Dissociative recombination of cold H D 3 and its interstellar implications

H + 3 plays a key role in interstellar chemistry as the initiator of ion molecule chemistry. The amount of H + 3 observed in dense interstellar clouds is consistent with expectations, but the large abundance of H + 3 seen in diffuse clouds is not easily explained by simple chemical models. A crucial parameter in predicting the abundance of H + 3 in diffuse clouds is the rate constant for dissociative recombination (DR) with electrons. The value of this constant has been very controversial, because different experimental techniques have yielded very different results, perhaps owing to varying degrees of rotational and vibrational excitation of the H + 3 ions. If the value of this rate constant under interstellar conditions were much lower than usually assumed, the large H + 3 abundance could be easily explained. In an attempt to pin down this crucial rate constant, we have performed DR measurements at the CRYRING ion storage ring in Stockholm, using a supersonic expansion ion source to produce rotationally cold H + 3 ions. These measurements suggest that the DR rate constant in diffuse clouds is not much lower than usually assumed and that the abundant H + 3 must be due to either a low electron fraction or a high ionization rate.