2017A&A...606A.126I

Context. High-mass star formation remains poorly understood due to observational difficulties (e.g. high dust extinction and large distances) hindering the resolution of disk-accretion and outflow-launching regions.
Aims. Orion Source I is the closest known massive young stellar object (YSO) and exceptionally powers vibrationally-excited SiO masers at radii within 100 AU, providing a unique probe of gas dynamics and energetics. We seek to observe and image these masers with Very Long Baseline Interferometry (VLBI).
Methods. We present the first images of the ²⁸SiO v=1, J=2-1 maser emission around Orion Source I observed at 86GHz (λ3mm) with the Very Long Baseline Array (VLBA). These images have high spatial (∼0.3mas) and spectral (∼0.054km/s) resolutions.
Results. We find that the λ3mm masers lie in an X-shaped locus consisting of four arms, with blue-shifted emission in the south and east arms and red-shifted emission in the north and west arms. Comparisons with previous images of the ²⁸SiO v=1,2, J=1-0 transitions at λ7mm (observed in 2001-2002) show that the bulk of the J=2-1 transition emission follows the streamlines of the J=1-0 emission and exhibits an overall velocity gradient consistent with the gradient at λ7mm. While there is spatial overlap between the λ3mm and λ7mm transitions, the λ3mm emission, on average, lies at larger projected distances from Source I (∼44AU compared with ∼35AU for λ7mm). The spatial overlap between the v=1, J=1-0 and J=2-1 transitions is suggestive of a range of temperatures and densities where physical conditions are favorable for both transitions of a same vibrational state. However, the observed spatial offset between the bulk of emission at λ3mm and λ7mm possibly indicates different ranges of temperatures and densities for optimal excitation of the masers. We discuss different maser pumping models that may explain the observed offset.
Conclusions. We interpret the λ3mm and λ7mm masers as being part of a single wide-angle outflow arising from the surface of an edge-on disk rotating about a northeast-southwest axis, with a continuous velocity gradient indicative of differential rotation consistent with a Keplerian profile in a high-mass proto-binary.