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.

Context. The methylidyne cation (CH + ) and hydroxyl (OH) are key molecules in the warm interstellar chemistry, but their formation and excitation mechanisms are not well understood. Their abundance and excitation are predicted to be enhanced by the presence of vibrationally excited H 2 or hot gas (∼500−1000 K) in photodissociation regions (PDRs) with high incident far-ultraviolet (FUV) radiation field. The excitation may also originate in dense gas (>10 5 cm −3 ) followed by nonreactive collisions with H 2 , H, and electrons. Previous observations of the Orion Bar suggest that the rotationally excited CH + and OH correlate with the excited CO, which is a tracer of dense and warm gas, and that formation pumping contributes to CH + excitation. Aims. Our goal is to examine the spatial distribution of the rotationally excited CH + and OH emission lines in the Orion Bar to establish their physical origin and main formation and excitation mechanisms. Methods. We present spatially sampled maps of the CH + J = 3-2 transition at 119.8 µm and the OH Λ doublet at 84 µm in the Orion Bar over an area of 110 × 110 with Herschel/PACS. We compare the spatial distribution of these molecules with those of their chemical precursors, C + , O and H 2 , and tracers of warm and dense gas (high-J CO). We assess the spatial variation of the CH + J = 2-1 velocity-resolved line profile at 1669 GHz with Herschel/HIFI spectrometer observations. Results. The OH and especially CH + lines correlate well with the high-J CO emission and delineate the warm and dense molecular region at the edge of the Bar. While notably similar, the differences in the CH + and OH morphologies indicate that CH + formation and excitation are strongly related to the observed vibrationally excited H 2 . This, together with the observed broad CH + line widths, indicates that formation pumping contributes to the excitation of this reactive molecular ion. Interestingly, the peak of the rotationally excited OH 84 µm emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas. Conclusions. The spatial distribution of CH + and OH revealed in our maps is consistent with previous modeling studies. Both formation pumping and nonreactive collisions in a UV-irradiated dense gas are important CH + J = 3-2 excitation processes. The excitation of the OH Λ doublet at 84 µm is mainly sensitive to the temperature and density.