A CSO Search for $l$-C$_3$H$^+$: Detection in the Orion Bar PDR
  The results of a Caltech Submillimeter Observatory (CSO) search for
$l$-C$_3$H$^+$, first detected by Pety et al. (2012) in observations toward the
Horsehead photodissociation region (PDR), are presented. A total of 39 sources
were observed in the 1 mm window. Evidence of emission from $l$-C$_3$H$^+$ is
found in only a single source - the Orion Bar PDR region, which shows a
rotational temperature of 178(13) K and a column density of 7(2) x $10^{11}$
cm$^{-2}$. In the remaining sources, upper limits of ~10$^{11} - 10^{13}$
cm$^{-2}$ are found. These results are discussed in the context of guiding
future observational searches for this species.
