AMBER and CRIRES observations of the binary sgB[e] star HD 327083: evidence of a gaseous disc traced by CO bandhead emission

Context. HD 327083 is a supergiant B[e] star thatforms a binary system with an orbital semi-major axis of approximately1.7 AU. Aims: Our previous observations using the VLTI and AMBERin the medium resolution K-band mode spatially resolved the environmentof HD 327083. The continuum visibilities obtained indicate the presenceof a circumbinary disc. CO bandhead emission was also observed. However,due to the limited spectral resolution of the previous observations, thekinematic structure of the emitting material could not be constrained.In this paper, we address this and probe the source of the CO emissionwith high spectral resolution and spatial precision. Methods: Todetermine the properties and kinematics of its CO emitting region, wehave observed HD 327083 with high spectral resolution (25 and 6 kms-1) using AMBER and CRIRES. The observations are compared tokinematical models to constrain the source of the emission. Results: The multi-epoch AMBER spectra obtained over 5 months contain noevidence that the CO 1st overtone emission of HD 327083 is variable.This indicates that the structure of the emitting region is not stronglydependent on orbital phase. It is shown that the CO bandhead emissioncan be reproduced using a model of a Keplerian disc with an inclinationand size consistent with our previous VLTI observations. The model iscompared to AMBER differential phase measurements, which have aprecision as high as ~30 μas. A differential phase signalcorresponding to 0.15 mas (~5σ) is seen over the bandheademission, which is in excellent agreement with the model that fits theCRIRES observations. In comparison, a model of an equatorial outflow, asenvisaged in the standard sgB[e] scenario, does not reproduce theobservations well. Conclusions: We present a direct test of thecircumstellar kinematics of the binary sgB[e] star HD 327083 using bothspatial and spectral information. The excellent agreement between thedisc model and observations in the spatial and spectral domains iscompelling evidence that the CO bandhead emission of HD 327083originates in a circumbinary Keplerian disc. In contrast, the model ofan equatorial outflow cannot reproduce the observations well. Thissuggests that the standard sgB[e] scenario is not applicable to HD327083, which supports the hypothesis that the B[e] behaviour of HD327083 is due to binarity. Based on data obtained at the European Southern Observatory, Paranal,Chile via the observing programmes 385.D-0513 and 087.D-0889.