The GRAVITY Young Stellar Object survey IV. The CO overtone emission in
  51 Oph at sub-au scales
  51 Oph is a Herbig Ae/Be star that exhibits strong near-infrared CO
ro-vibrational emission at 2.3 micron, most likely originating in the innermost
regions of a circumstellar disc. We aim to obtain the physical and geometrical
properties of the system by spatially resolving the circumstellar environment
of the inner gaseous disc. We used the second-generation VLTI/GRAVITY to
spatially resolve the continuum and the CO overtone emission. We obtained data
over 12 baselines with the auxiliary telescopes and derive visibilities, and
the differential and closure phases as a function of wavelength. We used a
simple LTE ring model of the CO emission to reproduce the spectrum and CO line
displacements. Our interferometric data show that the star is marginally
resolved at our spatial resolution, with a radius of 10.58+-2.65 Rsun.The
K-band continuum emission from the disc is inclined by 63+-1 deg, with a
position angle of 116+-1 deg, and 4+-0.8 mas (0.5+-0.1 au) across. The
visibilities increase within the CO line emission, indicating that the CO is
emitted within the dust-sublimation radius.By modelling the CO bandhead
spectrum, we derive that the CO is emitted from a hot (T=1900-2800 K) and dense
(NCO=(0.9-9)x10^21 cm^-2) gas. The analysis of the CO line displacement with
respect to the continuum allows us to infer that the CO is emitted from a
region 0.10+-0.02 au across, well within the dust-sublimation radius. The
inclination and position angle of the CO line emitting region is consistent
with that of the dusty disc. Our spatially resolved interferometric
observations confirm the CO ro-vibrational emission within the dust-free region
of the inner disc. Conventional disc models exclude the presence of CO in the
dust-depleted regions of Herbig AeBe stars. Ad hoc models of the innermost disc
regions, that can compute the properties of the dust-free inner disc, are
therefore required.
