[Paper Review] 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

T. Csengeri, N. Schneider+12 more|Repository of the Academy's Library (Library of the Hungarian Academy of Sciences)|Apr 17, 2018

Astrophysics and Star Formation Studies81 references21 citations

TL;DR

This study presents ALMA observations of the high-mass protostar G328.2551-0.5321, revealing a compact, marginally resolved dust continuum source and azimuthal elongations consistent with an accretion disk. Torsionally excited CH3OH emission at ±4.5 km s⁻¹ velocity offset from the systemic velocity traces shocks at the centrifugal barrier, indicating a disk with specific angular momentum an order of magnitude higher than in low-mass protostars, supporting global collapse models over core collapse scenarios.

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$ imes$10$^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 $v_{ m t}$=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 $v_{ m lsr}$. Local thermodynamic equilibrium analysis suggests N(CH$_3$OH)=1.2$-$2$ imes$10$^{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 $v_{ m 7}$=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.

Motivation & Objective

• To investigate the physical conditions and accretion structure of a high-mass protostar in its main accretion phase, prior to UC HII region formation.
• To determine whether high-mass star formation proceeds via a scaled-up version of low-mass star formation or follows a distinct mechanism.
• To identify tracers of accretion disks and centrifugal barriers in massive protostellar envelopes.
• To assess the role of angular momentum and collapse dynamics in high-mass star formation.
• To explore the potential of vibrationally excited molecular lines as tracers of compact accretion disks.

Proposed method

• High angular-resolution ALMA observations at ~0.17″ resolution (400 au physical scale) were used to map thermal dust continuum and molecular line emission.
• Rotational diagram analysis and local thermodynamic equilibrium (LTE) modeling were applied to CH3OH rotational transitions to derive kinetic temperature and column density.
• The velocity structure of CH3OH and CO (J=3–2) lines was compared to infer kinematics and locate the centrifugal barrier.
• The morphology and kinematics of SiO (J=8–7), SO2, and HC3N lines were analyzed to trace shocks and outflow activity.
• The vibrationally excited HC3N (υ7=1e, J=38–37) line was examined for potential association with the inner accretion disk.
• Dust continuum deconvolution and isophotal radius measurements (R90% ~250 au) were used to infer disk size and structure.

Experimental results

Research questions

• RQ1 Does the observed CH3OH emission at ±4.5 km s⁻¹ velocity offset trace shocks at a centrifugal barrier in a high-mass protostellar envelope?
• RQ2 What is the inferred specific angular momentum of the inner envelope, and how does it compare to low-mass protostars?
• RQ3 Can vibrationally excited HC3N lines serve as a new tracer for compact accretion disks around high-mass protostars?
• RQ4 Is the observed morphology and kinematics consistent with a global collapse model rather than a core collapse model?
• RQ5 What are the physical conditions (Tkin, N(CH3OH)) in the high-excitation CH3OH spots?

Key findings

• The CH3OH emission peaks are spatially offset from the dust continuum peak and exhibit a velocity shift of ±4.5 km s⁻¹ relative to the source's systemic velocity (vLSR).
• Rotational diagram analysis under LTE assumptions yields kinetic temperatures of 160–200 K and CH3OH column densities of 1.2–2.0 × 10¹⁹ cm⁻² in the high-excitation spots.
• The observed velocity shifts at the CH3OH peaks are consistent with Keplerian rotation around a central mass of ~15 M☉, matching the bolometric luminosity-based mass estimate.
• The dust continuum reveals a marginally resolved component with a deconvolved 90% radius of ~250 au, consistent with an accretion disk within the centrifugal barrier.
• Azimuthal elongations around the dust peak suggest a spiral pattern or disk-like structure in the inner envelope.
• The vibrationally excited HC3N (υ7=1e, J=38–37) line shows a compact morphology, suggesting it may trace the innermost disk region, offering a new potential disk tracer.