Probing discs around massive young stellar objects with CO first overtone emission

We present high-resolution (R? 50 000) spectroastrometry over the CO first overtone bandhead of a sample of seven intermediate/massive young stellar objects. These are primarily drawn from the Red MSX source (RMS) survey, a systematic search for young massive stars which has returned a large, well selected sample of such objects. The mean luminosity of the sample is approximately 5 × 104 L?, indicating the objects typically have a mass of ?15 M?. We fit the observed bandhead profiles with a model of a circumstellar disc, and find good agreement between the models and observations for all but one object. We compare the high angular precision (0.2–0.8 × 10?3 arcsec) spectroastrometric data to the spatial distribution of the emitting material in the best-fitting models. No spatial signatures of discs are detected, which is entirely consistent with the properties of the best-fitting models. Therefore, the observations suggest that the CO bandhead emission of massive young stellar objects originates in small-scale discs, in agreement with previous work. This provides further evidence that massive stars form via disc accretion, as suggested by recent simulations.