Probing the Evolutionary Status of Starless Cores through N2H+and N2D+Observations

We have undertaken a survey of N2H+ and N2D+ towards 31 low-mass starlesscores using the IRAM 30m telescope. Our main objective has been to determinethe abundance ratio of N2D+ and N2H+ towards the nuclei of these cores and thusto obtain estimates of the degree of deuterium enrichment, a symptom ofadvanced chemical evolution according to current models. We find that theN(N2D+)/N(N2H+) ratio is larger in more "centrally concentrated cores" withlarger peak H2 and N2H+ column density than the sample mean. The deuteriumenrichment in starless cores is presently ascribed to depletion of CO in thehigh density (> 3*10^4 cm-3) core nucleus. To substantiate this picture, wecompare our results with observations in dust emission at 1.2 mm and in twotransitions of C18O. We find a good correlation between deuterium fractionationand N(C18O)/N(H2) for the nuclei of 14 starless cores. We, thus, identified aset of properties that characterize the most evolved, or "pre-stellar",starless cores. These are: higher N2H+ and N2D+ column densities, higherN(N2D+)/N(N2H+), more pronounced CO depletion, broader N2H+ lines with infallasymmetry, higher central H2 column densities and a more compact densityprofile than in the average core. We conclude that this combination ofproperties gives a reliable indication of the evolutionary state of the core.Seven cores in our sample (L1521F, OphD, L429, L694, L183, L1544 and TMC2) showthe majority of these features and thus are believed to be closer to forming aprotostar than are the other members of our sample. Finally, we note that thesubsample of Taurus cores behaves more homogeneously than the total sample, anindication that the external environment could play an important role in thecore evolution.