Precise Canopus angular diameter measurement from AMBER/VLTI, photospheric structures suspected
Bendjoya, Ph.; Domiciano de Souza, A.; Vakili, F.; Millour, F.; Petrov, R. G.
SF2A-2008
Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics
Eds.: C. Charbonnel, F. Combes and R. Samadi., p.59
(2008)
Abstract
Direct measurements of fundamental parameters and photospheric structures of post-main-
sequence intermediate-mass stars are required for a deeper understanding of their evolution. 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. 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 (≃ 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. We
determined an angular diameter of = 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±4.0R⊙. 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 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.
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