2009MNRAS.395.2234Z

The aim of this study is to investigate systematic chemical differentiation of molecules in regions of high-mass star formation (HMSF). We observed five prominent sites of HMSF in HCN, HNC, HCO⁺, their isotopes, C¹⁸O, C³⁴S and some other molecular lines, for some sources both at 3 and 1.3 mm and in continuum at 1.3 mm. Taking into account earlier obtained data for N₂H⁺, we derive molecular abundances and physical parameters of the sources (mass, density, ionization fraction, etc.). The kinetic temperature is estimated from CH₃C₂ H observations. Then, we analyse correlations between molecular abundances and physical parameters and discuss chemical models applicable to these species. The typical physical parameters for the sources in our sample are the following: kinetic temperature in the range ∼30-50 K (it is systematically higher than that obtained from ammonia observations and is rather close to dust temperature), masses from tens to hundreds solar masses, gas densities ∼10⁵/cm3 and ionization fraction ∼10^–7. In most cases, the ionization fraction slightly (a few times) increases towards the embedded young stellar objects (YSOs). The observed clumps are close to gravitational equilibrium. There are systematic differences in distributions of various molecules. The abundances of CO, CS and HCN are more or less constant. There is no sign of CO and/or CS depletion as in cold cores. At the same time, the abundances of HCO⁺, HNC and especially N₂H⁺ strongly vary in these objects. They anticorrelate with the ionization fraction and as a result decrease towards the embedded YSOs. For N₂H⁺ this can be explained by dissociative recombination to be the dominant destroying process. N₂H⁺, HCO⁺ and HNC are valuable indicators of massive protostars.