Diameter and photospheric structures of Canopus from AMBER/VLTI interferometry

Free access article

Issue		A&A Volume 489, Number 2, October II 2008


Page(s)		L5 - L8
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:200810450
Published online		18 August 2008

A&A 489, L5-L8 (2008)
DOI: 10.1051/0004-6361:200810450

Letter

Diameter and photospheric structures of Canopus from AMBER/VLTI interferometry

A. Domiciano de Souza¹, P. Bendjoya¹, F. Vakili¹, F. Millour², and R. G. Petrov¹

¹ Lab. H. Fizeau, CNRS UMR 6525, Univ. de Nice-Sophia Antipolis, Obs. de la Côte d'Azur, Parc Valrose, 06108 Nice, France
e-mail: Armando.Domiciano@unice.fr
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 23 June 2008 / Accepted 25 July 2008

Abstract
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 ( $\simeq$ 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 $\diameter=6.93\pm0.15$ mas by adopting a linearly limb-darkened disk model. From this angular diameter and Hipparcos distance we derived a stellar radius $R=71.4\pm4.0~R_{\odot}$ . Depending on bolometric fluxes existing in the literature, the measured $\diameter$ provides two estimates of the effective temperature: $T_\mathrm{eff}= 7284\pm107$ K and $T_\mathrm{eff}= 7582\pm252$ 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.

Key words: stars: fundamental parameters -- stars: individual: Canopus -- supergiants -- methods: observational -- techniques: high angular resolution -- techniques: interferometric

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