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 1.3 \times 10 ^ { 4 } L _ { \odot } , which allows us to estimate its current protostellar mass to be between \sim 11 and 16 M _ { \odot } .
We show high angular-resolution observations with ALMA reaching a physical scale of \sim 400 au .
To reveal the structure of this high-mass protostellar envelope in detail at a \sim 0.17″ resolution , we use 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 , the SiO ( J =8–7 ) , and SO _ { 2 } ( J = 8 _ { 2 , 6 } -7 _ { 1 , 7 } ) lines tracing shocks along the outflow , as well as several CH _ { 3 } OH and HC _ { 3 } N lines that probe the gas of the inner envelope in the closest vicinity of the protostar .
The dust continuum emission reveals a single high-mass protostellar envelope , down to our resolution limit .
We find evidence for a compact , marginally resolved continuum source , which is surrounded by azimuthal elongations that could be consistent with a spiral pattern .
We also report on the detection of a rotational line of CH _ { 3 } OH within its \varv _ { t } = 1 torsionally excited state .
This shows two bright peaks of emission spatially offset from the dust continuum peak , and exhibiting a distinct velocity component \pm 4.5 km s ^ { -1 } offset compared to the source v _ { lsr } .
Rotational diagram analysis and models based on local thermodynamic equilibrium ( LTE ) assumption require high CH _ { 3 } OH column densities reaching N ( CH _ { 3 } OH ) = 1.2 - 2 \times 10 ^ { 19 } cm ^ { -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 SO _ { 2 } line suggests that the high excitation CH _ { 3 } OH spots are associated with the innermost regions of the envelope .
While the HC _ { 3 } N \varv _ { 7 } = 0 ( J =37–36 ) line is also detected in the outflow , the HC _ { 3 } N \varv _ { 7 } = 1 e ( J =38–37 ) rotational transition within the molecule ’ s vibrationally excited state shows a compact morphology .
We find that the velocity shifts at the position of the observed high excitation CH _ { 3 } OH spots correspond well to the expected Keplerian velocity around a central object with 15 M _ { \odot } consistent with the mass estimate based on the source ’ s bolometric luminosity .
We propose a picture where the CH _ { 3 } OH emission peaks trace the accretion shocks around the centrifugal barrier , pinpointing the interaction region between the collapsing envelope and an accretion disk .
The physical properties of the accretion disk inferred from these observations suggest a specific angular momentum several times larger 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 exited HC _ { 3 } N emission could be a new tracer for compact accretion disks around high-mass protostars .