No abstract

We describe the design and construction of GREAT (German REceiver for Astronomy at Terahertz frequencies) operated on the Stratospheric Observatory For Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne instrument for highresolution far-infrared (FIR) spectroscopy. Selected for SOFIA's Early Science demonstration, the instrument has successfully performed three Short and more than a dozen Basic Science flights since first light was recorded on its April 1, 2011 commissioning flight. We report on the in-flight performance and operation of the receiver that -in various flight configurations, with three different detector channels -observed in several science-defined frequency windows between 1.25 and 2.5 THz. The receiver optics was verified to be diffraction-limited as designed, with nominal efficiencies; receiver sensitivities are state-of-the-art, with excellent system stability. The modular design allows for the continuous integration of latest technologies; we briefly discuss additional channels under development and ongoing improvements for Cycle 1 observations. GREAT is a principal investigator instrument, developed by a consortium of four German research institutes, available to the SOFIA users on a collaborative basis.

During the dawn of chemistry 1,2 when the temperature of the young Universe had fallen below ~4000 K, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With its higher ionization potentials, He ++ (54.5 eV) and He + (24.6 eV) combined first with free electrons to form the first neutral atom, prior to the recombination of hydrogen (13.6 eV). At that time, in this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH + , by radiative association with protons (He + H + → HeH + + hν). As recombination progressed, the destruction of HeH + (HeH + + H → He + H 2 + ) created a first path to the formation of molecular hydrogen, marking the beginning of the Molecular Age. Despite its unquestioned importance for the evolution of the early Universe, the HeH + molecule has so far escaped unequivocal detection in interstellar space. In the laboratory the ion was discovered as long ago as 1925 3 , but only in the late seventies was the possibility that HeH + might exist in local astrophysical plasmas discussed 4,5,6,7 . In particular, the conditions in planetary nebulae were shown to be suitable for the production of potentially detectable HeH + column densities: the hard radiation field from the central hot white dwarf creates overlapping Strömgren spheres, where HeH + is predicted to form, primarily by radiative association of He + and H. With the GREAT spectrometer 8.9 on board SOFIA 10 the HeH + rotational ground-state transition at λ149.1 µm is now accessible. We report here its detection towards the planetary nebula NGC7027. The mere fact of its proven existence in nearby interstellar space constrains our understanding of the chemical networks controlling the formation of this very special molecular ion.To be published in Nature 568, pages 357-359 (2019) | KOSMA/Universität zu Köln, in cooperation with the DLR Institut für Optische Sensorsysteme.

We have observed five sulphur-bearing molecules in foreground diffuse molecular clouds lying along the sight-lines to five bright continuum sources. We have used the GREAT instrument on SOFIA to observe the SH 1383 GHz 2 Π 3/2 J = 5/2 ← 3/2 lambda doublet toward the star-forming regions W31C, G29.96-0.02, G34.3+0.1, W49N and W51, detecting foreground absorption towards all five sources; and the EMIR receivers on the IRAM 30 m telescope at Pico Veleta to detect the H 2 S 1 10 −1 01 (169 GHz), CS J = 2−1 (98 GHz) and SO 3 2 −2 1 (99 GHz) transitions. Upper limits on the H 3 S + 1 0 −0 0 (293 GHz) transition were also obtained at the IRAM 30 m. In nine foreground absorption components detected towards these sources, the inferred column densities of the four detected molecules showed relatively constant ratios, with N(SH)/N(H 2 S) in the range 1.1−3.0, N(CS)/N(H 2 S) in the range 0.32−0.61, and N(SO)/N(H 2 S) in the range 0.08−0.30. The column densities of the sulphur-bearing molecules are very well correlated amongst themselves, moderately well correlated with CH (a surrogate tracer for H 2 ), and poorly correlated with atomic hydrogen. The observed SH/H 2 ratios -in the range 5 to 26 × 10 −9 -indicate that SH (and other sulphur-bearing molecules) account for 1% of the gas-phase sulphur nuclei. The observed abundances of sulphur-bearing molecules, however, greatly exceed those predicted by standard models of cold diffuse molecular clouds, providing further evidence for the enhancement of endothermic reaction rates by elevated temperatures or ion-neutral drift. We have considered the observed abundance ratios in the context of shock and turbulent dissipation region (TDR) models. Using the TDR model, we find that the turbulent energy available at large scale in the diffuse ISM is sufficient to explain the observed column densities of SH and CS. Standard shock and TDR models, however, fail to reproduce the column densities of H 2 S and SO by a factor of about 10; more elaborate shock models -in which account is taken of the velocity drift, relative to H 2 , of SH molecules produced by the dissociative recombination of H 3 S + -reduce this discrepancy to a factor ∼3.

The GREAT observations need frequency-selective calibration across the passband for the residual atmospheric opacity at flight altitude. At these altitudes the atmospheric opacity has both narrow and broad spectral features. To determine the atmospheric transmission at high spectral resolution, GREAT compares the observed atmospheric emission with atmospheric model predictions, and therefore depends on the validity of the atmospheric models. We discuss the problems identified in this comparison with respect to the observed data and the models, and describe the strategy used to calibrate the science data from GREAT/SOFIA during the first observing periods.