Gravitational-darkening of Altair from interferometry

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: adomicia@mpifr-bonn.mpg.de
² LESIA, UMR 8109, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
³ Lab. Univ. d'Astrophysique de Nice (LUAN), UMR 6525, UNSA, Parc Valrose, 06108 Nice Cedex 02, France
⁴ Astronomical Observatory Belgrade, MNTRS 1940, Volgina 7, 11050 Beograd, Serbia and Montenegro
⁵ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Department of Physics, University of Redlands, 1200 East Colton Avenue, Redlands, CA 92373, USA

Received: 3 December 2004
Accepted: 13 June 2005

Abstract

Interferometric observations have revealed that the rapid rotator Altair is a flattened star with a non-centrally symmetric intensity distribution. In this work we perform for the first time a physically consistent analysis of all interferometric data available so far, corresponding to three different interferometers operating in several spectral bands. These observations include new data (squared visibilities in the H and K bands from VLTI-VINCI) as well as previously published data (squared visibilities in the K band from PTI and squared visibilities, triple amplitudes, and closure phases in the visible between 520 nm and 850 nm from NPOI). To analyze these data we perform a minimization using an interferometry-oriented model for fast rotators, which includes Roche approximation, limb-darkening, and von Zeipel-like gravity-darkening. Thanks to the rich interferometric data set available and to this physical model, the main uniqueness problems were avoided. As a result, we show that the observations can only be explained if Altair has a gravity-darkening compatible with the expected value for hot stars, i.e., the von Zeipel effect ().