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

Abstract

We have measured the deuterium fractionation (through the column density ratio N(N2D+)/N(N2H+)) and the CO depletion factor (ratio between expected and observed CO abundance) in a sample of 10 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. The observations were carried out with the IRAM-30m telescope and the JCMT. We have detected N2D+ emission in 7 of the 10 sources of our sample, and found an average value N(N2D+)/N(N2H+)~0.015. This value is 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. Also, 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. In principle, the cold gas that gives origin to 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 can be located into one (or more) starless core (cores) near the protostellar object. Due to the poor angular resolution of our data, we cannot decide which is the correct scenario.