What did the seahorse swallow? APEX 170 GHz observations of the chemical conditions in the Seahorse infrared dark cloud
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 (≥3σ) altogether, namely, SO(NJ=44-33), H13CN(J=2-1), H13CO+(J=2-1), SiO(J=4-3), HN13C(J=2-1), and C2H(N=2-1). While SO, H13CO+, and HN13C were detected in every source, the detection rates for C2H and H13CN 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, H13CN, H13CO+, HN13C, and C2H lines in absorption. We found three positive correlations among the derived molecular abundances, of which those between C2H and HN13C and HN13C and H13CO+ are the most significant (correlation coefficient r0.9). The statistically most significant evolutionary trends we uncovered are the drops in the C2H abundance and in the [ HN13C]/[ H13CN] 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 [C2H] evolutionary indicator we found is in agreement with previous studies, and can be explained by the conversion of C2H 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 [ HN13C]/[ H13CN] 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 13C isotopologue.