Searching for massive pre-stellar cores through observations of N2H+ and N2D+

Aims. We have measured the deuterium fractionation and the CO depletion factor (ratio between expected and observed CO abundance) in a sample of high-mass protostellar candidates, in order to understand whether the earliest evolutionary stages of high-mass stars have chemical characteristics similar to those of low-mass ones. It has been found that low-mass starless cores on the verge of star formation have large values both of the column density ratio N(N2D+)/N(N2H+) and of the CO depletion factor. Methods. With the IRAM-30 m telescope and the JCMT we have observed two rotational lines of N2H+ and N2D+, the (2-1) line of (CO)-O-17 and DCO+, and the sub-millimeter continuum towards a sample of 10 high-mass protostellar candidates. Results. We have detected N2D+ emission in 7 of the 10 sources of our sample, and found an average value N(N2D+)/N(N2H+) similar to 0.015. This value is similar to 3 orders of magnitude larger than the interstellar D/H ratio, indicating the presence of cold and dense gas, in which the physical-chemical conditions are similar to those observed in low-mass pre-stellar cores. The integrated CO depletion factors show that in the majority of the sources the expected CO abundances are larger than the observed values, with a median ratio of 3.2. Conclusions. In principle, the cold gas that generates the N2D+ emission can be the remnant of the massive molecular core in which the high-mass (proto-) star was born, not yet heated up by the central object. If so, our results indicate that the chemical properties of the clouds in which high-mass stars are born are similar to their low-mass counterparts. Alternatively, this cold gas could be located in one (or more) starless core (cores) near the protostellar object. Due to the poor angular resolution of our data, we cannot distinguish between the two scenarios.