Vol. 548
In section 6. Interstellar and circumstellar matter

The IRAM-30m line survey of the Horsehead PDR. II. First detection of the C3H+ hydrocarbon cation

The question whether long-chain carbon species can form in the gas phase in molecular clouds has been discussed since the late 70s. A relatively recent input to this discussion is the discovery that the abundance of some carbon-rich species is enhanced in certain photon-dominated regions (PDRs) such as the Horsehead nebula. This is thought to be either caused by the enhanced reactivity of ionized C in PDR conditions or by break-up of large aromatic species like PAHs. Now Pety et al. in the article highlighted here add to this debate with the discovery of a new interstellar molecular ion C₃H⁺ in the Horsehead PDR. The authors infer an abundance relative to H₂ of 2 10^-11, which is quite high considering that it is easily destroyed in PDR conditions. It is also noteworthy that this appears to be a PDR-specific phenomenon (i.e., not found in sources such as Orion, TMC1, or SgrB2), though clearly this claim will have to be tested in future work.