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Adaptive optics will enhance the precision of exoplanet surveys Adaptive optics (AO) systems correct for optical wavefront errors introduced by Earth's turbulent atmosphere, turning initially blurry images into intense diffraction-limited concentrations of light. The implementation of AO systems on the world's largest telescopes has revolutionized essentially all areas of astronomy ( 1 ). Instruments that receive a well-corrected beam of light can operate as if observing from space and thus benefit from an order-of-magnitude higher spatial and spectral resolution (see the figure). Given the benefits of working with nonfuzzy images, it may therefore be surprising to learn that one of the most important techniques for finding extrasolar planets, the Doppler radial velocity method, still uses “seeing-limited” observations—that is, measurements obtained without AO correction.

The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on their surfaces. We present the detection of Kepler-186f, a 1.11 ± 0.14 Earth-radius planet that is the outermost of five planets, all roughly Earth-sized, that transit a 0.47 ± 0.05 solar-radius star. The intensity and spectrum of the star's radiation place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form.

We describe a new instrument that forms the core of a long-term high contrast imaging program at the 200 inch (5 m) Hale Telescope at Palomar Observatory. The primary scientific thrust is to obtain images and low-resolution spectroscopy of brown dwarfs and young exoplanets of several Jupiter masses in the vicinity of stars within 50 pc of the Sun. The instrument is a microlens-based integral field spectrograph integrated with a diffraction-limited, apodized-pupil Lyot coronagraph. The entire combination is mounted behind the Palomar adaptive optics (AO) system. The spectrograph obtains imaging in 23 channels across the J J and H H bands (1.06–1.78 μm). The image plane of our spectrograph is subdivided by a200 × 200 200 × 200 element microlens array with a plate scale of 19.2 mas per microlens, critically sampling the diffraction-limited point-spread function at 1.06 μm. In addition to obtaining spectra, this wavelength resolution allows suppression of the chromatically dependent speckle noise, which we describe. In addition, we have recently installed a novel internal wave front calibration system that will provide continuous updates to the AO system every 0.5–1.0 minutes by sensing the wave front within the coronagraph. The Palomar AO system is undergoing an upgrade to a much higher order AO system (PALM-3000): a 3388-actuator tweeter deformable mirror working together with the existing 241-actuator mirror. This system, the highest-resolution AO corrector of its kind, will allow correction with subapertures as small as 8.1 cm at the telescope pupil using natural guide stars. The coronagraph alone has achieved an initial dynamic range in the H H band of2 × 10-4 2 × 10 - 4 at 1″, without speckle noise suppression. We demonstrate that spectral speckle suppression provides a factor of 10–20 improvement over this, bringing our current contrast at 1″ to∼2 × 10-5 ∼ 2 × 10 - 5 . This system is the first of a new generation of apodized-pupil coronagraphs combined with high-order adaptive optics and integral field spectrographs (e.g., GPI, SPHERE, HiCIAO), and we anticipate that this instrument will make a lasting contribution to high-contrast imaging in the Northern Hemisphere for years.

Correlations between stellar properties and the occurrence rate of exoplanets can be used to inform the target selection of future planet-search efforts and provide valuable clues about the planet-formation process. We analyze a sample of 1266 stars drawn from the California Planet Survey targets to determine the empirical functional form describing the likelihood of a star harboring a giant planet as a function of its mass and metallicity. Our stellar sample ranges from M dwarfs with masses as low as0.2 M ⊙ 0.2     M ⊙ to intermediate-mass subgiants with masses as high as1.9 M ⊙ 1.9     M ⊙ . In agreement with previous studies, our sample exhibits a planet-metallicity correlation at all stellar masses; the fraction of stars that harbor giant planets scales as f ∝ 101.2[Fe/H] f ∝ 10 1.2 [ Fe / H ] . We can rule out a flat metallicity relationship among our evolved stars (at 98% confidence), which argues that the high metallicities of stars with planets is not likely due to convective envelope “pollution.” Our data also rule out a constant planet occurrence rate for[Fe/H] < 0 [ Fe / H ] < 0 , indicating that giant planets continue to become rarer at sub-Solar metallicities. We also find that planet occurrence increases with stellar mass ( f ∝ M ⋆ f ∝ M ⋆ ), characterized by a rise from 3% around M dwarfs (0.5 M ⊙ 0.5     M ⊙ ) to 14% around A stars (2 M ⊙ 2     M ⊙ ), at Solar metallicity. We argue that the correlation between stellar properties and giant planet occurrence is strong supporting evidence of the core-accretion model of planet formation.

The Second Workshop on Extreme Precision Radial Velocities defined circa 2015 the state of the art Doppler precision and identified the critical path challenges for reaching 10 cm s(-1) measurement precision. The presentations and discussion of key issues for instrumentation and data analysis and the workshop recommendations for achieving this bold precision are summarized here. Beginning with the High Accuracy Radial Velocity Planet Searcher spectrograph, technological advances for precision radial velocity (RV) measurements have focused on building extremely stable instruments. To reach still higher precision, future spectrometers will need to improve upon the state of the art, producing even higher fidelity spectra. This should be possible with improved environmental control, greater stability in the illumination of the spectrometer optics, better detectors, more precise wavelength calibration, and broader bandwidth spectra. Key data analysis challenges for the precision RV community include distinguishing center of mass (COM) Keplerian motion from photospheric velocities (time correlated noise) and the proper treatment of telluric contamination. Success here is coupled to the instrument design, but also requires the implementation of robust statistical and modeling techniques. COM velocities produce Doppler shifts that affect every line identically, while photospheric velocities produce line profile asymmetries with wavelength and temporal dependencies that are different from Keplerian signals. Exoplanets are an important subfield of astronomy and there has been an impressive rate of discovery over the past two decades. However, higher precision RV measurements are required to serve as a discovery technique for potentially habitable worlds, to confirm and characterize detections from transit missions, and to provide mass measurements for other space-based missions. The future of exoplanet science has very different trajectories depending on the precision that can ultimately be achieved with Doppler measurements.

Optimizing on-sky single-mode fiber (SMF) injection is an essential part of developing precise Doppler spectrometers and new astrophotonics technologies. We installed and tested a prototype SMF-injection system at the Large Binocular Telescope in 2016 April. The fiber injection unit was built as part of the derisking process for a new instrument named iLocater that will use adaptive optics (AO) to feed a high resolution, near-infrared spectrograph. In this paper we report Y-band SMF coupling measurements for bright, M-type stars. We compare theoretical expectations for delivered Strehl ratio and SMF coupling to experimental results, and evaluate fundamental effects that limit injection efficiency. We find the pupil geometry of the telescope itself limits fiber coupling to a maximum efficiency of tel 0.78. Further analysis shows the individual impact of AO correction, tip-tilt residuals, and static (noncommon-path) aberrations contribute coupling coefficients of Strehl 0.33, tip tilt 0.84 , and ncpa 0.8 respectively. Combined, these effects resulted in an average Y-band SMF efficiency of 0.18 for all observations. Finally, we investigate the impact of fiber coupling on radial velocity precision as a function of stellar apparent magnitude.

iLocater is a near-infrared (NIR) radial velocity (RV) spectrograph that is being developed for the Large Binocular Telescope in Arizona. Unlike seeing-limited designs, iLocater uses adaptive optics to inject starlight directly into a single-mode fiber. This feature offers high spectral resolution while simultaneously maintaining a compact optical design. Although this approach shows promise to generate extremely precise RV measurements, it differs from conventional Doppler spectrographs, and therefore carries additional risk. To aid with the design of the instrument, we have developed a comprehensive simulator and data reduction pipeline. In this paper, we describe the simulation code and quantify its performance in the context of understanding terms in a RV error budget. We find that the program has an intrinsic precision of < 5 cm s−1, thereby justifying its use in a number of instrument trade studies. The code is written in Matlab and available for download on GitHub.

The use of single-mode fibers (SMFs) to illuminate radial velocity (RV) spectrographs shows promise to achieve extremely precise Doppler measurements. Due to their small core diameter, SMFs only propagate a single spatial mode which allows for diffraction-limited optical performance while simultaneously eliminating fiber modal noise. The single spatial mode however consists of two orthogonal polarization modes. In circular core fiber with a non-isotropic refractive index profile or asymmetries in the cross-sectional geometry, the two polarization modes propagate with different relative speeds inducing birefringence. Conditions at a telescope observatory will subject the fiber to mechanical (bending and twisting) and thermal stresses, inducing birefringence that varies in time. The interaction of variable birefringence combined with with polarization sensitive optics, such as diffraction gratings, results in an intensity modulation that causes unwanted Doppler shifts via \"polarization noise.\" In this paper, we characterize variable fiber birefringence both in the laboratory and at the Large Binocular Telescope using a Stokes parameters. We then combine the measured Stokes vector through a numerical model of a SMF spectrograph to understand the impact of variable polarization on RV precision. We find that polarization noise is a tens of cm s−1 to several m s−1 effect, which is exacerbated by the degree of polarization of the light source and the polarization response of the spectrograph optics. Finally we show experimentally mitigating the RV offset using polarization averaging methods and in-line fiber depolarizers and can reduce a several m s−1 polarization noise to ≤10 cm s−1.