Measuring the variability of directly imaged exoplanets using vector Apodizing Phase Plates combined with ground-based differential spectrophotometry

TL;DR

This paper introduces a novel ground-based differential spectrophotometric method using a vector Apodizing Phase Plate coronagraph to monitor brightness variability in directly imaged exoplanets, achieving high precision and potential for future atmospheric studies.

Contribution

The study demonstrates a new high-contrast imaging technique combining dgvAPP360 with integral field spectroscopy for precise variability measurements of exoplanets.

Findings

Detected 8.8% variability in HD 1160 B with a 3.24 h period.

Achieved 3.7% precision per 18-minute bin in ground-based high-contrast light curves.

Found no evidence of a systematic noise floor, indicating room for further improvement.

Abstract

Clouds and other features in exoplanet and brown dwarf atmospheres cause variations in brightness as they rotate in and out of view. Ground-based instruments reach the high contrasts and small inner working angles needed to monitor these faint companions, but their small fields-of-view lack simultaneous photometric references to correct for non-astrophysical variations. We present a novel approach for making ground-based light curves of directly imaged companions using high-cadence differential spectrophotometric monitoring, where the simultaneous reference is provided by a double-grating 360{\deg} vector Apodizing Phase Plate (dgvAPP360) coronagraph. The dgvAPP360 enables high-contrast companion detections without blocking the host star, allowing it to be used as a simultaneous reference. To further reduce systematic noise, we emulate exoplanet transmission spectroscopy, where the light is spectrally-dispersed and then recombined into white-light flux. We do this by combining the dgvAPP360 with the infrared ALES integral field spectrograph on the Large Binocular Telescope Interferometer. To demonstrate, we observed the red companion HD 1160 B (separation ~780 mas) for one night, and detect $8.8\%$ semi-amplitude sinusoidal variability with a ~3.24 h period in its detrended white-light curve. We achieve the greatest precision in ground-based high-contrast imaging light curves of sub-arcsecond companions to date, reaching $3.7\%$ precision per 18-minute bin. Individual wavelength channels spanning 3.59-3.99 $\mu$m further show tentative evidence of increasing variability with wavelength. We find no evidence yet of a systematic noise floor, hence additional observations can further improve the precision. This is therefore a promising avenue for future work aiming to map storms or find transiting exomoons around giant exoplanets.