Imaging search for the unseen companion to Eps Ind A -- Improving the detection limits with 4 micron observations

TL;DR

This study used advanced 4 micron imaging techniques to search for an unseen companion to Eps Ind A, improving detection limits and constraining the companion's properties despite not directly observing it.

Contribution

The paper introduces a novel application of active angular differential imaging and LOCI-based PSF subtraction at 4 microns for high-contrast imaging around nearby stars.

Findings

No companion detected within the sensitivity limits.

Constraints placed on the companion's mass and orbit, suggesting 5-20 Mjup at 10-20 AU.

Indications that the system is older than 1 Gyr.

Abstract

Eps Ind A is one of the nearest sun-like stars, located only 3.6 pc away. It is known to host a binary brown dwarf companion, Eps Ind Ba/Bb, at a large projected separation of 6.7", but radial velocity measurements imply that an additional, yet unseen component is present in the system, much closer to Eps Ind A. Previous direct imaging has excluded the presence of any stellar or high-mass brown dwarf companion at small separations, indicating that the unseen companion may be a low-mass brown dwarf or high-mass planet. We present the results of a deep high-contrast imaging search for the companion, using active angular differential imaging (aADI) at 4 micron, a particularly powerful technique for planet searches around nearby and relatively old stars. We also develop an additional PSF reference subtraction scheme based on locally optimized combination of images (LOCI) to further enhance the detection limits. No companion is seen in the images, although we are sensitive to significantly lower masses than previously achieved. Combining the imaging data with the known radial velocity trend, we constrain the properties of the companion to within approximately 5-20 Mjup, 10-20 AU, and i > 20 deg, unless it is an exotic stellar remnant. The results also imply that the system is probably older than the frequently assumed age of ~1 Gyr.