The search for high-mass protostars with ALMA revealed up to kilo-parsec scales (SPARKS): I. Indication for a centrifugal barrier in the environment of a single high-mass envelope

Abstract

The formation of the most massive O-type stars is poorly understood. We present a case study of a young massive clump from the ATLASGAL survey, G328.2551-0.5321. It exhibits a bolometric luminosity of 1.3$\times$10${}^4$ L${}_{\odot }$ corresponding to a current protostellar mass of $\sim$11 and 16 M${}_{\odot }$. We analyze high angular-resolution observations with ALMA at $\sim$0.17" corresponding a physical scale of $\sim$400 au in dust continuum and molecular lines. The dust continuum emission reveals a single high-mass protostellar envelope and shows evidence for a marginally resolved continuum source. We detect a rotational line of CH${}_3$OH within its $v_{\mathrm{t}}$=1 torsionally excited state revealing 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 $v_{\mathrm{lsr}}$. Local thermodynamic equilibrium analysis suggests N(CH${}_3$OH)=1.2$-$2$\times$10${}^{19}$ cm${}^{-2}$, and kinetic temperatures of 160$-$200 K at the position of these peaks. Their velocity shifts correspond well to the expected Keplerian velocity around a central object with 15M${}_{\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. Because the HC${}_3$N $v_7$=1e ($J$=38-37) line shows compact emission, and a velocity pattern consistent with models of Keplerian rotation, we suggest that this could be a new tracer for compact accretion disks around high-mass protostars. The estimated physical properties of the accretion disk suggest a specific angular momentum several times larger than typically observed towards low-mass protostars.

Manuscript submitted to A&A, and revised after first referee report (abstract abbreviated)