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 present observations of the ν 2 =0 and vibrationally excited ν 2 =1 J=9-8 rotational lines of HCN at 797 GHz toward the deeply embedded massive young stellar object GL 2591, which provide the missing link between the extended envelope traced by lower-J line emission and the small region of hot (T ex ≥ 300 K), abundant HCN seen in 14 µm absorption with the Infrared Space Observatory (ISO). The line ratio yields T ex = 720 +135 −100 K and the line profiles reveal that the hot gas seen with ISO is at the velocity of the protostar, arguing against a location in the outflow or in shocks. Radiative transfer calculations using a depth-dependent density and temperature structure show that the data rule out a constant abundance throughout the envelope, but that a model with a jump of the abundance in the inner part by two orders of magnitude matches the observations. Such a jump is consistent with the sharp increase in HCN abundance at temperatures > ∼ 230 K predicted by recent chemical models in which atomic oxygen is driven into water at these temperatures. Together with the evidence for ice evaporation in this source, this result suggests that we may be witnessing the birth of a hot core. Thus, GL 2591 may represent a rare class of objects at an evolutionary stage just preceding the 'hot core' stage of massive star formation.

Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in the Milky Way and of galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. Such a facility is essential to address key astrophysical questions, especially concerning galaxy evolution and the development of planetary systems.SPICA is a mission concept aimed at taking the next step in mid-and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultrasensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. Rather than using liquid cryogen, a combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With cooling not dependent on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. The combination of low telescope background and instruments with state-of-the-art detectors means that SPICA can provide a huge advance on the capabilities of previous missions.The SPICA instrument complement offers spectral resolving power ranging from R ∼50 through 11000 in the 17-230 µm domain as well as R ∼28.000 spectroscopy between 12 and 18 µm. Additionally SPICA will be capable of efficient 30-37 µm broad band mapping, and small field spectroscopic and polarimetric imaging in the 100-350 µm range. SPICA will enable far infrared spectroscopy with an unprecedented sensitivity of ∼ 5 × 10 −20 W/m 2 (5σ/1hr) -at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.

Aims. In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described. Methods. The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources. Results. The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies.

We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna at 4.3 THz. Using hot/cold blackbody loads and a beam splitter all in vacuum, we measured a double sideband receiver noise temperature of 1300 K at the optimum local oscillator ͑LO͒ power of 330 nW, which is about 12 times the quantum noise ͑h / 2k B ͒. Our result indicates that there is no sign of degradation of the mixing process at the superterahertz frequencies. Moreover, a measurement method is introduced which allows us for an accurate determination of the sensitivity despite LO power fluctuations. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2819534͔Superconducting mixers 1 play a key role in astrophysics at terahertz frequencies, where the early universe radiates strongly. The availability of low noise superconductorinsulator-superconductor ͑SIS͒ mixers and hot electron bolometer ͑HEB͒ mixers has made the realization of highly sensitive spectrometers on ground, airborne, and space telescopes possible. An example of this is the heterodyne instrument for far infrared on the Herschel space telescope, 2 to be launched in 2008, where the heterodyne spectrometers are operated up to 1.3 THz using SIS mixers and further up to 1.9 THz using HEB mixers. For the next generation of space telescopes, it becomes highly desirable to demonstrate sensitive mixers in the frequency range between 2 and 6 THz. HEB mixers, which are currently the only devices suitable for this frequency range, have been reported up to 5.3 THz. [3][4][5] However, only few experiments have so far been done at the frequencies above 3 THz, namely, superterahertz frequencies, and the performance is relatively poor. 3,4 The noise temperature of a receiver is a crucial parameter that defines the ultimate sensitivity of the heterodyne spectrometer and the observation time. To achieve the low noise at superterahertz, several challenges are expected either in the mixer itself or in the testing technique. First, it is unclear whether the performance of HEBs will degrade. The relaxation of highly excited electrons due to increased photon energy can be complicated by cascade processes of emission and absorption of phonons. This can compete with the electron-electron interaction and thus may decrease the mixing efficiency. 6 Also, there is a concern of the quantum noise. 7 Second, it becomes more difficult to couple terahertz radiation to the HEB. Third, there is lack of local oscillators ͑LOs͒. Optically pumped far infrared ͑FIR͒ gas lasers are commonly used, but achieving stable output power is cumbersome. Terahertz quantum cascade lasers 8 ͑QCLs͒ are promising, stable solid-state LOs, 9 but still in a development stage. Finally, there is an increase in the air loss due to the absorption of terahertz radiation by water vapor, which can increase the receiver noise temperature and may also cause instability.In this letter, we report the measurement of a quasioptical NbN HEB mixer at 4.3 THz using a hot/cold load built in vacuum an...