2018A&A...617A..89C

The conditions leading to the formation of the most massive O-type stars are still an enigma in modern astrophysics. To assess the physical conditions of high-mass protostars in their main accretion phase, here we present a case study of a young massive clump selected from the ATLASGAL survey, G328.2551-0.5321. The source exhibits a bolometric luminosity of 1.3x10⁴L_☉, which allows us to estimate that its current protostellar mass lies between ∼11 and 16M_☉. We show high angular resolution observations with ALMA that reach a physical scale of ∼400au. To reveal the structure of this high-mass protostellar envelope in detail at a ∼0.17'' resolution, we used the thermal dust continuum emission and spectroscopic information, amongst others from the CO (J=3-2) line, which is sensitive to the high-velocity molecular outflow of the source. We also used the SiO (J=8-7) and SO₂ (J=8_2,6-7_1,7) lines, which trace shocks along the outflow, as well as several CH₃OH and HC₃N lines that probe the gas of the inner envelope in the closest vicinity of the protostar. Our observations of the dust continuum emission reveal a single high-mass protostellar envelope, down to our resolution limit. We find evidence for a compact, marginally resolved continuum source that is surrounded by azimuthal elongations that could be consistent with a spiral pattern. We also report on the detection of a rotational line of CH₃OH within its v_t=1 torsionally excited state. This shows two bright emission peaks that are spatially offset from the dust continuum peak and exhibit a distinct velocity component ±4.5km/s offset from the systemic velocity of the source. Rotational diagram analysis and models based on local thermodynamic equilibrium assumption require high CH₃OH column densities that reach N(CH₃OH)=1.2-2x10¹⁹cm^–2, and kinetic temperatures of the order of 160-200 K at the position of these peaks. A comparison of their morphology and kinematics with those of the outflow component of the CO line and the SO₂ line suggests that the high-excitation CH₃OH spots are associated with the innermost regions of the envelope. While the HC₃N v₇=0 (J=37-36) line is also detected in the outflow, the HC₃N v₇=1e (J=38-37) rotational transition within the first vibrationally excited state of the molecule shows a compact morphology. We find that the velocity shifts at the position of the observed high-excitation CH₃ OH spots correspond well to the expected Keplerian velocity around a central object with 15M_☉ consistent with the mass estimate based on the bolometric luminosity of the source. We propose a picture where the CH₃ OH emission peaks trace the accretion shocks around the centrifugal barrier, pinpointing the interaction region between the collapsing envelope and an accretion disc. The physical properties of the accretion disc inferred from these observations suggest a specific angular momentum several times higher than typically observed towards low-mass protostars. This is consistent with a scenario of global collapse setting on at larger scales that could carry a more significant amount of kinetic energy compared to the core-collapse models of low-mass star formation. Furthermore, our results suggest that vibrationally excited HC₃ N emission could be a new tracer for compact accretion discs around high-mass protostars.