A 1.3 cm line survey toward Orion KL

Context. 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. Aims. The main goal is to systematically study the spectral characteristics of Orion KL in the lambda similar to 1.3 cm band. Methods. We carried out a spectral line survey with the Effelsberg-100 m telescope toward Orion KL. It covers the frequency range between 17.9 GHz and 26.2 GHz, i.e., the radio K band. We also examined ALMA maps to address the spatial origin of molecules detected by our 1.3 cm line survey. Results. In Orion KL, we find 261 spectral lines, yielding an average line density of about 32 spectral features per GHz above 3 sigma (a typical value of 3 sigma is 15 mJy). 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) (NH3)-N-15 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 H-2 are estimated for 12 molecules with local thermodynamic equilibrium (LTE) methods. Rotational diagrams of non-metastable (NH3)-N-14 transitions with J = K + 1 to J = K + 4 yield different results; metastable (J = K) (NH3)-N-15 is found to have a higher excitation temperature than non-metastable (NH3)-N-15, 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; C-12/C-13 = 63 +/- 17 from (CH3OH)-C-12/(CH3OH)-C-13; N-14/N-15 = 100 +/- 51 from (NH3)-N-14/(NH3)-N-15; and D/H = (8.3 +/- 4.5) x 10(-3) from NH2D/NH3. The dispersion of the He/H ratios derived from H alpha/He alpha pairs to H delta/He delta 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.