Space as a spectroscopic laboratory: High-resolution spectroscopy of the [13C II] hyperfine structure with SOFIA/upGREAT

The emission at 158,̊m μ m is a key cooling line of the interstellar medium and serves as a unique tracer of gas kinematics in spatially and spectrally resolved observations. However, its spectral profile is often modified by optical depth effects. The intrinsic line shape can be reconstructed via comparisons with emission from the less abundant singly ionized 13 C isotope. Due to the additional neutron spin, the þCII emission splits into three hyperfine structure (hfs) transitions. Laboratory measurements have provided the þCII centroid frequency and the frequency of the strongest hfs component (F=2-1), whereas the two weaker outer components (F=1-0 and F=1-1) have been inferred solely from quantum-mechanical calculations. Consequently, the magnetic-dipole hfs constants, from which the individual transition frequencies can be derived, have not been determined experimentally thus far. The high spectral resolution of observations with the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT) on board SOFIA enabled the simultaneous detection of all three þCII hfs transitions. Thus, we were able to determine, for the first time based on astronomical observations, the magnetic-dipole hyperfine structure constants, A_ 1/2 ^ ̊m hf = 810.71(11), ̊m MHz and A_ 3/2 ^ ̊m hf = 162.18(5), ̊m MHz , of the þCII 2s^2,2p, ^ 2 P^ ̧irc ground term with unprecedented precision. Combining these with the previously measured þCII centroid frequency in the laboratory, we derived the rest frequencies of all three hfs lines. Using the observed emission as a reference, we also determined the þCII centroid frequency with improved precision from astronomical observations. This work demonstrates that spectrally resolved astronomical observations can be used to constrain fundamental atomic properties. A high spectral resolution and signal-to-noise ratio enable a level of precision in determining hfs constants that rivals laboratory measurements. The approach is broadly applicable to other atomic and molecular transitions where laboratory data are difficult to obtain.