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4GREAT is an extension of the German receiver for astronomy at terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are coaligned on the sky. The frequency bands of 4GREAT cover 491-635, 890-1090, 1240-1525, and 2490-2590 GHz, respectively. This article presents the design and characterization of the instrument, and its in-flight performance. The first light of 4GREAT was on June 2018. It has been offered to the interested SOFIA communities starting with observing cycle 6.

We report NASA-DLR SOFIA upGREAT circumstellar [O i] 63.2 mu m and [C ii] 157.7 mu m emission profiles and FIFI-LS [O i] 63.2 mu m, [O i] 145.5 mu m, and [C ii] 157.7 mu m fluxes obtained shortly after Betelgeuse's 2019/2020 Great Dimming event. Haas et al. noted a potential correlation between the [O i] 63.2 mu m flux and V magnitude based on three Kuiper Airborne Observatory observations made with the CGS and FIFI instruments. The FIFI observation was obtained when V similar or equal to 0.88 and revealed a 3 sigma non-detection at a quarter of the previous CGS flux measurement made when V similar or equal to 0.35. A potential explanation could be a change in dust-gas drag heating by circumstellar silicates caused by variations in the photospheric radiation field. SOFIA observations provide a unique test of this correlation because the V-band brightness went to its lowest value on record, V similar or equal to 1.61, with the SOFIA observations being made when V (FIFI-LS) similar or equal to 1.51 and V (upGREAT) similar or equal to 1.36. The upGREAT spectra show a [O i] 63.2 mu m flux larger than previous space observatory measurements obtained when V similar or equal to 0.58. The profile is consistent with formation in the slower, more turbulent inner S1 outflow, while the [C ii] 157.7 mu m profile is consistent with formation farther out in the faster S2 outflow. Modeling of dust-gas drag heating, combined with 25 yr of Wing three-filter and V photometry, reveals that it is unlikely that the S1 circumstellar envelope and [O i] 63.2 mu m fluxes are dominated by the dust-gas drag heating and that another heating source is also active. The [O i] 63.2 mu m profile is hard to reconcile with existing outflow velocity models.

The far-infrared (FIR) regime is one of the wavelength ranges where no astronomical data with sub-arcsecond spatial resolution exist. None of the medium-term satellite projects like SPICA, Millimetron, or the Origins Space Telescope will resolve this malady. For many research areas, however, information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, from highly excited carbon monoxide (CO), light hydrides, and especially from water lines would open the door for transformative science. A main theme will be to trace the role of water in proto-planetary discs, to observationally advance our understanding of the planet formation process and, intimately related to that, the pathways to habitable planets and the emergence of life. Furthermore, key observations will zoom into the physics and chemistry of the star-formation process in our own Galaxy, as well as in external galaxies. The FIR provides unique tools to investigate in particular the energetics of heating, cooling, and shocks. The velocity-resolved data in these tracers will reveal the detailed dynamics engrained in these processes in a spatially resolved fashion, and will deliver the perfect synergy with ground-based molecular line data for the colder dense gas.

Aims. The aim of our study is to investigate the physical properties of the star-forming interstellar medium (ISM) in the Large Magellanic Cloud (LMC) by separating the origin of the emission lines spatially and spectrally. The LMC provides a unique local template to bridge studies in the Galaxy and high redshift galaxies because of its low metallicity and proximity, enabling us to study the detailed physics of the ISM in spatially resolved individual star-forming regions. Following Okada et al. (Okada, Y., Requena-Torres, M. A., Gusten, R., et al. 2015, A&A, 580, A54), we investigate different phases of the ISM traced by carbon-bearing species in four star-forming regions in the LMC, and model the physical properties using the KOSMA-tau PDR model. Methods. We mapped 3-13 arcmin(2) areas in 30 Dor, N158, N160, and N159 along the molecular ridge of the LMC in [C II] 158 mu m with GREAT on board SOFIA. We also observed the same area with CO(2-1) to (6-5), (CO)-C-13(2-1) and (3-2), [C I] P-3(1)-P-3(0) and P-3(2)-P-3(1) with APEX. For selected positions in N159 and 30 Dor, we observed [O I] 145 mu m and [O I] 63 mu m with upGREAT. All spectra are velocity resolved. Results. In all four star-forming regions, the line profiles of CO, (CO)-C-13, and [C I] emission are similar, being reproduced by a combination of Gaussian profiles defined by CO(3-2), whereas [C II] typically shows wider line profiles or an additional velocity component. At several positions in N159 and 30 Dor, we observed the velocity-resolved [O I] 145 and 63 mu m lines for the first time. At some positions, the [O I] line profiles match those of CO, at other positions they are more similar to the [C II] profiles. We interpret the different line profiles of CO, [C II] and [O I] as contributions from spatially separated clouds and/or clouds in different physical phases, which give different line ratios depending on their physical properties. We modeled the emission from the CO, [C I], [C II], and [O I] lin

During the dawn of chemistry(1,2), when the temperature of the young Universe had fallen below some 4,000 kelvin, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With their higher ionization potentials, the helium ions He2+ and He+ were the first to combine with free electrons, forming the first neutral atoms; the recombination of hydrogen followed. In this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen. Despite its unquestioned importance in the evolution of the early Universe, the HeH+ ion has so far eluded unequivocal detection in interstellar space. In the laboratory the ion was discovered3 as long ago as 1925, but only in the late 1970s was the possibility that HeH+ might exist in local astrophysical plasmas discussed4-7. In particular, the conditions in planetary nebulae were shown to be suitable for producing potentially detectable column densities of HeH+. Here we report observations, based on advances in terahertz spectroscopy8,9 and a high-altitude observatory10, of the rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027. This confirmation of the existence of HeH+ in nearby interstellar space constrains our understanding of the chemical networks that control the formation of this molecular ion, in particular the rates of radiative association and dissociative recombination.

We have performed a multi-wavelength study toward a quasi-sinusoidal filament (CFG028.68-0.28). A new large-scale (CO)-C-12 J = 3 - 2 map was obtained from the China-Cologne Observation for SubMillimeter Astronomy (CCOSMA) 3m radio telescope. Based on the ATLASGAL catalog, we have identified 27 dust clumps in the filament. Through the relationship between the mass and radius of these clumps, 67% of these clumps are dense and massive enough to potentially form massive stars. The obtained CFE is similar to 11% in the filament. The filament has a linear mass density of similar to 305 M(circle dot)pc(-1), which is smaller than its critical mass to length ratio. This suggests that the external pressure from the neighboring H II regions may help prevent the filament from dispersing under the effects of turbulence. Comparing the energy injection from outflows and H II regions in the filament, the ionization feedback from the H II regions can help maintain the observed turbulence.

Phase gratings are used as beam multiplexers in the submillimeter and terahertz spectral range. Two-dimensional beam arrangements can often be obtained most easily by the superposition of two one-dimensional grating structures. We show that in general this approach does not yield the maximum grating efficiency and propose a method to obtain significantly higher efficiencies for this grating class. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We present the details of a coupling structure to embed a 1.9-THz double-metal quantum cascade lasers (QCLs) into a 120-mu m-wide full-height rectangular waveguide. We describe the split-block manufacturing of the waveguide coupler together with a diagonal feed horn and present power and beam shape measurements of two different devices. The two devices differ in coupling factor, which can be chosen in a wide range by the mounting position of the QCL. Our waveguide embedding allows coupling of a large fraction of the laser power into the waveguide and the subsequent horn antenna emitting with a large Gaussian mode content, enabling efficient integration into a diffraction limited optics setup. This is illustrated by a self-mixing experiment and by using the embedded QCL as a local oscillator in a heterodyne receiver.

We present the upGREAT THz heterodyne arrays for far-infrared astronomy. The low-frequency array (LFA) is designed to cover the 1.9-2.5 THz range using 2 7-pixel wave-guide-based HEB mixer arrays in a dual polarization configuration. The high-frequency array (HFA) will perform observations of the [OI] line at 4.745 THz using a 7-pixel waveguide-based HEB mixer array. This paper describes the common design for both arrays, cooled to 4.5 K using closed-cycle pulse tube technology. We then show the laboratory and telescope characterization of the first array with its 14 pixels (LFA), which culminated in the successful commissioning in May 2015 aboard the SOFIA airborne observatory observing the [CII] fine structure transition at 1.9005 THz. This is the first successful demonstration of astronomical observations with a heterodyne focal plane array above 1 THz and is also the first time high-power closed-cycle coolers for temperatures below 4.5 K are operated on an airborne platform.

2 x 2 parallel fed and 3 x 3 serial fed patch antenna arrays on a benzocyclobutene (BCB) polymer layer are integrated with a 70 mu m wide, dry etched, double metal waveguide quantum cascade laser, operating at about 1.9 THz. The BCB surrounds the quantum cascade laser ridge and is planarized to fit precisely its height. The patch antenna arrays emit a linearly polarized, highly symmetric beam perpendicular to the antenna plane. The beams have a FWHM angle of 49 degrees (2 x 2) and 35 degrees (3 x 3). Both measurements and simulations indicate coupling factors to a Gaussian beam of over 90%. The antenna design is strongly governed by the high thickness (h = 13.6 mu m) and the low dielectric constant (epsilon(r) = 2.45) of the BCB substrate. Because the patch array has a very low input reflectivity of -13 to -20 dB over the 1.7-2.1 THz operation band, the laser needs a partially transmitting reflector to maintain the Q-factor of the active medium resonator to assure lasing in the antennas operation band. By changing the dimensions of the reflector, the facet transparency can be designed in a wide range from fully transmissive to highly reflective. (C) 2016 Optical Society of America

GaAs/AlGaAs quantum cascade lasers based on four quantum well structures operating at 4.7 THz are reported. A large current density dynamic range is observed, leading to a maximum operation temperature of 150 K for the double metal waveguide device and a high peak output power more than 200 mW for the single surface plasmon waveguide device. A continuous wave, single mode, third order distributed feedback laser with a low electrical power dissipation and a narrow far-field beam pattern, which is required for a local oscillator in astronomy heterodyne spectrometers, is also demonstrated.

We present the first superconducting hot electron bolometer (HEB) waveguide mixer operating at 4.7 THz. The 5.5-nm-thick, 300-nm-long, and 3600-nm-wide NbN HEB micro-bridge is integrated into a normal metal (Au) planar circuit on a 2 mu m thick silicon substrate. This circuit is integrated in a 24 mu m x 48 mu m x 21 mu m waveguide cavity and a 14 mu m x 7 mu m x 200 mu m substrate channel, which is directly machined into a CuTe alloy block. The power spectrum of the HEB mixer, measured with a Fourier transform spectrometer, is in good agreement with the results of 3-D EM circuit simulation. Measured mixer performance shows a state-of-the-art double sideband noise temperature of 1100 K, averaged over the IF bandwidth of 0.2-3.5 GHz. The 3-dB noise roll-off is 3.5 GHz. This mixer is used in the German REceiver for Astronomy at Terahertz frequencies (GREAT) at the airborne Stratospheric Observatory for Far Infrared Astronomy (SOFIA).