2015A&A...581A..48G

The nearby Orion Kleinmann-Low nebula is one of the most prolific sources of molecular line emission. It has served as a benchmark for spectral line searches throughout the (sub)millimeter regime. The main goal is to systematically study the spectral characteristics of Orion KL in the λ∼1.3cm band. We carried out a spectral line survey with the Effelsberg-100 m telescope toward Orion KL. It covers the frequency range between 17.9GHz and 26.2GHz, i.e., the radio ``K band''. We also examined ALMA maps to address the spatial origin of molecules detected by our 1.3cm line survey. In Orion KL, we find 261 spectral lines, yielding an average line density of about 32 spectral features per GHz above 3σ (a typical value of 3σ is 15mJy). The identified lines include 164 radio recombination lines (RRLs) and 97 molecular lines. The RRLs, from hydrogen, helium, and carbon, stem from the ionized material of the Orion Nebula, part of which is covered by our beam. The molecular lines are assigned to 13 different molecular species including rare isotopologues. A total of 23 molecular transitions from species known to exist in Orion KL are detected for the first time in the interstellar medium. Non-metastable (J>K) ¹⁵NH₃ transitions are detected in Orion KL for the first time. Based on the velocity information of detected lines and the ALMA images, the spatial origins of molecular emission are constrained and discussed. A narrow feature is found in SO₂ (8_1,7-7_2,6), but not in other SO₂ transitions, possibly suggesting the presence of a maser line. Column densities and fractional abundances relative to H₂ are estimated for 12 molecules with local thermodynamic equilibrium (LTE) methods. Rotational diagrams of non-metastable ¹⁴NH₃ transitions with J=K+1 to J=K+4 yield different results; metastable (J=K) ¹⁵NH₃ is found to have a higher excitation temperature than non-metastable ¹⁵NH₃, also indicating that they may trace different regions. Elemental and isotopic abundance ratios are also estimated: He/H=(8.7±0.7)% derived from the ratios between helium RRLs and hydrogen RRLs; ¹²C/¹³C=63±17 from ¹²CH₃OH/¹³CH₃OH; ¹⁴N/¹⁵N=100±51 from ¹⁴NH₃/¹⁵NH₃; and D/H=(8.3±4.5)x10^–3 from NH₂D/NH₃. The dispersion of the He/H ratios derived from Hα/Heα pairs to Hδ/Heδ pairs is very small, which is consistent with theoretical predictions that the departure coefficients b_n factors for hydrogen and helium are nearly identical. Based on a non-LTE code that neglects excitation by the infrared radiation field and a likelihood analysis, we find that the denser regions have lower kinetic temperature, which favors an external heating of the hot core.