Maser Emission from Sio Isotopologues Traces the Innermost 100 Au around Radio Source I in Orion Bn/kl

We have used the Very Large Array (VLA) at 7 mm wavelength to image five rotational transitions (J = 1 − 0) from three SiO isotopologues towards Orion BN/KL: SiO v = 0, 1, 2; and SiO and SiO v = 0. For the first time, we have mapped the SiO and SiO J = 1− 0 emission, established the maser nature of the emission, and confirmed association with the deeply embedded high-mass young stellar object commonly denoted radio Source I. The SiO v = 0 maser emission shows a bipolar structure that extends over ∼ 700 AU along a northeast-southwest axis, and we propose that it traces a bipolar outflow driven by Source I. The high-brightness isotopic SiO maser emission imaged with a .0. 2 resolution has a more compact distribution, generally similar to that of the SiO v = 1, 2 emission, and it probably traces bulk gas flows in a region of diameter .100 AU centered on Source I. On small scales of . 10 AU, however, compact SiO/SiO v = 0 and SiO v = 1, 2 emission features may be offset from one another in position and line-of-sight velocity. From a radiative transfer analysis based on a large velocity gradient (LVG) pumping model, we derive similar temperatures and densities for the optimum excitation of both SiO/SiO v = 0 and SiO v = 1, 2 masers, significantly higher than required for SiO v = 0 maser excitation. In order to account for the small-scale differences among the isotopologues (v = 0) and the main species (v = 1, 2), follow-up radiative transfer modeling that incorporates non-local line overlap among transitions of all SiO isotopic species may be required. Subject headings: line: profiles masers ISM: individual objects: Orion BN/KL stars: formation