Chemical Timescale of Dark Cloud Cores Estimated from Deuterium Fractionation

Abstract

The chemical evolutionary age of dark cloud cores is estimated from the degree of deuterium fractionation, on the assumption that the chemical model used is reasonable. The method is applied to dark cloud cores along the TMC-1 ridge, using the abundance ratios between DCO+ and H13CO+ combined with the new standard model network of gas-phase chemical reactions. The difference in deuterium fractionation between the ammonia peak and the cyanopolyyne peak, though its error transferred from the observed data is relatively large, is explained by a time difference in the evolutionary age of more than 105 yr, or by a small change in the depletion factor of carbon and oxygen, which also indicates the degree of core evolution. The method of determining the evolutionary age of dark cloud cores is somewhat free from the details of chemical reaction not directly related to deuterium fractionation. The present result is compared with those from other methods.