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$\gamma$ 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.
