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Chemical models predict that the deuterated fraction (the column densityratio between a molecule containing D and its counterpart containing H) ofN2H+, Dfrac(N2H+), is high in massive pre-protostellar cores and rapidly dropsof an order of magnitude after the protostar birth, while that of HNC,Dfrac(HNC), remains constant for much longer. We tested these predictions byderiving Dfrac(HNC) in 22 high-mass star forming cores divided in threedifferent evolutionary stages, from high-mass starless core candidates (HMSCs,8) to high-mass protostellar objects (HMPOs, 7) to Ultracompact HII regions(UCHIIs, 7). For all of them, Dfrac (N2H+) was already determined throughIRAM-30m Telescope observations, which confirmed the theoretical rapid decreaseof Dfrac(N2H+) after protostar birth (Fontani et al. 2011). Therefore ourcomparative study is not affected by biases introduced by the source selection.We have found average Dfrac(HNC) of 0.012, 0.009 and 0.008 in HMSCs, HMPOs andUCHIIs, respectively, with no statistically significant differences among thethree evolutionary groups. These findings confirm the predictions of thechemical models, and indicate that large values of Dfrac(N2H+) are moresuitable than large values of Dfrac(HNC) to identify cores on the verge offorming high-mass stars, likewise what found in the low-mass regime.

DNC/HNC and N2D+/N2H+ ratios in high-mass star forming cores