Shaken or Stirred: The Diffuse Interstellar Medium with Exceptionally High SiO Abundance

Abstract

Interstellar shocks, a key element of stellar feedback processes, shape the structure of the interstellar medium (ISM) and are essential for the chemistry, thermodynamics, and kinematics of interstellar gas. Powerful, high-velocity shocks are driven by stellar winds, young supernova explosions, more evolved supernova remnants, cloud–cloud collisions, and protostellar outflows, whereas the existence and origin of much-lower-velocity shocks (≲10 km s−1) are not understood. Direct observational evidence for interstellar shocks in diffuse and translucent ISM environments has been especially lacking. We present the most sensitive survey to date of SiO—often considered an unambiguous tracer of interstellar shocks—in absorption, obtained with the Northern Extended Millimeter Array interferometer. We detect SiO in five of eight directions probing diffuse and translucent environments without ongoing star formation. Our results demonstrate that SiO formation in the diffuse ISM (i.e., in the absence of significant star formation and stellar feedback) is more widespread and effective than previously reported. The observed SiO line widths are all ≲4 km s−1, excluding high-velocity shocks as a formation mechanism. Yet, the SiO abundances we detect are mostly 1–2 orders of magnitude higher than those typically assumed in quiescent environments and are often accompanied by other molecular transitions whose column densities cannot be explained with UV-dominated chemical models. Our results challenge the traditional view of SiO production via stellar feedback sources and emphasize the need for observational constraints on the distribution of Si in the gas phase and grain mantles, which are crucial for understanding the physics of grain processing and the diffuse interstellar chemistry.

Gas-Phase Production of Hydroxylated Silicon Oxide Cluster Cations: Structure, Infrared Spectroscopy, and Astronomical Relevance

“…Once formed, silicate dust grains leave their circumstellar birthpace and enter the ISM where energetic processing (e.g., by supernovae shocks) leads to sputtering of Si atoms and/or Si-bearing material, and subsequent reformation of SiO molecules . Despite this possible SiO formation route, recent detection of highly abundant SiO molecules in the diffuse and translucent ISM suggests that shocked dust may not be the primary source of interstellar SiO . The high abundance of SiO is, however, consistent with the expected high production of SiO predicted by models of gas phase silicon chemistry in translucent and dense molecular clouds .…”

“…48 Despite this possible SiO formation route, recent detection of highly abundant SiO molecules in the diffuse and translucent ISM suggests that shocked dust may not be the primary source of interstellar SiO. 49 The high abundance of SiO is, however, consistent with the expected high production of SiO predicted by models of gas phase silicon chemistry in translucent 50 and dense molecular clouds. 51 Here, the efficiency of SiO formation appears to be reliant on the proposed presence of a number of positively ionized species (e.g., Si + , SiO + , and SiOH + ) and their reactions with H 2 , H 2 O, and OH.…”