A Q-band Line Survey toward Orion KL Using the Tianma Radio Telescope

We have conducted a line survey toward Orion KL using the Q-band receiver of the Tianma 65 m radio telescope (TMRT), covering 34.8-50 GHz with a velocity resolution between 0.79 and 0.55 km s^-1, respectively. The observations reach a sensitivity of the level of 1-8 mK, proving that the TMRT is sensitive for conducting deep-line surveys. In total, 597 Gaussian features are extracted. Among them, 177 radio recombination lines (RRLs) are identified, including 126, 40, and 11 RRLs of hydrogen, helium, and carbon, with a maximum Δn of 16, 7, and 3, respectively. The carbon RRLs are confirmed to originate from photodissociation regions with a V _LSR ~ 9 km s^-1. In addition, 371 molecular transitions of 53 molecular species are identified. Twenty-one molecular species of this survey were not firmly detected in the Q band by Rizzo et al., including species such as H₂CS, HCOOH, C₂H₅OH, ${{\rm{H}}}_{2}^{13}$ CO, H₂CCO, CH₃CHO, CH₂OCH₂, HCN υ ₂ = 1, and CH₃OCHO υ _t = 1. In particular, the vibrationally excited states of ethyl cyanide (C₂H₅CN υ13/υ21) are for the first time firmly detected in the Q band. NH₃ (15,15) and (16,16) are identified, and they are so far the highest transitions of the NH₃ inversion lines detected toward Orion KL. All of the identified lines can be reproduced by a radiative transfer model.