Chemical properties of two dense cores in a Planck Galactic Cold Clump G168.72-15.48

TL;DR

This study investigates the chemical compositions of two dense cores within a Planck Galactic Cold Clump, revealing differences in molecular abundances and suggesting variations in evolutionary stages and physical conditions.

Contribution

First detailed chemical analysis of two dense cores in PGCC G168.72-15.48, highlighting molecular abundance differences and their relation to core evolution.

Findings

N2H+ depletion likely caused by N2 depletion, not CO destruction

Younger core G168-H1 shows higher carbon-chain molecule abundance

G168-H1 resembles cold dark clouds, G168-H2 is more evolved

Abstract

To deepen our understanding of the chemical properties of the Planck Galactic Cold Clump (PGCC) G168.72-15.48, we performed observations of nine molecular species, namely, \ce{c-C3H}, \ce{H2CO}, \ce{HC5N}, \ce{HC7N}, \ce{SO}, \ce{CCH}, \ce{N2H+}, \ce{CH3OH}, and \ce{CH3CCH}, toward two dense cores in PGCC G168.72-15.48 using the Tianma Radio Telescope and Purple Mountain Observatory Telescope. We detected \ce{c-C3H}, \ce{H2CO}, \ce{HC5N}, \ce{N2H+}, \ce{CCH}, and \ce{CH3OH} in both G168-H1 and G168-H2 cores, whereas \ce{HC7N} and \ce{CH3CCH} were detected only in G168-H1 and SO was detected only in G168-H2. Mapping observations reveal that the \ce{CCH}, \ce{N2H+}, \ce{CH3OH}, and \ce{CH3CCH} emissions are well coupled with the dust emission in G168-H1. Additionally, \ce{N2H+} exhibits an exceptionally weak emission in the denser and more evolved G168-H2 core, which may be attributed to the \ce{N2H+} depletion. We suggest that the \ce{N2H+} depletion in G168-H2 is dominated by \ce{N2} depletion, rather than the destruction by CO. The local thermodynamic equilibrium calculations indicate that the carbon-chain molecules of \ce{CCH}, \ce{HC5N}, \ce{HC7N}, and \ce{CH3CCH} are more abundant in the younger G168-H1 core. We found that starless core G168-H1 may have the properties of cold dark clouds based on its abundances of carbon-chain molecules. While, the prestellar core G168-H2 exhibits lower carbon-chain molecular abundances than the general cold dark clouds. With our gas-grain astrochemical model calculations, we attribute the observed chemical differences between G168-H1 and G168-H2 to their different gas densities and different evolutionary stages.