L'-band AGPM vector vortex coronagraph's first light on VLT/NACO: Discovery of a late-type companion at two beamwidths from an F0V star

TL;DR

This paper reports the first on-sky demonstration of an L'-band AGPM vector vortex coronagraph on VLT/NACO, achieving high contrast imaging close to the star and opening new possibilities for exoplanet discovery.

Contribution

It introduces a diamond-based AGPM coronagraph optimized for the L' band and demonstrates its successful deployment and high-contrast performance on the VLT/NACO system.

Findings

Achieved >7.5 magnitude contrast at 0.09 arcseconds from the star

First on-sky test of an L'-band AGPM coronagraph on VLT/NACO

Demonstrated potential for high contrast imaging in the inner working angle

Abstract

Context. High contrast imaging has thoroughly combed through the limited search space accessible with first-generation ground-based adaptive optics instruments and the Hubble Space Telescope. Only a few objects were discovered, and many non-detections reported and statistically interpreted. The field is now in need of a technological breakthrough. Aim. Our aim is to open a new search space with first-generation systems such as NACO at the Very Large Telescope, by providing ground-breaking inner working angle (IWA) capabilities in the L' band. The L' band is a sweet spot for high contrast coronagraphy since the planet-to-star brightness ratio is favorable, while the Strehl ratio is naturally higher. Methods. An annular groove phase mask (AGPM) vector vortex coronagraph optimized for the L' band, made from diamond subwavelength gratings was manufactured and qualified in the lab. The AGPM enables high contrast imaging at very small IWA, potentially being the key to unexplored discovery space. Results. Here we present the installation and successful on-sky tests of an L'-band AGPM coronagraph on NACO. Using angular differential imaging, which is well suited to the rotational symmetry of the AGPM, we demonstrated a \Delta L' > 7.5 mag contrast from an IWA ~ 0".09 onwards, during average seeing conditions, and for total integration times of a few hundred seconds.