APEX observations of ortho-H${}_2$D${}^{+}$ towards dense cores in the Orion B9 filament

Abstract

We used the APEX telescope to observe the 372 GHz o-H${}_2$D${}^{+}(J_{K_a,K_c}=1_{1,0}-1_{1,1})$ line towards three prestellar cores and three protostellar cores in Orion B9. We also employed our previous APEX observations of C${}^{17}$O, C${}^{18}$O, N${}_2$H${}^{+}$, and N${}_2$D${}^{+}$ line emission, and 870 $\mu$m dust continuum emission towards the target sources. The o-H${}_2$D${}^{+}$ line was detected in all three prestellar cores, but in only one of the protostellar cores. The corresponding o-H${}_2$D${}^{+}$ abundances were derived to be $\sim (12-30)\times 10^{-11}$ and $6\times 10^{-11}$. Two additional spectral lines, DCO${}^{+}(5-4)$ and N${}_2$H${}^{+}(4-3)$, 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-H${}_2$D${}^{+}$ abundance and gas temperature obeyed by other low-mass dense cores. The o-H${}_2$D${}^{+}$ abundance was found to decrease as the core evolves. The o-H${}_2$D${}^{+}$ 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-H${}_2$D${}^{+}$ 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 H${}_2$D${}^{+}$. The validity of using the [o-H${}_2$D${}^{+}$]/[N${}_2$D${}^{+}$] 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-H${}_2$D${}^{+}$ and D${}_2$H${}^{+}$ would be of particular interest.

arXiv comment15 pages (incl. an appendix), 8 figures, 7 tables, accepted for publication in A&A, abstract abridged for arXiv

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