Chemical compositions in the vicinity of protostars in Ophiuchus

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 ∼ 19 in Oph-emb9, while the difference in this column density ratio between these two positions is a factor of ∼ 2.6 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.