C ₂H N = 1 ? 0 and N ₂H + J = 1 ? 0 observations of Planck Galactic cold clumps

摘要

A survey of C_(2)H N = 1 ? 0 and N_(2)H~(+) J = 1 ? 0 toward Planck Galactic cold clumps (PGCCs) was performed using the Purple Mountain Observatory’s 13.7 m telescope. C_(2)H and N_(2)H~(+)were chosen to study the chemical evolutionary states of PGCCs. Among 121 observed molecular cores associated with PGCCs, 71 and 58 are detected with C_(2)H N = 1 ? 0 and N_(2)H~(+) J = 1 ? 0, respectively. The detected lines of most sources can be fitted with a single component with compatible V _(LSR)and line widths, which confirms that these PGCC cores are very cold (with gas temperatures 9–21 K) and quiescent while still dominanted by turbulence. The ratio between the column densities of C_(2)H and N_(2)H~(+)( N (C_(2)H)/ N (N_(2)H~(+))) is found to be a good tracer for the evolutionary states of PGCC cores. Gas-grain chemical model can reproduce the decreasing trend of N (C_(2)H)/ N (N_(2)H~(+)) as a function of time. The cores with the lowest abundances of N_(2)H~(+)( X [N_(2)H~(+)] < 10~(?10)) are the youngest, and have nearly constant abundances of C_(2)H. In evolved cores with X [N_(2)H~(+)] ~10~(?9), abundances of C_(2)H drop quickly as the exhaustion of carbon atoms. Although these PGCC cores are in different evolutionary states, they are all quite young (< 5 × 10~(5)yr) with N (C_(2)H) > N (N_(2)H~(+)). Mapping observations are carried out toward 20 PGCC cores. The PGCC cores in Cepheus have lower N (C_(2)H)/ N (N_(2)H~(+)) and larger line widths compared with those in Taurus. This implies that PGCC cores in Taurus are less chemically evolved than those in Cepheus.