Beyond the diffraction limit of optical/IR interferometers^{⋆,⋆⋆,⋆⋆⋆}

I. Angular diameter and rotation parameters of Achernar from differential phases

¹ Laboratoire J.-L. Lagrange, UMR 7293, Observatoire de la Côte d’Azur (OCA), Université de Nice-Sophia Antipolis (UNS), Centre National de la Recherche Scientifique (CNRS), Campus Valrose, 06108 Nice Cedex 2, France
e-mail: Armando.Domiciano@oca.eu
² Centre de Recherche en Astronomie, Astrophysique et Géophysique (CRAAG), Route de l’Observatoire, BP 63, Bouzareah 16340, Alger, Algérie
³ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo (USP), Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil
⁴ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 Place Jules Janssen, 92195 Meudon, France
⁵ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 6 January 2012
Accepted: 26 July 2012

Abstract

Context. Spectrally resolved long-baseline optical/IR interferometry of rotating stars opens perspectives to investigate their fundamental parameters and the physical mechanisms that govern their interior, photosphere, and circumstellar envelope structures.

Aims. Based on the signatures of stellar rotation on observed interferometric wavelength-differential phases, we aim to measure angular diameters, rotation velocities, and orientation of stellar rotation axes.

Methods. We used the AMBER focal instrument at ESO-VLTI in its high-spectral resolution mode to record interferometric data on the fast rotator Achernar. Differential phases centered on the hydrogen Br γ line (K band) were obtained during four almost consecutive nights with a continuous Earth-rotation synthesis during  ~5 h/night, corresponding to  ~60° position angle coverage per baseline. These observations were interpreted with our numerical code dedicated to long-baseline interferometry of rotating stars.

Results. By fitting our model to Achernar’s differential phases from AMBER, we could measure its equatorial radius R_eq = 11.6  ±  0.3 R_⊙, equatorial rotation velocity V_eq = 298 ± 9 km s^-1, rotation axis inclination angle i = 101.5  ±  5.2°, and rotation axis position angle (from North to East) PA_rot = 34.9  ±  1.6°. From these parameters and the stellar distance, the equatorial angular diameter ⌀_eq of Achernar is found to be 2.45  ±  0.09 mas, which is compatible with previous values derived from the commonly used visibility amplitude. In particular, ⌀_eq and PA_rot measured in this work with VLTI/AMBER are compatible with the values previously obtained with VLTI/VINCI.

Conclusions. The present paper, based on real data, demonstrates the super-resolution potential of differential interferometry for measuring sizes, rotation velocities, and orientation of rotating stars in cases where visibility amplitudes are unavailable and/or when the star is partially or poorly resolved. In particular, we showed that differential phases allow the measurement of sizes up to  ~4 times smaller than the diffraction-limited angular resolution of the interferometer.