Aims. We investigated the chemistry ofnitrogen-containing species, principally isotopologues of CN, HCN, andHNC, in a sample of pre-protostellar cores. Methods. We used the IRAM 30m telescope tomeasure the emission in rotational and hyperfine transitions of CN,HCN, , H, HN, and HC in L1544, L183,OphD, L1517B, L310. The observations weremade along axial cuts through the dust emission peak, at a number ofregularly-spaced offset positions. The observations were reduced andanalyzed to obtain the column densities, using the measurements of theless abundant isotopic variants in order to minimize the consequencesof finite optical depths in the lines. The observations were comparedwith the predictions of a free-fall gravitational collapse model, whichincorporates a non-equilibrium treatment of the relevant chemistry. Results. We found that CN, HCN, and HNC remainpresent in the gas phase at densities well above that at whichCOdepletes on to grains. The CN:HCN and the HNC:HCN abundanceratios are larger than unity in all the objects of our sample.Furthermore, there is no observational evidence for large variations ofthese ratios with increasing offset from the dust emission peak andhence with density. Whilst the differential freeze-out of CN and CO canbe understood in terms of the current chemistry, the behaviour of theCN:HCN ratio is more difficult to explain. Models suggest that mostnitrogen is not in the gas phase but may be locked in ices. Unambiguousconclusions require measurements of the rate coefficients of the keyneutral-neutral reactions at low temperatures. Key words: ISM: abundances - ISM:molecules - ISM: clouds
展开▼
