Mapping the core of the Tarantula Nebula with VLT-MUSE II. The
  spectroscopic Hertzsprung-Russell diagram of OB stars in NGC 2070
  We present the spectroscopic analysis of 333 OB-type stars extracted from
VLT-MUSE observations of the central 30 x 30 pc of NGC 2070 in the Tarantula
Nebula on the Large Magellanic Cloud, the majority of which are analysed for
the the first time. The distribution of stars in the spectroscopic
Hertzsprung-Russell diagram (sHRD) shows 281 stars in the main sequence. We
find two groups in the main sequence, with estimated ages of 2.1$\pm$0.8 and
6.2$\pm$2 Myr. A subgroup of 52 stars is apparently beyond the main sequence
phase, which we consider to be due to emission-type objects and/or significant
nebular contamination affecting the analysis. As in previous studies, stellar
masses derived from the sHRD are systematically larger than those obtained from
the conventional HRD, with the differences being largest for the most massive
stars. Additionally, we do not find any trend between the estimated projected
rotational velocity and evolution in the sHRD. The projected rotational
velocity distribution presents a tail of fast rotators that resembles findings
in the wider population of 30 Doradus. We use published spectral types to
calibrate the HeI$\lambda$4921/HeII$\lambda$5411 equivalent-width ratio as a
classification diagnostic for early-type main sequence stars when the classical
blue-visible region is not observed. Our model-atmosphere analyses demonstrate
that the resulting calibration is well correlated with effective temperature.
