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.

A direct measurement of the expansion of W 3(OH) is made by comparing Very Large Array images taken ∼ 10 yr apart. The expansion is anisotropic with a typical speed of 3 to 5 km s −1 , indicating a dynamical age of only 2300 yr. These observations are inconsistent with either the freely expanding shell model or a simple bow shock model. The most favored model is a slowly expanding shell-like H ii region, with either a fast rarefied flow or another less massive diffuse ionized region moving towards the observer. There is also a rapidly evolving source near the projected center of emission, perhaps related to the central star.

We report on the initial analysis of a Herschel-PACS full range spectrum of Neptune, covering the 51-220 μm range with a mean resolving power of ∼3000, and complemented by a dedicated observation of CH 4 at 120 μm. Numerous spectral features due to HD (R(0) and R(1)), H 2 O, CH 4 , and CO are present, but so far no new species have been found. Our results indicate that (i) Neptune's mean thermal profile is warmer by ∼3 K than inferred from the Voyager radio-occultation; (ii) the D/H mixing ratio is (4.5 ± 1) × 10 −5 , confirming the enrichment of Neptune in deuterium over the protosolar value (∼2.1 × 10 −5 ); (iii) the CH 4 mixing ratio in the mid stratosphere is (1.5 ± 0.2) × 10 −3 , and CH 4 appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH 4 stratospheric injection from Neptune's warm south polar region; (iv) the H 2 O stratospheric column is (2.1 ± 0.5) × 10 14 cm −2 but its vertical distribution is still to be determined, so the H 2 O external flux remains uncertain by over an order of magnitude; and (v) the CO stratospheric abundance is about twice the tropospheric value, confirming the dual origin of CO suspected from ground-based millimeter/submillimeter observations.

No abstract