Diameter and photospheric structures of Canopus from AMBER/VLTI interferometry

Context. Direct measurements of fundamental parameters and photospheric structures of post-main-sequence intermediate-mass stars are required for a deeper understanding of their evolution. Aims. Based on near-IR long-baseline interferometry we aim to resolve the stellar surface of the F0 supergiant star Canopus, and to precisely measure its angular diameter and related physical parameters. Methods. We used the AMBER/VLTI instrument to record interferometric data on Canopus: visibilities and closure phases in the H and K bands with a spectral resolution of 35. The available baselines ( similar or equal to 60-110 m) and the high quality of the AMBER/VLTI observations allowed us to measure fringe visibilities as far as in the third visibility lobe. Results. We determined an angular diameter of circle divide = 6.93 +/- 0.15 mas by adopting a linearly limb-darkened disk model. From this angular diameter and Hipparcos distance we derived a stellar radius R = 71.4 +/- 4.0 R(circle dot). Depending on bolometric fluxes existing in the literature, the measured circle divide provides two estimates of the effective temperature: T(eff) = 7284 +/- 107 K and T(eff) = 7582 +/- 252 K. Conclusions. In addition to providing the most precise angular diameter obtained to date, the AMBER interferometric data point towards additional photospheric structures on Canopus beyond the limb-darkened model alone. A promising explanation for such surface structures is the presence of convection cells. We checked such a hypothesis using first order star-cell models and concluded that the AMBER observations are compatible with the presence of surface convective structures. This direct detection of convective cells on Canopus from interferometry can provide strong constraints to radiation-hydrodynamics models of photospheres of F-type supergiants.