A 1.3 cm Line Survey toward Orion KL
  Orion KL has served as a benchmark for spectral line searches throughout the
(sub)millimeter regime. The main goal is to systematically study spectral
characteristics of Orion KL in the 1.3 cm band. We carried out a spectral line
survey (17.9 GHz to 26.2 GHz) with the Effelsberg-100 m telescope towards Orion
KL. We find 261 spectral lines, yielding an average line density of about 32
spectral features per GHz above 3$\sigma$. The identified lines include 164
radio recombination lines (RRLs) and 97 molecular lines. 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 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
($8_{1,7}-7_{2,6}$), possibly suggesting the presence of a maser line. Column
densities and fractional abundances relative to H2 are estimated for 12
molecules with LTE methods. Rotational diagrams of non-metastable 14NH3
transitions with J=K+1 to J=K+4 yield different results; metastable 15NH3 is
found to have a higher excitation temperature than non-metastable 15NH3,
indicating that they may trace different regions. Elemental and isotopic
abundance ratios are estimated: 12C/13C=63+-17, 14N/15N=100+-51,
D/H=0.0083+-0.0045. 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 bn
factors for hydrogen and helium are nearly identical. Based on a non-LTE code
neglecting 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.
