Tracing jet emission at the base of a high-mass YSO. First AMBER/VLTI
  observations of the Br\gamma emission in IRAS 13481-6124
  To probe the circumstellar environment of IRAS 13481-6124, a 20 M_sun
high-mass young stellar object (HMYSO) with a parsec-scale jet and accretion
disc, we investigate the origin of its Br\gamma-emission line through NIR
interferometry. We present the first AMBER/VLTI observations of the
Br\gamma-emitting region in an HMYSO at R~1500. Our AMBER/VLTI observations
reveal a spatially and spectrally resolved Br\gamma-line in emission with a
strong P Cygni profile, indicating outflowing matter with a terminal velocity
of ~500 km/s. Visibilities, differential phases, and closure phases are
detected in our observations within the spectral line and in the adjacent
continuum. Both total visibilities (continuum plus line emitting region) and
pure-line visibilities indicate that the Br\gamma-emitting region is more
compact (2-4 mas in diameter or ~6-13 au at 3.2 kpc) than the
continuum-emitting region (~5.4 mas or ~17 au). The absorption feature is also
spatially resolved at the longest baselines (81 and 85 m) and has a visibility
that is slightly smaller than the continuum-emitting region. The differential
phases at the four longest baselines display an \u2018S\u2019-shaped structure
across the line, peaking in the blue- and red-shifted high-velocity components.
The calibrated photocentre shifts are aligned with the known jet axis, i.e they
are probably tracing an ionised jet. The high-velocity components (v_r~100-500
km/s) are located far from the source, whereas the low-velocity components
(0-100 km/s) are observed to be closer, indicating a strong acceleration of the
gas flow in the inner 10 au. Finally, a non-zero closure phase along the
continuum is detected. By comparing our observations with the synthetic images
of the continuum around 2.16 um, we confirm that this feature originates from
the asymmetric brightness distribution of the continuum owing to the
inclination of the inner disc.
