Probing the innermost region of the AU~Microscopii debris disk

Abstract

AU Mic is a young and nearby M-dwarf star harbouring a circumstellar debris disk and one recently discovered planet on an 8d orbit. Large-scale structures within the disk were also discovered and are moving outward at high velocity. We aim at studying this system with the highest spatial resolution in order to probe the innermost regions and to search for additional low-mass companion or set detection limits. The star was observed with two different techniques probing complementary spatial scales. We obtained new SAM observations with SPHERE, which we combined with data from NACO, PIONIER and GRAVITY. We did not detect additional companions within 0.02-7au from the star. We determined magnitude upper limits for companions of H~9.8mag within 0.02-0.5au, Ks~11.2mag within 0.4-2.4au and L'~10.7mag within 0.7-7au. Using theoretical isochrones, we converted into mass upper limits of ~17Mjup, ~12Mjup and ~9jup, respectively. The PIONIER observations allowed us to determine the angular diameter of AU Mic, 0.825+/-0.050mas, which converts to R = 0.862+/-0.052Rsun. We did not detect the newly discovered planets, but we derived upper limit masses for the innermost region of AU Mic. We do not have any detection with a significance beyond 3sigma, the most significant signal with PIONIER being 2.9sigma and with SPHERE being 1.6\sigma. We applied the pyMESS2 code to estimate the detection probability of companions by combining radial velocities, SPHERE imaging and our interferometric detection maps. We show that 99% of the companions down to ~0.5Mjup can be detected within 0.02au or 1Mjup down to 0.2au. The low-mass planets orbiting at <0.11au will not be directly detectable with the current AO and interferometric instruments due to its close orbit and very high contrast (~10e-10 in K). It will be also below the angular resolution and contrast limit of the next ELT IR imaging instruments.