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.

The Pt magnetization depth profile at a single buried Pt/Co interface was investigated by x-ray resonant magnetic reflectivity measurements. The asymmetry as function of angle of incidence has been measured in the Pt L 3 -near-edge absorption region at two energies. Observed asymmetry ratios in the order of 0.5% are described on the basis of a magnetically modified Parratt algorithm. Excellent agreement between simulations and experiment was achieved for a Pt magnetic moment of 0.21 B at the rough interface followed by an exponential decay of the induced polarization within 1 nm.

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.

We investigate the origin of self-absorption in [O i] 63 μm line emission, which is very clearly seen in approximately half of the 12 Galactic giant molecular cloud (GMC)/H ii regions observed. For this study, we observed velocity-resolved spectra of photon-dominated region (PDR) and H ii region tracers, the [O i] 63 μm, [N ii] 205 μm, and CO J  =  5–4 and 8–7 lines, with the upGREAT instrument in the 4GREAT configuration on the NASA/DLR Stratospheric Observatory For Infrared Astronomy (SOFIA). To probe the origin of the [O i] absorption and line shape and what they tell us about the physical conditions, we focus on the W3 region, for which we obtained data for eight positions along a line near the H ii region W3 A. We derive the foreground column density of low-excitation atomic oxygen to be in the range 2–7 × 1018 cm − 2 . At the position of strongest [O i] emission and greatest absorbing column density, 24% of the oxygen in the PDR is in the form of low-excitation atomic oxygen. We employ the Meudon PDR code to study the chemical and thermal structure of the PDR and to understand the large column density of neutral oxygen throughout the PDR. The reduction in the integrated intensity of the [O i] 63 μm emission is a factor of ≃2–4 in directions with strong [O i] emission. The results from our sample, if general, would significantly impact the use of the [O i] 63 μm line as a tracer of massive star formation and could play a significant role in explaining the “63 μm [O i] deficit” seen in very luminous extragalactic sources.

Radiative and mechanical feedback of massive stars regulates star formation and galaxy evolution. Positive feedback triggers the creation of new stars by collecting dense shells of gas, while negative feedback disrupts star formation by shredding molecular clouds. Although key to understanding star formation, their relative importance is unknown. Here, we report velocity-resolved observations from the SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program FEEDBACK of the massive star-forming region RCW 120 in the [CII] 1.9-THz fine-structure line, revealing a gas shell expanding at 15 km/s. Complementary APEX (Atacama Pathfinder Experiment) CO J = 3-2 345-GHz observations exhibit a ring structure of molecular gas, fragmented into clumps that are actively forming stars. Our observations demonstrate that triggered star formation can occur on much shorter time scales than hitherto thought (<0.15 million years), suggesting that positive feedback operates on short time periods.