Outflow structure and velocity field of Orion source

I. ALMA imaging of SiO isotopologue maser and thermal emission

European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
e-mail: fniederh@eso.org; ehumphre@eso.org; cgoddi@eso.org

Received: 27 August 2012
Accepted: 28 September 2012

Abstract

Using Science Verification data from the Atacama Large Millimeter/Submillimeter Array (ALMA), we have identified and imaged five rotational transitions (J = 5–4 and J = 6–5) of the three silicon monoxide isotopologues ²⁸SiO v = 0, 1, 2 and ²⁹SiO v = 0 and ²⁸Si¹⁸O v = 0 in the frequency range from 214 to 246 GHz towards the Orion BN/KL region. The emission of the ground-state ²⁸SiO, ²⁹SiO and ²⁸Si¹⁸O shows an extended bipolar shape in the northeast-southwest direction at the position of Radio Source I, indicating that these isotopologues trace an outflow (~18 km s^-1, PA ~ 50°,  ~5000 AU in diameter) that is driven by this embedded high-mass young stellar object (YSO). Whereas on small scales (10–1000 AU) the outflow from Source I has a well-ordered spatial and velocity structure, as probed by Very Long Baseline Interferometry (VLBI) imaging of SiO masers, the large scales (500–5000 AU) probed by thermal SiO with ALMA reveal a complex structure and velocity field, most likely related to the effects of the environment of the BN/KL region on the outflow emanating from Source I. The emission of the vibrationally-excited species peaks at the position of Source I. This emission is compact and not resolved at an angular resolution of  ~1$\stackrel{\mathrm{″}}{.}$5 (~600 AU at a distance of 420 pc). 2D Gaussian fitting to individual velocity channels locates emission peaks within radii of 100 AU, i.e. they trace the innermost part of the outflow. A narrow spectral profile and spatial distribution of the v = 1 J = 5–4 line similar to the masing v = 1 J = 1–0 transition, provide evidence for the most highly rotationally excited (frequency  > 200 GHz) SiO maser emission associated with Source I known to date. The maser emission will enable studies of the Source I disk-outflow interface with future ALMA longest baselines.