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

Wheelwright, H. E., de Wit, W. J., Weigelt, G., Oudmaijer, R. D., Ilee, J. D.

A&A, 543, id.A77 (2012)


Context. HD 327083 is a supergiant B[e] star that forms a binary system with an orbital semi-major axis of approximately 1.7 AU.

Aims: Our previous observations using the VLTI and AMBER in the medium resolution K-band mode spatially resolved the environment of HD 327083. The continuum visibilities obtained indicate the presence of a circumbinary disc. CO bandhead emission was also observed. However, due to the limited spectral resolution of the previous observations, the kinematic structure of the emitting material could not be constrained. In this paper, we address this and probe the source of the CO emission with high spectral resolution and spatial precision.

Methods: To determine the properties and kinematics of its CO emitting region, we have observed HD 327083 with high spectral resolution (25 and 6 km s-1) using AMBER and CRIRES. The observations are compared to kinematical models to constrain the source of the emission.

Results: The multi-epoch AMBER spectra obtained over 5 months contain no evidence that the CO 1st overtone emission of HD 327083 is variable. This indicates that the structure of the emitting region is not strongly dependent on orbital phase. It is shown that the CO bandhead emission can be reproduced using a model of a Keplerian disc with an inclination and size consistent with our previous VLTI observations. The model is compared to AMBER differential phase measurements, which have a precision as high as ~30 µas. A differential phase signal corresponding to 0.15 mas (~5σ) is seen over the bandhead emission, which is in excellent agreement with the model that fits the CRIRES observations. In comparison, a model of an equatorial outflow, as envisaged in the standard sgB[e] scenario, does not reproduce the observations well.

Conclusions: We present a direct test of the circumstellar kinematics of the binary sgB[e] star HD 327083 using both spatial and spectral information. The excellent agreement between the disc model and observations in the spatial and spectral domains is compelling evidence that the CO bandhead emission of HD 327083 originates in a circumbinary Keplerian disc. In contrast, the model of an equatorial outflow cannot reproduce the observations well. This suggests that the standard sgB[e] scenario is not applicable to HD 327083, which supports the hypothesis that the B[e] behaviour of HD 327083 is due to binarity.

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