摘要：Aims.We aim to investigate the chemistry of internal photon-dominated regions (PDRs) surrounding deeply embedded hypercompact (HC) and ultracompact (UC) HIIregions. We search for specific tracers of this evolutionary stage of massive star formation that can be detected with current astronomical facilities.Methods.We modeled hot cores with embedded HC/UCHIIregions (called HIIregion models in the article despite the fact that we do not model the HIIregion itself), by coupling the astrochemical codeSaptarsyto a radiative transfer framework obtaining the spatio-temporal evolution of abundances as well as time-dependent synthetic spectra. In these models where we focused on the internal PDR surrounding the HIIregion, the gas temperature is set to the dust temperature and we do not include dynamics thus the density structure is fixed. We compared this to hot molecular core (HMC) models and studied the effect on the chemistry of the radiation field which is included in the HIIregion models only during the computation of abundances. In addition, we investigated the chemical evolution of the gas surrounding HIIregions with models of different densities at the ionization front, different sizes of the ionized cavity and different initial abundances.Results.We obtain the time evolution of synthetic spectra for a dozen of selected species as well as ratios of their integrated intensities. We find that some molecules such as C, N2H+, CN, and HCO do not trace the inner core and so are not good tracers to distinguish the HII/PDR regions to the HMCs phase. On the contrary, C+and O trace the internal PDRs, in the two models starting with different initial abundances, but are unfortunately currently unobservable with the current achievable spatial resolution because of the very thin internal PDR (ΔrPDR< 100 AU). The emission of these two tracers is very dependent on the size of the HIIregion and on the density in the PDR. In addition, we find that the abundance profiles are highly affected by the choice of the initial abundances, hence the importance to properly define them.
关键词：Key wordsenastrochemistrystars: formationstars: massiveISM: moleculesISM: lines and bandsevolution
