[Paper Review] Chasing discs around O-type (proto)stars - ALMA evidence for an SiO disc and disc wind from G17.64+0.16

L. T. Maud, R. Cesaroni+29 more|UvA-DARE (University of Amsterdam)|Oct 9, 2018

Astrophysics and Star Formation Studies144 references18 citations

TL;DR

This paper presents ALMA observations of the massive protostar G17.64+0.16 at 0.2 arcsecond resolution, detecting a compact, Keplerian SiO disc (~200 au radius) and a radially expanding disc wind. The data, combined with H30α recombination line emission, indicate a 20–30 M⊙ O-type star in the final stages of protostellar contraction, with a disc and wind consistent with a late-stage massive star formation scenario.

ABSTRACT

We present high angular resolution 0.2 arcsec continuum and molecular emission line Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of G17.64+0.16 in Band 6 (220GHz) taken as part of a campaign in search of circumstellar discs around (proto)-O-stars. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. We detect SiO (5-4) emission that is marginally resolved and elongated in a direction perpendicular to the large-scale outflow seen in the 13CO (2-1) line using the main ALMA array in conjunction with the Atacama Compact Array (ACA). Morphologically, the SiO appears to represent a disc-like structure. Using parametric models we show that the position-velocity profile of the SiO is consistent with the Keplerian rotation of a disc around an object between 10-30Mo in mass, only if there is also radial expansion from a separate structure. The radial motion component can be interpreted as a disc wind from the disc surface. Models with a central stellar object mass between 20 and 30Mo are the most consistent with the stellar luminosity (100000 Lo) and indicative of an O-type star. The H30a millimetre recombination line (231.9GHz) is also detected, but spatially unresolved, and is indicative of a very compact, hot, ionised region co-spatial with the dust continuum core. Accounting for all observables, we suggest that G17.64 is consistent with a O-type young stellar object in the final stages of protostellar assembly, driving a wind, but that has not yet developed into a compact HII region. The existance and detection of the disc in G17.64 is likely related to its isolated and possibly more evolved nature, traits which may underpin discs in similar sources.

Motivation & Objective

• To search for circumstellar discs around (proto)-O-type stars using high-resolution ALMA observations.
• To investigate the kinematic and morphological structure of molecular emission in G17.64+0.16 to identify disc-like features.
• To determine the mass of the central object and assess the evolutionary stage of the protostar using SiO and H30α line profiles.
• To explore the origin of the wide-angle outflow and its interaction with the disc and surrounding natal cloud.

Proposed method

• High angular resolution (0.2 arcsecond) ALMA Band 6 observations at 220–230 GHz to image dust continuum and molecular line emission.
• Parametric modeling of SiO (5–4) emission to fit Keplerian rotation and radial expansion, distinguishing disc and wind components.
• Comparison of CH3CN and CH3OH thermal emission with OH maser positions to trace inner outflow cavity walls.
• Analysis of H30α recombination line width and spatial morphology to infer ionized core size and bulk motions.
• Use of 13CO (2–1) emission to map large-scale outflow structure and cavity walls.
• Joint modeling with main ALMA array and Atacama Compact Array (ACA) to recover extended emission and improve dynamic range.

Experimental results

Research questions

• RQ1 Does G17.64+0.16 host a circumstellar disc around a massive protostar, as indicated by SiO emission morphology and kinematics?
• RQ2 What is the mass of the central object inferred from Keplerian rotation of the SiO disc and consistency with stellar luminosity?
• RQ3 How do the broad H30α line width and compact emission relate to ionization and bulk motions in the central region?
• RQ4 What is the origin of the arc-shaped CH3CN and CH3OH plumes, and how do they relate to the outflow and maser emission?
• RQ5 Is the radial expansion in SiO emission best explained by a disc wind, and how does it relate to the large-scale outflow direction?

Key findings

• The SiO (5–4) emission is marginally resolved and elongated perpendicular to the large-scale CO outflow (PA ~135°), with position-velocity profile consistent with Keplerian rotation in a disc.
• Modeling indicates a central object mass between 10 and 30 M⊙, with best-fit models at 20–30 M⊙, consistent with an O-type star and luminosity of 1×10⁵ L⊙.
• The radial velocity component in SiO emission is best explained by a separate radially expanding structure, interpreted as a disc wind from the disc surface.
• Compact H30α emission (FWHM = 81.9 km s⁻¹) is spatially unresolved and consistent with a hot, ionized core, indicating ongoing ionization and wind driving.
• CH3CN and CH3OH emission trace arc-shaped plumes curving away from the disc plane, coinciding with OH masers and likely marking the inner working surfaces of a wide-angle wind.
• The source is isolated and lacks a compact H II region, suggesting it is in the final stages of protostellar contraction, prior to full H II region development.