Probing the innermost region of the AU Microscopii debris disc

Abstract

Context. AU Mic is a young and nearby M-dwarf star harbouring a circumstellar debris disc and one recently discovered planet on an eight-day orbit. Large-scale structures within the disc were also discovered and are moving outwards at high velocity. Aims. We aim to study this system with the highest spatial resolution in order to probe the innermost regions and to search for additional low-mass companions or set detection limits. Methods. The star was observed with two different high-angular resolution techniques probing complementary spatial scales. We obtained new Ks-band sparse aperture masking observations with VLT/SPHERE, which we combined with data from VLT/NACO, VLTI/PIONIER and VLTI/GRAVITY. Results. We did not detect additional close companions within the separation range 0.02–7 au from the parent star. We determined magnitude upper limits for companions of H ~ 9.8 mag within 0.02-0.5 au, Ks ~ 11.2 mag within 0.4–2.4 au, and L ~ 10.7 mag within 0.7–7 au. Using theoretical isochrones, we converted these magnitudes into upper limits on the mass of ~17 Mjup, ~12 Mjup, and ~9 Mjup, respectively. The PIONIER observations also allowed us to determine the angular diameter of AU Mic, θLD = 0.825 ± 0.033stat ± 0.038sys mas, which converts to a linear radius R = 0.862 ± 0.052 R⊙ when combined with the Gaia parallax. Conclusions. We did not detect the newly discovered planets orbiting AU Mic (M < 0.2 Mjup), but we derived upper limit masses for the innermost region of AU Mic. We do not have any detection with a significance beyond 3σ, the most significant signal with PIONIER being 2.9σ and that with SPHERE being 1.6σ. We applied the pyMESS2 code to estimate the detection probability of companions by combining radial velocities, multi-band SPHERE imaging, and our interferometric detection maps. We show that 99% of the companions down to ~0.5 Mjup can be detected within 0.02 au or 1 Mjup down to 0.2 au. The low-mass planets orbiting at ≲0.11 au (≲11 mas) from the star will not be directly detectable with the current adaptive optics (AO) and interferometric instruments because of its close orbit and very high contrast (~10−10 K). It will also be below the angular resolution and contrast limit of the next Extremely Large Telescope Infrared (ELT IR) imaging instruments.