2020A&A...639A..65M

Context. Infrared dark clouds (IRDCs) are useful target sources for the studies of molecular cloud substructure evolution and early stages of star formation. Determining the chemical composition of IRDCs helps to constrain the initial conditions and timescales (via chemical clocks) of star formation in these often filamentary, dense interstellar clouds.
Aims. We aim to determine the fractional abundances of multiple different molecular species in the filamentary IRDC G304.74+01.32, nicknamed the Seahorse IRDC, and to search for relationships between the abundances and potential evolutionary trends.
Methods. We used the Atacama Pathfinder EXperiment (APEX) telescope to observe spectral lines occurring at about 170 GHz frequency towards 14 positions along the full extent of the Seahorse filament. The sample is composed of five clumps that appear dark in the mid-IR, eight clumps that are associated with mid-IR sources, and one clump that is already hosting an HII region and is, hence, likely to be in the most advanced stage of evolution of all the target sources. We also employed our previous 870 µm dust continuum imaging data of the Seahorse.
Results. Six spectral line transitions were detected (≥3σ) altogether, namely, SO(N_J=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. Of the detected species, C₂H was found to be the most abundant one with respect to H₂ (a few times 10^–9 on average), while HN¹³C was found to be the least abundant species (a few times 10^–11). 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 r ≃ 0.9). 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.
Conclusions. The absorption lines detected towards SMM 6 and SMM 7 could arise from continuum radiation from an embedded young stellar object and an extragalactic object seen along the line of sight. However, the cause of absorption lines in the IR dark clump SMM 5 remains unclear. 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. With the exception of H¹³CN and H¹³CO⁺, the fractional abundances of the detected molecules in the Seahorse are relatively low compared to those in other IRDC sources. 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. Both single-dish and high-resolution interferometric imaging of molecular line emission (or absorption) of the Seahorse filament are required to understand the large-scale spatial distribution of the gas and to search for possible hot, high-mass star-forming cores in the cloud.