What did the seahorse swallow? APEX 170 GHz observations of the chemical conditions in the Seahorse infrared dark cloud

TL;DR

This study used 170 GHz spectral observations of the Seahorse infrared dark cloud to analyze molecular abundances and their evolution, revealing key chemical changes during star formation stages.

Contribution

It provides new insights into chemical abundance trends and molecular correlations in IR dark clouds, highlighting evolutionary indicators based on molecular ratios.

Findings

C$_2$H abundance decreases with evolution from IR dark to HII regions.

H$^{13}$CN/H$^{13}$CO$^+$ ratio increases as the cloud evolves.

Detected correlations among molecular abundances indicating chemical evolution.

Abstract

We used the APEX telescope to observe spectral lines occurring at about 170 GHz frequency towards 14 positions along the full extent of the filamentary Seahorse infrared dark cloud. Six spectral line transitions were detected ($\geq3\sigma$) altogether, namely, SO$(N_J=4_4-3_3)$, H$^{13}$CN$(J=2-1)$, H$^{13}$CO$^+(J=2-1)$, SiO$(J=4-3)$, HN$^{13}$C$(J=2-1)$, and C$_2$H$(N=2-1)$. While SO, H$^{13}$CO$^+$, and HN$^{13}$C were detected in every source, the detection rates for C$_2$H and H$^{13}$CN were 92.9% and 85.7%, respectively. Only one source (SMM 3) showed detectable SiO emission (7.1% detection rate). Three clumps (SMM 5, 6, and 7) showed the SO, H$^{13}$CN, H$^{13}$CO$^+$, HN$^{13}$C, and C$_2$H lines in absorption. We found three positive correlations among the derived molecular abundances, of which those between C$_2$H and HN$^{13}$C and HN$^{13}$C and H$^{13}$CO$^+$ are the most significant (correlation coefficient $r\simeq0.9$). The statistically most significant evolutionary trends we uncovered are the drops in the C$_2$H abundance and in the $[{\rm HN^{13}C}]/[{\rm H^{13}CN}]$ ratio as the clump evolves from an IR dark stage to an IR bright stage and then to an HII region. The correlations we found between the different molecular abundances can be understood as arising from the gas-phase electron (ionisation degree) and atomic carbon abundances. The [C$_2$H] evolutionary indicator we found is in agreement with previous studies, and can be explained by the conversion of C$_2$H to other species (e.g. CO) when the clump temperature rises, especially after the ignition of a hot molecular core in the clump. The decrease of $[{\rm HN^{13}C}]/[{\rm H^{13}CN}]$ as the clump evolves is also likely to reflect the increase in the clump temperature, which leads to an enhanced formation of HCN and its $^{13}$C isotopologue.