Paper detail

Mirror, mirror on the outflow cavity wall. Near-infrared CO overtone disc emission of the high-mass YSO IRAS 11101-5829

Aims: The inner regions of high-mass protostars are often invisible in the near-infrared. We aim to investigate the inner gaseous disc of IRAS11101-5829 through scattered light from the outflow cavity walls. Methods: We observed the environment of the high-mass young stellar object IRAS11101-5829 and the closest knots of its jet, HH135-136, with the VLT/SINFONI. We also retrieved archival data from the high-resolution long-slit spectrograph VLT/X-shooter. Results: We detect the first three bandheads of the $\upsilon=2-0$ CO vibrational emission for the first time in this object. It is coincident with continuum and Br$γ$ emission and extends up to $\sim10000$ au to the north-east and $\sim10 000$ au to the south-west. The line profiles have been modelled as a Keplerian rotating disc assuming a single ring in LTE. The model output gives a temperature of $\sim3000$ K, a CO column density of $\sim1\times10^{22}\mathrm{ cm^{-2}}$, and a projected Keplerian velocity $v_\mathrm{K}\sin i_\mathrm{disc} \sim 25\mathrm{ km s^{-1}}$, which is consistent with previous modelling in other high-mass protostars. In particular, the low value of $v_\mathrm{K}\sin i_\mathrm{disc}$ suggests that the disc is observed almost face-on, whereas the well-constrained geometry of the jet imposes that the disc must be close to edge-on. This apparent discrepancy is interpreted as the CO seen reflected in the mirror of the outflow cavity wall. Conclusions: From both jet geometry and disc modelling, we conclude that all the CO emission is seen through reflection by the cavity walls and not directly. This result implies that in the case of highly embedded objects, as for many high-mass protostars, line profile modelling alone might be deceptive and the observed emission could affect the derived physical and geometrical properties; in particular the inclination of the system can be incorrectly interpreted.