pyramid wavefront sensor and a holographic dispersed fringe sensor in turbulence with the Giant Magellan Telescope high contrast adaptive optics phasing testbed

The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m diameter mirrors, together forming a single 25.4-m diameter primary mirror. This large aperture and collecting area can help extreme adaptive optics (ExAO) systems such as GMT’s GMagAO-X achieve the small angular resolutions and contrasts required to image habitable zone earth-like planets around late type stars and possibly lead to the discovery of life outside of our solar system. However, the GMT primary mirror segments are separated by large >30 cm gaps, creating the possibility of fluctuations in optical path differences (piston) due to flexure, segment vibrations, wind buffeting, temperature effects, and atmospheric seeing. To utilize the full diffraction-limited aperture of the GMT for high-contrast, natural guide star-adaptive optics science, the seven mirror segments must be co-phased to well within a fraction of a wavelength. The current design of the GMT involves seven adaptive secondary mirrors, a slow (∼0.03 Hz) off-axis dispersed fringe sensor (part of the acquisition guiding and wavefront sensing system’s active optics off-axis guider), and a pyramid wavefront sensor [PyWFS; part of the natural guide star wavefront sensor (NGWS) adaptive optics] to measure and correct the total path length between segment pairs, but these methods have yet to be tested “end-to-end” in a lab environment. We present the design and working prototype of a “GMT high contrast adaptive optics phasing testbed” that leverages the existing MagAO-X ExAO instrument to demonstrate segment phase sensing and simultaneous AO-control for high-contrast GMT natural guide star science [i.e., testing the NGWS wavefront sensor (WFS) architecture]. We present the first test results of closed-loop piston control with one GMT segment using MagAO-X’s PyWFS with and without simulated atmospheric turbulence. We show that the PyWFS was able to successfully control segment piston without turbulence within 12- to 33-nm RMS for 0 λ / D to 5 λ / D modulation, but was unsuccessful at controlling segment piston with generated ∼0.6 arcsec (median seeing conditions at the GMT site) and ∼1.2 arcsec seeing turbulence due to nonlinear modal cross-talk and poor pixel sampling of the segment gaps on the PyWFS detector. These results suggest that a PyWFS alone is not an ideal piston sensor for the GMT (and likely the TMT and ELT). Hence, a dedicated “second channel” piston sensor is required. We report the success of an alternate solution to control piston using a holographic dispersed fringe sensor (HDFS) while controlling all other modes with the PyWFS purely as a slope sensor (piston mode removed). This “second channel” WFS method worked well to control segment piston to within 50 nm RMS and ±10 μm dynamic range under simulated 0.6 arcsec atmospheric seeing (median seeing conditions at the GMT site). These results led to the inclusion of the HDFS as the official second channel piston sensor for the GMT NGWS WFS. This HDFS + PyWFS architecture should also work well to control piston petal modes on the ELT and TMT telescopes.

Phasing the Giant Magellan Telescope with the holographic dispersed fringe sensor

The next generation of Giant Segmented Mirror Telescopes (GSMT) will have large gaps between the segments either caused by the shadow of the mechanical structure of the secondary mirror [European Extremely Large Telescope (E-ELT) and Thirty Meter Telescope (TMT)] or intrinsically by design [Giant Magellan Telescope (GMT)]. These gaps are large enough to fragment the aperture into independent segments that are separated by more than the typical Fried parameter. This creates piston and petals modes that are not well sensed by conventional wavefront sensors such as the Shack–Hartmann wavefront sensor or the pyramid wavefront sensor. We propose to use a new optical device, the holographic dispersed fringe sensor (HDFS), to sense and control these petal/piston modes. The HDFS uses a single pupil-plane hologram to interfere the segments onto different spatial locations in the focal plane. Numerical simulations show that the HDFS is very efficient and that it reaches a differential piston root-mean-square (rms) smaller than 10 nm for GMT/E-ELT/TMT for guide stars up to 13th J + H band magnitude. The HDFS has also been validated in the lab with Magellan adaptive optics extreme and high-contrast adaptive optics phasing testbed, the GMT phasing testbed. The lab experiments reached 5-nm rms piston error on the Magellan telescope aperture. The HDFS also reached 50-nm rms of piston error on a segmented GMT-like aperture while the pyramid wavefront sensor was compensating simulated atmosphere under median seeing conditions. The simulations and lab results demonstrate the HDFS as an excellent piston sensor for the GMT. We find that the combination of a pyramid slope sensor with an HDFS piston sensor is a powerful architecture for the GMT.

The First High-contrast Images of X-Ray Binaries: Detection of Candidate Companions in the γ Cas Analog RX J1744.7-2713

X-ray binaries provide exceptional laboratories for understanding the physics of matter under the most extreme conditions. Until recently, there were few, if any, observational constraints on the circumbinary environments of X-ray binaries at ∼100–5000 au scales. It remains unclear how the accretion onto the compact objects or the explosions giving rise to the compact objects interact with their immediate surroundings. Here, we present the first high-contrast adaptive optics images of X-ray binaries. These observations target all X-ray binaries within ∼3 kpc accessible with the Keck/NIRC2 vortex coronagraph. This paper focuses on one of the first key results from this campaign; our images reveal the presence of 21 sources potentially associated with the γ Cassiopeiae analog high-mass X-ray binary RX J1744.7−2713. By conducting different analyses—a preliminary proper motion analysis, a color–magnitude diagram, and a probability of chance alignment calculation—we found that three of these 21 sources have a high probability of being bound to the system. If confirmed, they would be in wide orbits (∼450 to 2500 au). While follow-up astrometric observations will be needed in ∼5–10 yr to confirm further the bound nature of these detections, these discoveries emphasize that such observations may provide a major breakthrough in the field. In fact, they would be useful not only for our understanding of stellar multiplicity, but also for our understanding of how planets, brown dwarfs, and stars can form even in the most extreme environments.

Concept validation of a high dynamic range point-diffraction interferometer for wavefront sensing in adaptive optics.

The direct detection and imaging of exoplanets requires the use of high-contrast adaptive optics (AO). In these systems quasi-static aberrations need to be highly corrected and calibrated. To achieve this, the pupil-modulated point-diffraction interferometer (m-PDI) was presented in an earlier paper. This present paper focuses on m-PDI concept validation through three experiments. First, the instrument's accuracy and dynamic range are characterized by measuring the spatial transfer function at all spatial frequencies and at different amplitudes. Then, using visible monochromatic light, an AO control loop is closed on the system's systematic bias to test for precision and completeness. In a central section of the pupil with 72% of the total radius, the residual error is 7.7 nm rms. Finally, the control loop is run using polychromatic light with a spectral FWHM of 77 nm around the R-band. The control loop shows no drop in performance with respect to the monochromatic case, reaching a final Strehl ratio larger than 0.7.

Effects of mirror seeing on high-contrast adaptive optics instruments

Ground-based direct imaging surveys like the Gemini Planet Imager Exoplanet Survey (GPIES) rely on Adaptive Optics (AO) systems to image and characterize exoplanets that are up to a million times fainter than their host stars. One factor that can reduce AO performance is turbulence induced by temperature differences in the instrument's immediate surroundings (e.g.: "dome seeing" or "mirror seeing"). In this analysis we use science observations, AO telemetry, and environmental data from September 2014 to February 2017 of the GPIES campaign to quantify the effects of "mirror seeing" on the performance of the GPI instrument. We show that GPI performance is optimal when the primary mirror (M1) is in equilibrium with the outside air temperature. We then examine the characteristics of mirror seeing by calculating the power spectral densities (PSD) of spatial and temporal Fourier modes. Inside the inertial range of the PSDs, we find that the spatial PSD amplitude increases when M1 is out of equilibrium and that the integrated turbulence may exhibit deviations from Kolmogorov atmospheric turbulence models and from the 1-layer frozen flow model. We conclude with an assessment of the current temperature control and ventilation strategy at Gemini South.

A 2 R⊕ Planet Orbiting the Bright Nearby K Dwarf Wolf 503

Abstract Since its launch in 2009, the Kepler telescope has found thousands of planets with radii between that of Earth and Neptune. Recent studies of the distribution of these planets have revealed a gap in the population near 1.5–2.0 R ⊕, informally dividing these planets into “super-Earths” and “sub-Neptunes.” The origin of this division is difficult to investigate directly because the majority of planets found by Kepler orbit distant, dim stars and are not amenable to radial velocity follow-up or transit spectroscopy, making bulk density and atmospheric measurements difficult. Here, we present the discovery and validation of a newly found planet in direct proximity to the radius gap, orbiting the bright (J = 8.32 mag), nearby (D = 44.5 pc) high proper motion K3.5V star Wolf 503 (EPIC 212779563). We determine the possibility of a companion star and false positive detection to be extremely low using both archival images and high-contrast adaptive optics images from the Palomar observatory. The brightness of the host star makes Wolf 503b a prime target for prompt radial velocity follow-up, and with the small stellar radius (0.690 ± 0.025R ⊙), it is also an excellent target for HST transit spectroscopy and detailed atmospheric characterization with JWST. With its measured radius near the gap in the planet radius and occurrence rate distribution, Wolf 503b offers a key opportunity to better understand the origin of this radius gap as well as the nature of the intriguing populations of “super-Earths” and “sub-Neptunes” as a whole.

Calibration of quasi-static aberrations in high-contrast astronomical adaptive optics with a pupil-modulated point-diffraction interferometer.

The direct detection and imaging of exoplanets requires the use of high-contrast adaptive optics (AO). In these systems quasi-static aberrations need to be highly corrected and calibrated. In order to achieve this, a high-sensitivity wavefront sensor, the pupil-modulated point-diffraction interferometer (m-PDI), is presented. This sensor modulates and retrieves both the phase and the amplitude of an incoming electric field. The theory behind the wavefront reconstruction, the visibility of fringes, chromatic effects and noise propagation are developed. Results show this interferometer has a wide chromatic bandwidth. For a bandwidth of Δλ = 50% in units of central wavelength, the visibility of fringes and the response of the WFS to low and high-order aberrations are almost unaffected with respect to the monochromatic case. The WFS is, in contrast, very sensitive to variations in the size of its pinhole. The size of the pinhole is shown to affect the sensor's linearity, the dynamic range and the amount of noise. Larger pinholes make the sensor less sensitive to low-order aberrations, but in turn also decrease the effects of misalignments.

PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS

We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (>1 MJup) around 122 newly identified nearby (<40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). Our H- and K-band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12-14 mag and 9-13 mag at 1", respectively, which corresponds to limiting planet masses of 0.5-10 MJup at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 $\pm$ 6 MJup; L0$^{+2}_{-1}$; 120 $\pm$ 20 AU), GJ 3629 B (64$^{+30}_{-23}$ MJup; M7.5 $\pm$ 0.5; 6.5 $\pm$ 0.5 AU), 1RXS J034231.8+121622 B (35 $\pm$ 8 MJup; L0 $\pm$ 1; 19.8 $\pm$ 0.9 AU), and 2MASS J15594729+4403595 B (43 $\pm$ 9 MJup; M8.0 $\pm$ 0.5; 190 $\pm$ 20 AU). Over 150 candidate planets were identified; we obtained follow-up imaging for 56% of these but all are consistent with background stars. Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs. We infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13 MJup planets between 10-100 AU for hot-start and cold-start (Fortney) evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor massive gas giants in the 5-13 MJup range like those orbiting HR 8799 and $\beta$ Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. Although the first directly imaged planets were found around massive stars, there is currently no statistical evidence for a trend of giant planet frequency with stellar host mass at large separations as predicted by the disk instability model of giant planet formation.

THE TRENDS HIGH-CONTRAST IMAGING SURVEY. V. DISCOVERY OF AN OLD AND COLD BENCHMARK T-DWARF ORBITING THE NEARBY G-STAR HD 19467

The nearby Sun-like star HD 19467 shows a subtle radial velocity (RV) acceleration of -1.37+/-0.09 m/s/yr over an 16.9 year time baseline (an RV trend), hinting at the existence of a distant orbiting companion. We have obtained high-contrast adaptive optics images of the star using NIRC2 at Keck Observatory and report the direct detection of the body that causes the acceleration. The companion, HD 19467 B, is dK=12.57+/-0.09 mag fainter than its parent star (contrast ratio of 9.4e-6), has blue colors J-K_s=-0.36+/-0.14 (J-H=-0.29+/-0.15), and is separated by 1.653+/-0.004" (51.1+/-1.0 AU). Follow-up astrometric measurements obtained over an 1.1 year time baseline demonstrate physical association through common parallactic and proper motion. We calculate a firm lower-limit of m>51.9^{+3.6}_{-4.3}Mjup for the companion mass from orbital dynamics using a combination of Doppler observations and imaging. We estimate a model-dependent mass of m=56.7^{+4.6}_{-7.2}Mjup from a gyrochronological age of 4.3^{+1.0}_{-1.2} Gyr. Isochronal analysis suggests a much older age of $9\pm1$ Gyr, which corresponds to a mass of m=67.4^{+0.9}_{-1.5}Mjup. HD 19467 B's measured colors and absolute magnitude are consistent with a late T-dwarf [~T5-T7]. We may infer a low metallicity of [Fe/H]=-0.15+/-0.04 for the companion from its G3V parent star. HD 19467 B is the first directly imaged benchmark T-dwarf found orbiting a Sun-like star with a measured RV acceleration.

THE TRENDS HIGH-CONTRAST IMAGING SURVEY. II. DIRECT DETECTION OF THE HD 8375 TERTIARY

We present the direct imaging detection of a faint tertiary companion to the single-lined spectroscopic binary HD 8375 AB. Initially noticed as an 53 m/s/yr Doppler acceleration by Bowler et al. 2010, we have obtained high-contrast adaptive optics observations at Keck using NIRC2 that spatially resolve HD 8375 C from its host(s). Astrometric measurements demonstrate that the companion shares a common proper-motion. We detect orbital motion in a clockwise direction. Multiband relative photometry measurements are consistent with a spectral-type of M1V. Our combined Doppler and imaging observations place a lower-limit of m>0.297Msun on its dynamical mass. We also provide a refined orbit for the inner pair using recent RV measurements obtained with HIRES. HD 8375 is one of many triple-star systems that are apparently missing in the solar neighborhood.

PLANETS AROUND LOW-MASS STARS (PALMS). II. A LOW-MASS COMPANION TO THE YOUNG M DWARF GJ 3629 SEPARATED BY 0.″2

We present the discovery of a 0.2" companion to the young M dwarf GJ 3629 as part of our high contrast adaptive optics imaging search for giant planets around low-mass stars with the Keck-II and Subaru telescopes. Two epochs of imaging confirm the pair is co-moving and reveal signs of orbital motion. The primary exhibits saturated X-ray emission, which together with its UV photometry from GALEX point to an age younger than ~300 Myr. At these ages the companion lies below the hydrogen burning limit with a model-dependent mass of 46 +/- 16 Mjup based on the system's photometric distance of 22 +/- 3 pc. Resolved YJHK photometry of the pair indicates a spectral type of M7 +/- 2 for GJ 3629 B. With a projected separation of 4.4 +/- 0.6 AU and an estimated orbital period of 21 +/- 5 yr, GJ 3629 AB is likely to yield a dynamical mass in the next several years, making it one of only a handful of brown dwarfs to have a measured mass and an age constrained from the stellar primary.

THE GEMINI NICI PLANET-FINDING CAMPAIGN: DISCOVERY OF A CLOSE SUBSTELLAR COMPANION TO THE YOUNG DEBRIS DISK STAR PZ Tel

We report the discovery of a tight substellar companion to the young solar analog PZ Tel, a member of the β Pic moving group observed with high-contrast adaptive optics imaging as part of the Gemini Near-Infrared Coronagraphic Imager Planet-Finding Campaign. The companion was detected at a projected separation of 16.4 ± 1.0 AU (033 ± 001) in 2009 April. Second-epoch observations in 2010 May demonstrate that the companion is physically associated and shows significant orbital motion. Monte Carlo modeling constrains the orbit of PZ Tel B to eccentricities >0.6. The near-IR colors of PZ Tel B indicate a spectral type of M7 ± 2 and thus this object will be a new benchmark companion for studies of ultracool, low-gravity photospheres. Adopting an age of 12+8−4 Myr for the system, we estimate a mass of 36 ± 6 MJup based on the Lyon/DUSTY evolutionary models. PZ Tel B is one of the few young substellar companions directly imaged at orbital separations similar to those of giant planets in our own solar system. Additionally, the primary star PZ Tel A shows a 70 μm emission excess, evidence for a significant quantity of circumstellar dust that has not been disrupted by the orbital motion of the companion.

The effect of a small heat source on PSF stability for high-contrast imaging

High-contrast adaptive optics systems, such as those needed to image extrasolar planets, are known to require excellent wavefront control and diffraction suppression. The Laboratory for Adaptive Optics at UC Santa Cruz is investigating limits to high-contrast imaging in support of the Gemini Planet Imager (GPI). In this paper we examine the effect of heat sources in the testbed on point-spread-function (PSF) stability. Introducing a heat source primarily introduces image motion. The GPI error budget requires image motion to be less than 0.1 lambda /D. Systematic motion of the PSF core is typically 0.01 lambda /D rms and with a 20 watt heat source introduced near the pupil plane image motion is increased to 0.02 lambda /D rms. Therefore, even a heat source as large as 20 watts near the pupil plane causes errors below the GPI requirement, but the combination of the heat source and additional air turbulence on the system introduced by changes to the enclosure or the fan of other components can produce significantly more motion. Heat also can affect the speckle pattern in the high-contrast region, but in the final instrument other sources of error should be more significant.

High Contrast Adaptive Optics for Exoplanet Detections: performance limits and optimal wavefront sensing strategy

High Contrast Adaptive Optics for Exoplanet Detections: performance limits and optimal wavefront sensing strategy

A dynamical calibration of the mass–luminosity relation at very low stellar masses and young ages

Mass is the most fundamental parameter of a star, yet it is also one of the most difficult to measure directly. In general, astronomers estimate stellar masses by determining the luminosity and using the 'mass-luminosity' relationship, but this relationship has never been accurately calibrated for young, low-mass stars and brown dwarfs. Masses for these low-mass objects are therefore constrained only by theoretical models. A new high-contrast adaptive optics camera enabled the discovery of a young (50 million years) companion only 0.156 arcseconds (2.3 au) from the more luminous (> 120 times brighter) star AB Doradus A. Here we report a dynamical determination of the mass of the newly resolved low-mass companion AB Dor C, whose mass is 0.090 +/- 0.005 solar masses. Given its measured 1-2-micrometre luminosity, we have found that the standard mass-luminosity relations overestimate the near-infrared luminosity of such objects by about a factor of approximately 2.5 at young ages. The young, cool objects hitherto thought to be substellar in mass are therefore about twice as massive, which means that the frequency of brown dwarfs and planetary mass objects in young stellar clusters has been overestimated.