Upper limit for the D_2H^+ ortho-to-para ratio in the prestellar core 16293E (CHESS)

Abstract

The H^+_3 ion plays a key role in the chemistry of dense interstellar gas clouds where stars and planets are forming. The low temperatures and high extinctions of such clouds make direct observations of H^+_3 impossible, but lead to large abundances of H_2D^+ and D_2H^+, which are very useful probes of the early stages of star and planet formation. The ground-state rotational ortho-D_2H^+ 1_(1,1)–0_(0,0) transition at 1476.6 GHz in the prestellar core 16293E has been searched for with the Herschel HIFI instrument, within the CHESS (Chemical HErschel Surveys of Star forming regions) Key Program. The line has not been detected at the 21 mK km s^(-1) level (3σ integrated line intensity). We used the ortho-H_2D^+ 1_(1,0)–1_(1,1) transition and para-D_2H^+ 1_(1,0)–1_(0,1) transition detected in this source to determine an upper limit on the ortho-to-para D_2H^+ ratio as well as the para-D_2H^+/ortho-H_2D^+ ratio from a non-local thermodynamic equilibrium analysis. The comparison between our chemical modeling and the observations suggests that the CO depletion must be high (larger than 100), with a density between 5 × 10^5 and 10^6 cm^(-3). Also the upper limit on the ortho-D_2H^+ line is consistent with a low gas temperature (~11 K) with a ortho-to-para ratio of 6 to 9, i.e. 2 to 3 times higher than the value estimated from the chemical modeling, making it impossible to detect this high frequency transition with the present state of the art receivers.

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