Deuterium Chemistry in the Young Massive Protostellar Core NGC 2264 CMM3
  In this work we present the first attempt of modelling the deuterium
chemistry in the massive young protostellar core NGC 2264 CMM3. We investigated
the sensitivity of this chemistry to the physical conditions in its surrounding
environment. The results showed that deuteration, in the protostellar gas, is
affected by variations in the core density, the amount of gas depletion onto
grain surfaces, the CR ionisation rate, but it is insensitive to variations in
the H$_2$ ortho-to-para ratio.
  Our results, also, showed that deuteration is often enhanced in less-dense,
partially depleted ($<$ 85\%), or cores that are exerted to high CR ionisation
rates ($\ge$ 6.5 $\times$ 10$^{-17}$ s$^{-1}$ ). However, in NGC 2264 CMM3,
decreasing the amount of gas depleted onto grains and enhancing the CR
ionisation rate are often overestimating the observed values in the core. The
best fit time to observations occurs around (1 - 5) $\times$ 10$^4$ yrs for
core densities in the range (1 - 5) $\times$ 10$^6$ cm$^{-3}$ with CR
ionisation rate between (1.7 - 6.5)$\times$ 10$^{-17}$ s$^{-1}$. These values
are in agreement with the results of the most recent theoretical chemical model
of CMM3, and the time range of best fit is, also, in-line with the estimated
age of young protostellar objects.
  We conclude that deuterium chemistry in protostellar cores is: (i) sensitive
to variations in the physical conditions in its environment, %(ii) deuteration
in the protostellar gas is (ii) insensitive to changes in the H$_2$
ortho-to-para ratio. %, but is sensitive to these variations in the dark cloud
phase. We also conclude that the core NGC 2264 CMM3 is in its early stages of
chemical evolution with an estimated age of (1 - 5) $\times$ 10$^4$ yrs.
