C₂H N = 1 − 0 and N₂H⁺ J = 1 − 0 observations of Planck Galactic cold clumps

¹ Department of Astronomy, School of Physics, Peking University, 100871 Beijing, PR China
e-mail: 1501110219@pku.edu.cn; ywu@pku.edu.cn
² Kavli Institute for Astronomy and Astrophysics, Peking University, 100871 Beijing, PR China
³ Department of Astronomy, Yunnan University, Kunming, 650091, PR China
⁴ Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Korea
⁵ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁶ Purple Mountain Observatory, Qinghai Station, 817000, Delingha, PR China
⁷ Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, PR China

Received: 10 October 2018
Accepted: 17 November 2018

Abstract

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 13.7 m 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 9–21 K) 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 (< 5 × 10⁵ yr) 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.