APEX observations of ortho-H2D+ towards dense cores in the Orion B9 filament
Abstract
We used the APEX telescope to observe the 372 GHz o-H2D+(JKa,\,Kc=11,\,0-11,\,1) line towards three prestellar cores and three protostellar cores in Orion B9. We also employed our previous APEX observations of C17O, C18O, N2H+, and N2D+ line emission, and 870 μm dust continuum emission towards the target sources. The o-H2D+ line was detected in all three prestellar cores, but in only one of the protostellar cores. The corresponding o-H2D+ abundances were derived to be (12-30)×10-11 and 6×10-11. Two additional spectral lines, DCO+(5-4) and N2H+(4-3), were detected in the observed frequency bands with high detection rates of 100\% and 83\%, respectively. The Orion B9 cores were found to be consistent with the relationship between the o-H2D+ abundance and gas temperature obeyed by other low-mass dense cores. The o-H2D+ abundance was found to decrease as the core evolves. The o-H2D+ abundances in the Orion B9 cores are in line with those found in other low-mass cores and larger than derived for high-mass star-forming regions. The higher o-H2D+ abundance in prestellar cores compared to that in cores hosting protostars is to be expected from chemical reactions where higher concentrations of gas-phase CO and elevated gas temperature accelerate the destruction of H2D+. The validity of using the [o-H2D+]/[N2D+] ratio as an evolutionary indicator, which has been proposed for massive clumps, remains to be determined when applied to these target cores. Overall, as located in a dynamic environment of Orion B, the Orion B9 filament provides an interesting target system to investigate the deuterium-based chemistry, and further observations of species like para-H2D+ and D2H+ would be of particular interest.