The abundance of HNCO and its use as a diagnostic of environment

摘要

Aims. We aim to investigate the chemistry and gasphase abundance of HNCO and the variation of the HNCO/CS abundanceratio as a diagnostic of the physics and chemistry in regions ofmassive star formation. Methods. A numerical-chemical model has beendeveloped which self-consistently follows the chemical evolution of ahot core. The model comprises of two distinct stages. The first stagefollows the isothermal, modified free-fall collapse of a molecular darkcloud. This is immediately followed by an increase in temperature whichrepresents the switch on of a central massive star and the subsequentevolution of the chemistry in a hot, dense gas cloud (the hot core).During the collapse phase, gas species are allowed to accrete on tograin surfaces where they can participate in further reactions. Duringthe hot core phase surface species thermally desorb back in to theambient gas and further chemical evolution takes place. For comparison,the chemical network was also used to model a simple dark cloud andphotodissociation regions. Results. Our investigation reveals that HNCO isinefficiently formed when only gas-phase formation pathways areconsidered in the chemical network with reaction rates consistent withexisting laboratory data. This is particularly true at low temperaturesbut also in regions with temperatures up to 200K. Using currently measured gasphase reaction rates, obtaining the observed HNCO abundances requiresits formation on grain surfaces - similar to other ``hot core'' speciessuch as CH3OH. However our model shows that thegas phase HNCO in hot cores is not a simple direct product of theevaporation of grain mantles. We also show that the HNCO/CS abundanceratio varies as a function of time in hot cores and can match the rangeof values observed. This ratio is not unambiguously related to theambient UV field as been suggested - our results are inconsistent withthe hypothesis of Martín etal. (2008, ApJ, 678, 245). Inaddition, our results show that this ratio is extremely sensitive tothe initial sulphur abundance. We find that the ratio growsmonotonically with time with an absolute value which scalesapproximately linearly with the S abundance at early times. Key words: astrochemistry - stars:formation