Hint of curvature in the orbital motion of the exoplanet 51 Eridani b using 3 yr of VLT/SPHERE monitoring

Context. The 51 Eridani system harbors a complex architecture with its primary star forming a hierarchical system with the binary GJ 3305AB at a projected separation of 2000 au, a giant planetorbiting the primary star at 13 au, and a low-mass debris diskaround the primary star with possible cold and warm components inferred from the spectral energy distribution. Aims: We aim to better constrain the orbital parameters of the known giant planet. Methods: We monitored the system over three years from 2015 to 2018 with the Spectro-Polarimetric High-contrast Exoplanet REsearch(SPHERE) instrument at the Very Large Telescope(VLT). Results: We measure an orbital motionfor the planetof 130 mas with a slightly decreasing separation ( 10 mas) and find a hint of curvature. This potential curvature is further supported at 3σ significance when including literature Gemini Planet Imager(GPI) astrometrycorrected for calibration systematics. Fits of the SPHERE and GPI data using three complementary approaches provide broadly similar results. The data suggest an orbital periodof 32-9+17 yr (i.e., 12-2+4 au in semi-major axis), an inclination of 133-7+14 deg, an eccentricity of 0.45-0.15+0.10, and an argument of periastron passage of 87-30+34 deg [mod 180°]. The time at periastron passage and the longitude of node exhibit bimodal distributions because we do not yet detect whether the planetis accelerating or decelerating along its orbit. Given the inclinations of the orbit and of the stellar rotationaxis (134-144°), we infer alignment or misalignment within 18° for the star- planetspin-orbit. Further astrometric monitoring in the next 3-4 yr is required to confirm at a higher significance the curvature in the motion of the planet, determine if the planetis accelerating or decelerating on its orbit, and further constrain its orbital parameters and the star- planetspin-orbit. The fitted orbits and the histogram distributions of the orbital parameters are only available at the CDS via anonymous ftpto http://cdsarc.u-strasbg.fr ( ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A118Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 095.C-0298, 096.C-0241, 198.C-0209, and 1100.C-0481.