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.

Aims. This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) that was launched onboard ESA's Herschel Space Observatory in May 2009. Methods. The instrument is a set of 7 heterodyne receivers that are electronically tuneable, covering 480−1250 GHz with SIS mixers and the 1410−1910 GHz range with hot electron bolometer (HEB) mixers. The local oscillator (LO) subsystem comprises a Ka-band synthesizer followed by 14 chains of frequency multipliers and 2 chains for each frequency band. A pair of auto-correlators and a pair of acousto-optical spectrometers process the two IF signals from the dual-polarization, single-pixel front-ends to provide instantaneous frequency coverage of 2 × 4 GHz, with a set of resolutions (125 kHz to 1 MHz) that are better than 0.1 km s −1 . Results. After a successful qualification and a pre-launch TB/TV test program, the flight instrument is now in-orbit and completed successfully the commissioning and performance verification phase. The in-orbit performance of the receivers matches the pre-launch sensitivities. We also report on the in-orbit performance of the receivers and some first results of HIFI's operations.

We report the first extragalactic detection of the neutral carbon [CI] 3 P 2 → 3 P 1 fine structure line at 809 GHz. The line was observed towards M 82 simultaneously with the 3 P 1 → 3 P 0 line at 492 GHz, providing a precise measurement of the J = 2 → 1/J = 1 → 0 integrated line ratio of 0.96 (on a [K km s −1 ]-scale). This ratio constrains the [CI] emitting gas to have a temperature of at least 50 K and a density of at least 10 4 cm −3 . Already at this minimum temperature and density, the beam averaged CI-column density is large, 2.1 10 18 cm −2 , confirming the high CI/CO abundance ratio of ≈ 0.5 estimated earlier from the 492 GHz line alone. We argue that the [CI] emission from M 82 most likely arises in clouds of linear size around a few pc with a density of about 10 4 cm −3 or slightly higher and temperatures of 50 K up to about 100 K.

We present a new multi-pixel high resolution (R > ∼ 10 7 ) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receiver uses 2 × 7-pixel subarrays in orthogonal polarization, each in an hexagonal array around a central pixel. We present the first results for this new instrument after commissioning campaigns in May and December 2015 and after science observations performed in May 2016. The receiver is designed to ultimately cover the full 1.8−2.5 THz frequency range but in its first implementation, the observing range was limited to observations of the [CII] line at 1.9 THz in 2015 and extended to 1.83−2.07 THz in 2016. The instrument sensitivities are state-of-the-art and the first scientific observations performed shortly after the commissioning confirm that the time efficiency for large scale imaging is improved by more than an order of magnitude as compared to single pixel receivers. An example of large scale mapping around the Horsehead Nebula is presented here illustrating this improvement. The array has been added to SOFIA's instrument suite already for ongoing observing cycle 4.

We present the performance of the upGREAT heterodyne array receivers on the SOFIA telescope after several years of operations. This instrument is a multi-pixel high resolution (R 10 7 ) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receivers use 7-pixel subarrays configured in a hexagonal layout around a central pixel. The low frequency array receiver (LFA) has 2x7 pixels (dual polarization), and presently covers the 1.83-2.06 THz frequency range, which allows to observe the [CII] and [OI] lines at 158 µm and 145 µm wavelengths. The high frequency array (HFA) covers the [OI] line at 63 µm and is equipped with one polarization at the moment (7 pixels, which can be upgraded in the near future with a second polarization array). The 4.7 THz array has successfully flown using two separate quantum-cascade laser local oscillators from two different groups. NASA completed the development, integration and testing of a dual-channel closed-cycle cryocooler system, with two independently operable He compressors, aboard SOFIA in early 2017 and since then, both arrays can be operated in parallel using a frequency separating dichroic mirror. This configuration is now the prime GREAT configuration and has been added to SOFIA's instrument suite since observing cycle 6.