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 have experimentally investigated the quasiparticle shot noise in NbN/MgO/NbN superconductor -insulator -superconductor tunnel junctions. The observed shot noise is significantly larger than theoretically expected. We attribute this to the occurrence of multiple Andreev reflection processes in pinholes present in the MgO barrier. This mechanism causes the current to flow in large charge quanta (Andreev clusters), with a voltage dependent average value of m ≈ 1 + 2∆ eV times the electron charge. Because of this charge enhancement effect, the shot noise is increased by the factor m.PACS numbers: 72.70.+m, 74.50.+r, 74.80.Fp A dc current I flowing through a vacuum tube or a tunnel junction generates shot noise, time dependent fluctuations of the current due to the discreteness of charge carriers. Shot noise studies provide information on the nature of conduction not obtainable by conductance studies, e.g. the electric charge of carriers or the degree of correlation in the conducting system. For an uncorrelated system in which the electrons do not interact, the passage of carriers can be described by a Poisson distribution. The spectral density of current fluctuations S I then equals full shot noise: S I = 2qI = P Poisson for zero frequency and temperature 1 . The charge quantum q is normally the electron charge e.In superconductor -normal metal (SN) systems Andreev reflection occurs, causing an effective charge to be transferred of 2e. Due to this doubling of the charge, the shot noise in such a system is predicted to have a maximum of twice the Poisson noise 2-4 . More recently, giant shot noise in the supercurrent is predicted in a singlechannel superconductor -normal metal -superconductor (SNS) point contact 5 which is attributed to transport of large charge quanta (q ≫ e) at finite voltages caused by Multiple Andreev Reflection (MAR)6 . Observation of enhanced charge quanta in SN or SNS structures requires a combination of conductance and shot noise measurements. Despite extensive theoretical work, experimental results in this field are rare. A recent experiment 7 is performed on a NbN/c/Nb structure in which c is assumed to be a Nb constriction with a length of 7 nm and a diameter of 15 nm. At 9.5 K the structure acts like an NS interface but doubled shot noise is not observed. The predicted giant supercurrent shot noise is not observed either (at 4.2 K).From an applied point of view, shot noise in superconducting structures is of interest since this noise forms a major limitation to the sensitivity of NbN/MgO/NbN Superconductor-Insulator-Superconductor (SIS) THz radiation detectors 8 . For these reasons, we have investigated quasiparticle current transport and shot noise in an SNS structure in which the quasiparticle current is carried by MAR. Anticipating the experimental shot noise results presented in this paper we demonstrate that in a system in which multiple Andreev reflections occurs the quasiparticle shot noise at V < 2∆/e has a maximum value given by S I = (1 +

During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the highresolution HIFI spectrometer onboard Herschel. We report on the first detection of H 16 2 O and the rarer isotopologues H 17 2 O and H 18 2 O in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 × 10 −5 , and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H 2 O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of 12 CO, 13 CO, 28 SiO, 29 SiO, 30 SiO, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.

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.