tokens: analysis of photochemical models of Titan (Dobrijevic et al. 2008 ). It was found that the chemistry of C 2 H 2 and C 2 H 4 could be bistable although the underlying chemical pathways, involving neutral–neutral processes, differ from those studied here. The physical environment of Titan is significantly different from that of dense interstellar clouds; nevertheless, as H 2 is much less abundant on Titan, this work suggests that Titan’s ion–molecule chemistry could also lead to bistable solutions. Finally, the interstellar chemistry of carbon is much more complex than studied here, where we have only considered hydrocarbons up to C 2 . Autocatalytic processes in addition to those described above are likely present. For example, ethene, when viewed as a dimer (CH 2 ) 2 takes part in autocatalysis for CH 2 in reaction sequences similar to those for acetylene. Although present in our network, we found this played no role in producing bistable solutions. Analysis of larger, more extensive, carbon chemistry networks may lead to the discovery of more deeply embedded, and so less apparent, autocatalytic cycles. If one identifies the higher hydrocarbons as dimers then diacetylene (C 4 H 2 ) and the carbon tetramer (C 4 ) could act as respective intermediates for C 2 H and C 2 autocatalysis. The possibility for autocatalysis involving larger organic molecules remains to be explored; anion chemistry (e.g., Cordiner Charnley 2012 ; McElroy et al. 2013 ) may become as important as the cationic reactions considered here. 5. Conclusions We have found new bistable solutions in reduced chemical sets comparable to simplistic gas-phase chemical models of dense molecular clouds. We have identified several autocatalytic cycles in the ion–molecule chemistry of the major volatile elements: O, N, and C. In nitrogen chemistry there are two possible autocatalytic processes. An autocatalytic cycle driven by the reaction of He + with N 2 produces bistability. We find that this model has bistable solutions at high densities ( n H > 10 5 cm −3 ), which are present at all values of the elemental N abundance considered. The bistable solutions in the reduced model of the oxygen chemistry occur at low densities but are similarly present at all values of elemental O depletion, including none. Thus, bistable solutions are a fundamental property of interstellar chemistry; introduction of other elements, carbon in particular, acts to suppress these solutions by inhibiting autocatalysis through reactions with O 2 and OH. The carbon chemistry exhibits bistable solutions that appear around 10 2 cm −3 , or less, but only when an elemental depletion equivalent to that of the diffuse interstellar medium is adopted. We have identified three possible autocatalytic processes. We find that two autocatalyses occur in the acetylene chemistry, with CH, rather than atomic C, being the autocatalyst. These involve and C 2 H 2 in processes analogous to those found in the oxygen autocatalysis; their
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