2019A&A...622A..32L

A survey of C₂H N=1-0 and N₂H⁺ J=1-0 toward Planck Galactic cold clumps (PGCCs) was performed using the Purple Mountain Observatory's 13.7m telescope. C₂H and N₂H⁺ were chosen to study the chemical evolutionary states of PGCCs. Among 121 observed molecular cores associated with PGCCs, 71 and 58 are detected with C₂H N=1-0 and N₂H⁺ 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 9-21K) and quiescent while still dominanted by turbulence. The ratio between the column densities of C₂H and N₂H⁺ (N(C₂H)/N(N₂H⁺)) 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(C₂H)/N(N₂H⁺) as a function of time. The cores with the lowest abundances of N₂H⁺ (X[N₂H⁺]<10^–10) are the youngest, and have nearly constant abundances of C₂H. In evolved cores with X[N₂H⁺]∼10^–9, abundances of C₂H drop quickly as the exhaustion of carbon atoms. Although these PGCC cores are in different evolutionary states, they are all quite young (<5x10⁵yr) with N(C₂H)>N(N₂H⁺). Mapping observations are carried out toward 20 PGCC cores. The PGCC cores in Cepheus have lower N(C₂H)/N(N₂H⁺) and larger line widths compared with those in Taurus. This implies that PGCC cores in Taurus are less chemically evolved than those in Cepheus.