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 _ { 2 } D ^ { + } and D _ { 2 } H ^ { + } , which are very useful probes of the early stages of star and planet formation .
The ground-state rotational ortho–D _ { 2 } H ^ { + } 1 _ { 1 , 1 } –0 _ { 0 , 0 } transition at 1476.6 GHz in the prestellar core 16293E has been searched for with the Herschel Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA . 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 ( 3 \sigma integrated line intensity ) .
We used the ortho–H _ { 2 } D ^ { + } 1 _ { 1 , 0 } –1 _ { 1 , 1 } transition and para–D _ { 2 } H ^ { + } 1 _ { 1 , 0 } –1 _ { 0 , 1 } transition detected in this source to determine an upper limit on the ortho-to-para D _ { 2 } H ^ { + } ratio as well as the para–D _ { 2 } H ^ { + } /ortho–H _ { 2 } D ^ { + } ratio from a non-LTE 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 \times 10 ^ { 5 } and 10 ^ { 6 } cm ^ { -3 } .
Also the upper limit on the ortho–D _ { 2 } H ^ { + } line is consistent with a low gas temperature ( \sim 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 .