Chemical Compositions in the Vicinity of Protostars in Ophiuchus

Abstract

Abstract We have analyzed Atacama Large Millimeter/submillimeter Array 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-C3H2 (c-C3H2), H2CO, CH3OH, 13CO, C18O, and DCO+ have been detected from both sources, while DCN is detected only in Oph-emb9. Around Oph-emb5, c-C3H2 is enhanced at the west side, relative to the IR source, whereas H2CO and CH3OH are abundant at the east side. In the field of Oph-emb9, moment 0 maps of the c-C3H2 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 CH3OH and H2CO show peaks farther from the bright star. We derive the N(c-C3H2)/N(CH3OH) column density ratios at the peak positions of c-C3H2 and CH3OH near each YSO, which are identified based on their moment 0 maps. The N(c-C3H2)/N(CH3OH) ratio at the c-C3H2 peak is significantly higher than at the CH3OH 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 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.