2009ApJ...698.1165G

We have used the Very Large Array at 7 mm wavelength to image five rotational transitions (J = 1-0) from three SiO isotopologues toward Orion Becklin-Neugebauer/Kleinmann-Low (BN/KL): ²⁸SiO v = 0, 1, 2; and ²⁹SiO and ³⁰SiO v = 0. For the first time, we have mapped the ²⁹SiO and ³⁰SiO 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 ²⁸SiO 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 ²⁸SiO 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 ²⁹SiO/³⁰SiO v = 0 and ²⁸SiO 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 pumping model, we derive similar temperatures and densities for the optimum excitation of both ²⁹SiO/³⁰SiO v = 0 and ²⁸SiO v = 1, 2 masers, significantly higher than required for ²⁸SiO 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 nonlocal line overlap among transitions of all SiO isotopic species may be required.