Beyond the diffraction limit of optical/IR interferometers. II. Stellar parameters of rotating stars from differential phases

Description

Context. As previously demonstrated on Achernar, one can derive the angular radius, rotational velocity, axis tilt, and orientation of a fast-rotating star from the differential phases obtained by spectrally resolved long baseline interferometry using earth-rotation synthesis.
Aims: We applied this method on a small sample of stars for different spectral types and classes, in order to generalize the technique to other rotating stars across the H-R diagram and determine their fundamental parameters.
Methods: We used differential phase data from the AMBER/VLTI instrument obtained prior to refurbishing its spectrometer in 2010. With the exception of Fomalhaut, which has been observed in the medium-resolution mode of AMBER (λ/δλ ≈ 1500), our three other targets, Achernar, Altair, and δ Aquilae offered high-resolution (λ/δλ ≈ 12 000) spectro-interferometric data around the Brγ absorption line in K band. These data were used to constrain the input parameters of an analytical, still realistic model to interpret the observations with a systematic approach for the error budget analysis in order to robustly conclude on the physics of our 4 targets. We applied the super resolution provided by differential phases φ_diff to measure the size (equatorial radius R_eq and angular diameter ⌀_eq), the equatorial rotation velocity (V_eq), the inclination angle (i), and the rotation axis position angle (PA_rot) of 4 fast-rotating stars: Achernar, Altair, δ Aquilae, and Fomalhaut. The stellar parameters of the targets were constrained using a semi-analytical algorithm dedicated to fast rotators SCIROCCO.
Results: The derived parameters for each star were R_eq = 11.2 ± 0.5 R_⊙, V_eqsini = 290 ± 17 km s^-1, PA_rot = 35.4° ± 1.4°, for Achernar; R_eq = 2.0 ± 0.2 R_⊙, V_eqsini = 226 ± 34 km s^-1, PA_rot = -65.5° ± 5.5°, for Altair; R_eq = 2.2 ± 0.3 R_⊙, V_eqsini = 74 ± 35 km s^-1, PA_rot = -101.2° ± 14°, for δ Aquilae; and R_eq = 1.8 ± 0.2 R_⊙, V_eqsini = 93 ± 16 km s^-1, PA_rot = 65.6° ± 5°, for Fomalhaut. They were found to be compatible with previously published values from differential phase and visibility measurements, while we were able to determine, for the first time, the inclination angle i of Fomalhaut (i = 90° ± 9°) and δ Aquilae (i = 81° ± 13°), and the rotation-axis position angle PA_rot of δ Aquilae.
Conclusions: Beyond the theoretical diffraction limit of an interferometer (ratio of the wavelength to the baseline), spatial super resolution is well suited to systematically estimating the angular diameters of rotating stars and their fundamental parameters with a few sets of baselines and the Earth-rotation synthesis provided a high enough spectral resolution.

Based on observations performed at the European Southern Observatory, Chile, under ESO AMBER-consortium GTO program IDs 084.D-0456 081.D-0293 and 082.C-0376.Figure 5 is available in electronic form at http://www.aanda.org

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