High contrast imaging with Fizeau interferometry: The case of Altair

TL;DR

This paper demonstrates the use of Fizeau interferometry with the Large Binocular Telescope to achieve high-resolution imaging of Altair, setting new constraints on its companions and paving the way for future extremely large telescope observations.

Contribution

First application of filled-aperture LBT Fizeau interferometry with active phase control for high-contrast imaging of a star, exploring its potential for future ELT observations.

Findings

Achieved sensitivity to companions down to 0.5 solar masses at 1 arcsecond.

Placed constraints on companions of 1.3 solar masses at 0.15 arcseconds.

Demonstrated the feasibility and limitations of Fizeau interferometry for high-contrast imaging.

Abstract

The Large Binocular Telescope (LBT) has two 8.4-m primary mirrors that produce beams that can be combined coherently in a "Fizeau" interferometric mode. In principle, the Fizeau PSF enables the probing of structure at a resolution up to three times better than that of the adaptive-optics-corrected PSF of a single 8.4-m telescope. In this work, we examined the nearby star Altair (5.13 pc, type A7V, $\sim$100s Myr to $\approx$1.4 Gyr) in the Fizeau mode with the LBT at Br-$\alpha$ (4.05 $\mu$m) and carried out angular differential imaging to search for companions. This work presents the first filled-aperture LBT Fizeau science dataset to benefit from a correcting mirror which provides active phase control. In the analysis of the $\lambda/D$ angular regime, the sensitivity of the dataset is down to $\approx$0.5 $M_{\odot}$ at 1" for a 1.0 Gyr system. This sensitivity remains limited by the small amount of integration time, which is in turn limited by the instability of the Fizeau PSF. However, in the Fizeau fringe regime we attain sensitivities of $\Delta m \approx 5$ at 0.2" and put constraints to companions of 1.3 $M_{\odot}$ down to an inner angle of $\approx$0.15", closer than any previously published direct imaging of Altair. This analysis is a pathfinder for future datasets of this type, and represents some of the first steps to unlocking the potential of the first ELT. Fizeau observations will be able to reach dimmer targets with upgrades to the instrument, in particular the phase detector.