Phenol in High-mass Star-forming Regions

Phenol,which belongs to the C_(6)H_(6)O isomeric group,is the simplest molecule in the family of alcohol of the aromatic series.Although phenol has yet to be detected in the interstellar medium,a tentative identification was reported toward the Orion KL hot core using the IRAM-30 m line survey.To explore some more species of this isomeric group,we consider ten species to study the fate of their astronomical detection.It is noticed that phenol is the most energetically favorable isomer of this group.In contrast,propargyl ether is the least favorable(having relative energy~103kcal mol^(-1)compared to phenol)species of this group.So far,the studies associated with the formation of phenol are heavily concentrated on combustion chemistry.Here,we suggest a few key reactions(C_(6)H_(6)+OH→C_(6)H_(5)+H_(2)O,C_(6)H_(6)+O→C_(6)H_(5)OH,C_(6)H_(6)+H→C_(6)H_(5)+H_(2),and C_(6)H_(5)+OH→C_(6)H_(5)OH+hν)for the formation of phenol.All these pathways are included in a large gas-grain chemical network to study its formation in high mass star-forming regions and dark cloud environments.It is noticed that the phenyl(-C_(6)H_(5))formation by the ice-phase hydrogen abstraction reaction of benzene(i.e.,C_(6)H_(6)+OH→C_(6)H_(5)+H_(2)O if allowed at~10 K)could serve as the starting point for the formation of phenol in the gas phase by radiative association reaction C_(6)H_(5)+OH→C_(6)H_(5)OH+hν.The gasphase reaction C_(6)H_(6)+O→C_(6)H_(5)OH significantly contributes to the formation of phenol,when the ice-phase reaction C_(6)H_(6)+OH→C_(6)H_(5)+H_(2)O is not considered at low temperature.Band 4 ALMA archival data of a hot molecular core,G10.47+0.03,are analyzed.It yields an upper limit on phenol abundance of 5.19×10^(-9).Our astrochemical model delivers an upper limit on phenol abundance of~2.20×10^(-9)in the hot molecular core,whereas its production in the dark cloud is not satisfactory.