Felix

We present a study of the origin of the argonium ((mathrm{ArH}^+)) emission discovered by Herschel in the Crab Nebula (Barlow et al. 2013). The argonium molecule is believed to be formed principally by the reaction of singly ionised argon ((mathrm{Ar}^+)) with molecular hydrogen ((mathrm{H}_2)), and to be destroyed by reactions with (mathrm{H}_2) and UV photons. For the case of the argonium ground state absorption lines seen by Herschel along several interstellar sightlines (Schilke et al. 2014), those authors argued that the presence of (mathrm{H}_2) in both the formation and destruction mechanisms means that (mathrm{ArH}^+) must form in largely atomic interstellar hydrogen clouds containing only trace amounts of (mathrm{H}_2). However, In the case of the Crab Nebula the observed argonium emission might originate either from transition regions containing both (mathrm{Ar}^+) and (mathrm{H}_2), or alternatively from inside the Crab Nebula’s (mathrm{H}_2) knots into which X-ray photons or charged particles from the pulsar wind nebula have penetrated to produce (mathrm{Ar}^+) and other ions. We report the results of our numerical studies that have used a combination of photoionisation and photodissociation region codes to investigate these alternative scenarios for producing (mathrm{ArH}^+) in the Crab Nebula.