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