A line confusion limited millimeter survey of Orion KL. II: Silicon-bearing species

TL;DR

This study analyzes silicon-bearing molecules in Orion KL using millimeter-wave observations, modeling their physical and chemical properties, detecting masering effects, and exploring their formation pathways.

Contribution

It provides the first detection of SiO J=4-3 maser emission in Orion KL and models the chemical processes leading to SiO and SiS formation.

Findings

Detected SiO J=4-3 maser emission.

Mapped SiO J=5-4 line and isotopologues.

Estimated SiO/SiS ratio of ~13.

Abstract

We present a study of the Silicon-bearing species detected in a line confusion limited survey towards Orion KL performed with the IRAM 30-m telescope, which ranges from 80 to 280 GHz. Our aim is to derive physical and chemical conditions for each family taking into account all observed lines from all isotopologues of each species. We have modeled the lines of the detected molecules using a radiative transfer code, which permit us to choose between LVG and LTE approximations depending on the physical conditions of the gas. We have used appropriate collisional rates for the LVG calculations. For the v=1 state of SiO we have detected the J=2-1 line and, for the first time in this source, emission in the J=4-3 transition, both of them showing strong masering effect. For SiO v=0, we have detected 28SiO, 29SiO, and 30SiO; in addition, we have mapped the J = 5-4 SiO line. For SiS, we have detected the main species, 29SiS, and SiS v=1. Unlikely other species detected in Orion KL (IRc2), the emission peak of SiS appears at a velocity of ~15.5 km s-1; a study of the 5-4 SiO line around IRc2 shows this feature as an extended component that probably arises from the interaction of the outflow with the ambient cloud. We derive a SiO/SiS column density ratio of ~13 in the plateau component. Besides, we provide upper limits to the column density of several non-detected Silicon-bearing species. The results of our chemical models show that while it is possible to reproduce SiO in the gas phase (as well as on the grains), SiS is a product of surface reactions, most likely involving direct reactions of S with Si.