Chemical compositions in the vicinity of protostars in Ophiuchus

TL;DR

This study investigates the chemical compositions near two young stellar objects in Ophiuchus using ALMA data, revealing how UV irradiation influences molecular abundances and PDR chemistry in protostellar environments.

Contribution

It provides the first detailed comparison of molecular line distributions and column density ratios around protostars in different irradiation conditions in Ophiuchus.

Findings

Higher c-C3H2/CH3OH ratio in irradiated source Oph-emb9.

Detection of DCN only in the irradiated protostar Oph-emb9.

Enhanced DCO+ column density linked to UV irradiation flux.

Abstract

We have analyzed Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 4 Band 6 data toward two young stellar objects (YSOs), Oph-emb5 and Oph-emb9, in the Ophiuchus star-forming region. The YSO Oph-emb5 is located in a relatively quiescent region, whereas Oph-emb9 is irradiated by a nearby bright Herbig Be star. Molecular lines from $cyclic$-C$_{3}$H$_{2}$ ($c$-C$_{3}$H$_{2}$), H$_{2}$CO, CH$_{3}$OH, $^{13}$CO, C$^{18}$O, and DCO$^{+}$ have been detected from both sources, while DCN is detected only in Oph-emb9. Around Oph-emb5, $c$-C$_{3}$H$_{2}$ is enhanced at the west side, relative to the IR source, whereas H$_{2}$CO and CH$_{3}$OH are abundant at the east side. In the field of Oph-emb9, moment 0 maps of the $c$-C$_{3}$H$_{2}$ lines show a peak at the eastern edge of the field of view, which is irradiated by the Herbig Be star. Moment 0 maps of CH$_{3}$OH and H$_{2}$CO show peaks farther from the bright star. We derive the $N$($c$-C$_{3}$H$_{2}$)/$N$(CH$_{3}$OH) column density ratios at the peak positions of $c$-C$_{3}$H$_{2}$ and CH$_{3}$OH near each YSO, which are identified based on their moment 0 maps. The $N$($c$-C$_{3}$H$_{2}$)/$N$(CH$_{3}$OH) ratio at the $c$-C$_{3}$H$_{2}$ peak is significantly higher than at the CH$_{3}$OH peak by a factor of $\sim 19$ in Oph-emb9, while the difference in this column density ratio between these two positions is a factor of $\sim2.6 $ in Oph-emb5. These differences are attributed to the efficiency of the photon-dominated region (PDR) chemistry in Oph-emb9. The higher DCO$^{+}$ column density and the detection of DCN in Oph-emb9 are also discussed in the context of UV irradiation flux.