The circumstellar disc, envelope and bipolar outflow of the massive young stellar object W33A

The young stellar object (YSO) W33A is one of the best known examples of a massive star still in the process of forming. Here we present Gemini North Altitude conjugate Adaptive optics for the Infrared (ALTAIR)/Near-Infrared Integral Field Spectrograph laser-guide star adaptive-optics assisted K-band integral-field spectroscopy of W33A and its inner reflection nebula. In our data, we make the first detections of a rotationally flattened outer envelope and fast bipolar jet of a massive YSO at near-infrared wavelengths. The predominant spectral features observed are Br gamma, H(2) and a combination of emission and absorption from CO gas. We perform a 3D spectro-astrometric analysis of the line emission, the first study of its kind. We find that the object's Br gamma emission reveals evidence for a fast bipolar jet on sub-milliarcsecond scales, which is aligned with the larger scale outflow. The hybrid CO features can be explained as a combination of hot CO emission arising in a disc close to the central star, while cold CO absorption originates in the cooler outer envelope. Kinematic analysis of these features reveals that both structures are rotating and consistent with being aligned perpendicular to both the ionized jet and the large-scale outflow. Assuming Keplerian rotation, we find that the circumstellar disc orbits a central mass of greater than or similar to 10 M(circle dot), while the outer envelope encloses a mass of similar to 15 M(circle dot). Our results suggest a scenario of a central star accreting material from a circumstellar disc at the centre of a cool extended rotating torus, while driving a fast bipolar wind. These results therefore provide strong supporting evidence for the hypothesis that the formation mechanism for high-mass stars is qualitatively similar to that of low-mass stars.