Shaken or stirred: the diffuse interstellar medium with exceptionally high SiO abundance

TL;DR

This study presents the most sensitive survey of SiO in diffuse interstellar environments, revealing widespread SiO formation without high-velocity shocks, challenging traditional models of shock-driven chemistry.

Contribution

It provides new observational evidence of SiO in diffuse ISM, demonstrating its abundance and formation mechanisms outside of high-velocity shocks and stellar feedback.

Findings

SiO detected in 5 out of 8 diffuse regions

SiO linewidths are all ≤ 4 km/s, indicating low-velocity shocks

Detected SiO abundances are 10-100 times higher than in quiescent environments

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 ($\lesssim $ 10 km$~$s$^{-1}$) are not understood. Direct observational evidence for interstellar shocks in diffuse and translucent ISM environments have 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 5/8 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 linewidths are all $\lesssim$ 4 km$~$s$^{-1}$, excluding high-velocity shocks as a formation mechanism. Yet, the SiO abundances we detect are mostly 1 to 2 orders of magnitude higher than typically assumed in quiescent environments and are often accompanied with 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 diffuse interstellar chemistry.