Tracing shocks and photodissociation in the Galactic center region

TL;DR

This study analyzes molecular emissions in the Galactic center to differentiate between shock and radiation effects using the HNCO to CS abundance ratio as a diagnostic tool.

Contribution

It introduces the HNCO to CS abundance ratio as a novel diagnostic for distinguishing shock and radiation influences in galactic nuclei.

Findings

HNCO abundance varies by over a factor of 20 among sources.

The HNCO/13CS ratio differs by up to a factor of 30 between shock-dominated and radiation-dominated regions.

The ratio effectively indicates the dominant physical process in molecular clouds.

Abstract

We present a systematic study of the HNCO, C18O, 13CS, and C34S emission towards 13 selected molecular clouds in the Galactic center region. The molecular emission in these positions are used as templates of the different physical and chemical processes claimed to be dominant in the circumnuclear molecular gas of galaxies. The relative abundance of HNCO shows a variation of more than a factor of 20 amo ng the observed sources. The HNCO/13CS abundance ratio is highly contrasted (up to a factor of 30) between the shielded molecular clouds mostly affected by shocks, where HNCO is released to gas-phase from grain mantles, and those pervaded by an intense UV radiation field, where HNCO is photo-dissociated and CS production favored via ion reactions. We propose the relative HNCO to CS abundance ratio as a highly contrasted diagnostic tool to distinguish between the influence of shocks and/or the radiation field in the nuclear regions of galaxies and their relation to the evolutionary state of their nuclear star formation bursts.