2008ApJ...678..245M

We present a systematic study of the HNCO, C¹⁸O, ¹³ CS, and C³⁴S emission toward 13 selected molecular clouds in the Galactic center region. The molecular emission in these positions is used as a template 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 among 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 photodissociated 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.