Maser emission from SiO isotopologues traces the innermost 100 AU around Radio Source I in Orion BN/KL

TL;DR

This study uses VLA observations to map SiO isotopologue masers around Orion Source I, revealing detailed structures within 100 AU and providing insights into the physical conditions and outflow dynamics of this high-mass star-forming region.

Contribution

First mapping of 29SiO and 30SiO J=1-0 masers, confirming their association with Source I and analyzing their spatial and velocity structures with high resolution.

Findings

28SiO v=0 maser traces bipolar outflow over ~700 AU

High-brightness isotopic SiO masers are compact, within 100 AU

Similar temperatures and densities for 29SiO/30SiO v=0 and 28SiO v=1,2 masers

Abstract

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: 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 arcsec 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 (LVG) 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 non-local line overlap among transitions of all SiO isotopic species may be required.