High-dispersion coronagraphy (HDC) optimally combines high-contrast imaging techniques such as adaptive optics/wavefront control plus coronagraphy to high spectral resolution spectroscopy. HDC is a critical pathway toward fully characterizing exoplanet atmospheres across a broad range of masses from giant gaseous planets down to Earth-like planets. In addition to determining the molecular composition of exoplanet atmospheres, HDC also enables Doppler mapping of atmosphere inhomogeneities (temperature, clouds, wind), as well as precise measurements of exoplanet rotational velocities. Here, we demonstrate an innovative concept for injecting the directly imaged planet light into a single-mode fiber, linking a high-contrast adaptively corrected coronagraph to a high-resolution spectrograph (diffraction-limited or not). Our laboratory demonstration includes three key milestones: close-to-theoretical injection efficiency, accurate pointing and tracking, and on-fiber coherent modulation and speckle nulling of spurious starlight signal coupling into the fiber. Using the extreme modal selectivity of single-mode fibers, we also demonstrated speckle suppression gains that outperform conventional image-based speckle nulling by at least two orders of magnitude.

Reference star differential imaging (RDI) is a powerful strategy for high contrast imaging. Using example observations taken with the vortex coronagraph mode of Keck/NIRC2 in L band, we demonstrate that RDI provides improved sensitivity to point sources at small angular separations compared to angular differential imaging (ADI). Applying RDI to images of the low-mass stellar companions HIP 79124 C (192 mas separation, ∆L =4.01) and HIP 78233 B (141 mas separation, ∆L =4.78), the latter a first imaging detection, increases the significance of their detections by up to a factor of 5 with respect to ADI. We compare methods for reference frames selection and find that pre-selection of frames improves detection significance of point sources by up to a factor of 3. In addition, we use observations of the circumstellar disks around MWC 758 and 2MASS J16042165-2130284 to show that RDI allows for accurate mapping of scattered light distributions without self-subtraction artifacts.

π Men hosts a transiting super Earth (P ≈ 6.27 d, m ≈ 4.82 M⊕, R ≈ 2.04 R⊕) discovered by TESS and a cold Jupiter (P ≈ 2093 d, msin I ≈ 10.02 MJup, e ≈ 0.64) discovered from radial velocity. We use Gaia DR2 and Hipparcos astrometry to derive the star’s velocity caused by the orbiting planets and constrain the cold Jupiter’s sky-projected inclination (Ib = 41 − 65○). From this we derive the mutual inclination (ΔI) between the two planets, and find that 49○ < ΔI < 131○ (1σ), and 28○ < ΔI < 152○ (2σ). We examine the dynamics of the system using N-body simulations, and find that potentially large oscillations in the super Earth’s eccentricity and inclination are suppressed by General Relativistic precession. However, nodal precession of the inner orbit around the invariable plane causes the super Earth to only transit between 7-22 per cent of the time, and to usually be observed as misaligned with the stellar spin axis. We repeat our analysis for HAT-P-11, finding a large ΔI between its close-in Neptune and cold Jupiter and similar dynamics. π Men and HAT-P-11 are prime examples of systems where dynamically hot outer planets excite their inner planets, with the effects of increasing planet eccentricities, planet-star misalignments, and potentially reducing the transit multiplicity. Formation of such systems likely involves scattering between multiple giant planets or misaligned protoplanetary discs. Future imaging of the faint debris disc in π Men and precise constraints on its stellar spin orientation would provide strong tests for these formation scenarios.

The NIRC2 vortex coronagraph is an instrument on Keck II designed to directly image exoplanets and circumstellar disks at mid-infrared bands L (3.4-4.1 µm) and M s (4.55-4.8 µm). We analyze imaging data and corresponding adaptive optics telemetry, observing conditions, and other metadata over a three year time period to characterize the performance of the instrument and predict the detection limits of future observations. We systematically process images from 359 observations of 304 unique stars to subtract residual starlight (i.e., the coronagraphic point spread function) of the target star using two methods: angular differential imaging (ADI) and reference star differential imaging (RDI). We find that for the typical parallactic angle (PA) rotation of our dataset (∼10 • ), RDI provides gains over ADI for angular separations smaller than 0.25 . Furthermore, we find a power-law relation between the angular separation from the host star and the minimum PA rotation required for ADI to outperform RDI, with a power-law index of -1.18±0.08. Finally, we use random forest models to estimate ADI and RDI post-processed detection limits a priori. These models, which we provide publicly on a website, explain 70%-80% of the variance in ADI detection limits and 30%-50% of the variance in RDI detection limits. Averaged over a range of angular separations, our models predict both ADI and RDI contrast to within a factor of 2. These results illuminate important factors in high-contrast imaging observations with the NIRC2 vortex coronagraph, help improve observing strategies, and inform future upgrades to the hardware.