The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). VII. Chemistry of Embedded Dense Cores

Abstract

We present a study of the chemistry toward 294 dense cores in 12 molecular clumps, using data obtained from the ALMA Survey of 70 μ m dark High-mass clumps in Early Stages. We identified 97 protostellar cores and 197 prestellar core candidates, based on the detection of outflows and molecular transitions of high upper-energy levels ( E u / k > 45 K). The detection rate of the N 2 D + emission toward the protostellar cores is 38%, which is higher than 9% for the prestellar cores, indicating that N 2 D + does not exclusively trace prestellar cores. The detection rates of the DCO + emission are 35% for the prestellar cores and 49% for the protostellar cores, which are higher than those for N 2 D + , implying that DCO + appears more frequently than N 2 D + in both prestellar and protostellar cores. Both the N 2 D + and DCO + abundances appear to decrease from the prestellar to the protostellar stage. The DCN, C 2 D, and 13 CS emission lines are rarely seen in the dense cores of early evolutionary phases. The detection rate of the H 2 CO emission toward dense cores is 52%, three times higher than that for CH 3 OH (17%). In addition, the H 2 CO detection rate, abundance, line intensities, and line widths increase with the core evolutionary status, suggesting that the H 2 CO line emission is sensitive to protostellar activity.