| 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 ± 0.033 stat ± 0.038 sys 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 3 σ, the most significant signal with PIONIER being 2.9 σ and that with SPHERE being 1.6 σ. 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.
|
