The search for high-mass protostars with ALMA revealed up to kilo-parsec scales (SPARKS)

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 × 104 L?, which allows us to estimate its current protostellar mass to be between ?11 and 16 M?. We show high angular-resolution observations with ALMA reaching a physical scale of ?400 au. To reveal the structure of this high-mass protostellar envelope in detail at a ?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 SO2 (J=82,6 ? 71,7) lines tracing 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. 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 CH3OH within its 3t = 1 torsionally excited state. This shows two bright peaks of emission spatially offset from the dust continuum peak, and exhibiting a distinct velocity component ±4.5 km s?1 offset compared to the source vlsr. Rotational diagram analysis and models based on local thermodynamic equilibrium (LTE) assumption require high CH3OH column densities reaching N(CH3OH)=1.2 ? 2 × 1019 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 SO2 line suggests that the high excitation CH3OH spots are associated with the innermost regions of the envelope. While the HC3N 37 = 0 (J=37–36) line is also detected in the outflow, the HC3N 37 = 1e (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 CH3OH spots correspond well to the expected Keplerian velocity around a central object with 15 M? consistent with the mass estimate based on the source’s bolometric luminosity. We propose a picture where the CH3OH 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 HC3N emission could be a new tracer for compact accretion disks around high-mass protostars.