Deuterium fractionation and the degree of ionisation in massive clumps
  within infrared dark clouds
  (Abridged) We aim to determine the degrees of CO depletion, deuterium
fractionation, and ionisation in a sample of seven massive clumps associated
with IRDCs. The APEX telescope was used to observe the C17O(2-1), H13CO+(3-2),
DCO+(3-2), N2H+(3-2), and N2D+(3-2) transitions towards the clumps. The CO
molecules do not appear to be significantly depleted in the observed clumps.
The DCO+/HCO+ and N2D+/N2H+ column density ratios are about 0.0002-0.014 and
0.002-0.028, respectively. The former ratio is found to decrease as a function
of gas kinetic temperature. A simple chemical analysis suggests that the lower
limit to the ionisation degree is in the range x(e)~10^{-8}-10^{-7}, whereas
the estimated upper limits range from a few 10^{-6} up to ~10^{-4}. Lower
limits to x(e) imply the cosmic-ray ionisation rate of H2 to lie between
zeta_H2~10^{-17}-10^{-15} s^{-1}. These are the first estimates of x(e) and
zeta_H2 towards massive IRDCs reported so far. The finding that CO is not
depleted in the observed sources conforms to the fact that they show evidence
of star formation activity which is believed to release CO from the icy grain
mantles back into the gas phase. The observed degree of deuteration is lower
than in low-mass starless cores and protostellar envelopes. Decreasing
deuteration with increasing temperature is likely to reflect the clump
evolution. On the other hand, the association with young high-mass stars could
enhance zeta_H2 and x(e) above the levels usually found in low-mass
star-forming regions. On the scale probed by our observations, ambipolar
diffusion cannot be a main driver of clump evolution unless it occurs on
timescales >>10^6 yr.
