The ALMA Survey of 70 $μ\rm m$ Dark High-mass Clumps in Early Stages (ASHES). VII: Chemistry of Embedded Dense Cores

Abstract: We present a study of chemistry toward 294 dense cores in 12 molecular clumps using the data obtained from the ALMA Survey of 70 $\mu \rm m$ dark High-mass clumps in Early Stages (ASHES). 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 of N${2}$D$^{+}$, implying that DCO$^{+}$ appears more frequently than N${2}$D$^{+}$ in both prestellar and protostellar cores. Both N${2}$D$^{+}$ and DCO$^{+}$ abundances appear to decrease from the prestellar to 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 of 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.