Search

The spectrum of Saturn from 43 to 197 μm was measured with the ISO Long Wavelength Spectrometer (LWS) during the performance verification phase of the mission. The measurements were made using the LWS in grating mode, with spectral resolutions of 0.29 μm from 43 to 90 μm and 0.6 μm from 90 to 197 μm. The spectrum was compared with an atmospheric radiative-transfer model and four results were obtained: first, the slope of the measured continuum within each detector passband is in good agreement with the model; second, absorption features due to ammonia and phosphine were unambiguously detected, and all detected features were attributed to these two molecules; third, the ammonia absorption features agree reasonably well with the nominal model (based on Voyager IRIS measurements); and fourth, the phosphine absorption features disagree with the nominal model. Superior agreement with the measured spectrum was obtained with a modified PH3 profile in which the tropospheric mixing ratio was increased to 7 × 10-6 and the cutoff due to photodissociation was lowered to 300 mbar. These results are based on trial observations during performance verification of the LWS, and provide an indication of the results we expect to obtain when the spectrum of Saturn is measured comprehensively later in the mission.

The Infrared Space Observatory is opening the 2.5 to 200 mu m band for detailed studies of galaxies. Based an the first results on ionic emission lines obtained with the Short-Wavelength-Spectrometer me discuss the nature of ultra-luminous IR, galaxies.

This paper gives an overview of the main past and future space experiments for infrared and sub-millimetre astronomy. The scientific objectives for these missions are briefly described and a summary is given of the capabilities and characteristics of their payload. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

Observations made with the short-wavelength spectrometer of the Infrared Space Observatory are used to investigate the composition of interstellar dust in the line of sight to Cygnus OB2 No. 12, commonly taken as representative of the diffuse (low-density) interstellar medium. Results are compared with data for the Galactic center source Sgr A*. Nondetections of the 3.0 and 4.27 mu m features of H2O and CO2 ices in Cyg OB2 No. 12 confirm the absence of dense molecular material in this line of sight, whereas the presence of these features in Sgr A* indicates that molecular clouds may contribute as much as 10 mag of visual extinction toward the Galactic center. The spectrum of Cyg OB2 No. 12 is dominated by the well-known 9.7 mu m silicate feature; detection of a shallow feature near 2.75 mu m indicates that the silicates are at least partially hydrated, with composition possibly similar to that of terrestrial phyllosilicates such as serpentine or chlorite. However, the 2.75 mu m feature is not seen in the Galactic center spectrum, suggesting that silicates in this line of sight are less hydrated or of different composition. The primary spectral signatures of C-rich dust in the diffuse ISM are weak absorptions at 3.4 mu m (the aliphatic C-H stretch) and 6.2 mu m (the aromatic C=C stretch). We conclude, based on infrared spectroscopy, that the most probable composition of the dust toward Cyg OB2 No. 12 is a mixture of silicates and carbonaceous solids in a volume ratio of approximately 3:2, with the carbonaceous component primarily in an aromatic form such as amorphous carbon.

We report the first detection of thermal water vapour emission from a circumstellar outflow. We have observed four far-infrared rotational emission lines of water vapour and one water absorption feature toward the evolved star W Hydrae, using the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). Three of the emission lines were observed in the instrument's Fabry-Perot mode at a resolving power lambda/Delta lambda of approximately 30 000: the 7(25) - 6(16) line at 29.84 mu m, the 4(41) - 3(12) line at 31.77 mu m, and the 4(32) - 3(03) line at 40.69 mu m One additional emission line, the 4(41) - 4(14) line at 37.98 mu m, and one absorption feature at 38.08 mu m that we attribute to a blend of the 13(13,0) - 13(12,1) and the 13(13,1) - 13(12,2) water lines were observed in grating mode at a resolving power of about 2 000. The observed emission line fluxes were 3.2 x 10(-19) 6.3 x 10(-19), 2.3 x 10(-19) and 2.8 x 10(-19) W cm(-2) respectively, and the equivalent width of the absorption feature was similar to 10 km s(-1). To within the possible errors in the flux calibration, the observed emission line fluxes can be accounted for simultaneously by a model similar to that of Chen & Neufeld (1995), given a mass-loss rate in the range (0.5 - 3) x 10(-5) M. yr(-1) This range lies at least a factor similar to 2 above an independent estimate of the mass-loss rate that may be derived from dynamical considerations, and at least a factor similar to 30 above previous estimates based upon the interpretation of CO observations.

We report the first 2.4 - 45 mu m spectroscopic study of Arp 220 obtained with the Short Wavelength Spectrometer onboard ISO. Observations of mid infrared lines penetrate deep into the highly obscured regions where the luminosity originates and give direct clues to their sources of excitation. From the observed Brackett line ratios and from the [S III] 18.7 mu m/33.5 mu m line ratio we derive a mean extinction of A(V) approximate to 50 mag, significantly higher than most earlier values. No AGN indicators such as the high excitation [Ne V] or [O IV] lines are detected to an upper limit significantly less than in bona fide AGN galaxies, indicating that a central AGN could only be a minor contributor to the total luminosity. The intensities of the low excitation [Ne II] and [S III] lines, and the extinction corrected ratio of the Lyman continuum and bolometric luminosity imply that massive star-formation is the dominant source of luminosity in Arp 220. The spectra include bright emission of the pure rotational S(1) and S(5) lines of molecular hydrogen. If the extinction to the emitting region is the same as to the ionized medium, as much as 10% Of the ISM of Arp 220 could be in the warm phase sampled by these lines.

We present mid-infrared spectroscopy of the prototypical interacting galaxies NGC 4038/39 (the 'Antennae') obtained with the ISO Short Wavelength Spectrometer (SWS). Our observations focus on the interaction zone where the two galaxies overlap, providing new constraints on the properties of the young and vigorous extranuclear starburst triggered by the recent interaction of the two galaxies. We use hydrogen recombination lines to derive an extinction of A(v) approximate to 70 to the starburst region, concluding that a model where emitting gas and absorbing dust are mixed best fits the data. We discuss the use of mid-infrared fine structure line ratios for determining the average effective temperature of the stellar ionizing radiation field, and derive an average temperature of 44000 K from the extinction-corrected [NeIII]15.6 mu m/[NeII]12.8 mu m line ratio. The observations are well described by starburst models for a recent starburst with an initial mass function extending up to 100 Mg.. Observations of the pure rotational (0-0) S(1) and S(2) lines of molecular hydrogen are used to constrain the fraction of molecular gas that is at temperatures of a few hundred Kelvin.

An integrated quasioptical receiver consisting of a planar double dipole antenna, SIS mixer and a superconducting local oscillator with matching circuits has been designed, fabricated and tested. A Flux-Flow Oscillator (FFO) based on unidirectional viscous flow of magnetic vortices in a long Josephson tunnel junction is employed as a local oscillator. All components of the receiver are integrated on a 4 mm x 4 mm x 0.2 mm crystalline quartz substrate on a base of the same Nb-AlOx-Nb trilayer in one technological run. The receiver has been studied in the frequency range 360 - 490 GHz. A lowest DSB noise temperature of 470 - 560 K has been achieved within the frequency range 425 - 455 GHz. Test circuits each comprising a FFO and a SIS detector have been experimentally investigated at frequencies up to 850 GHz. A new reliable technique for measuring the spectral linewidth of the integrated oscillators has been developed; the possibility of frequency locking of a FFO to an external microwave source has been demonstrated. The spectral linewidth of a FFO has been measured in the frequency range 250 - 580 GHz; a linewidth as low as 200 kHz is obtained at 450 GHz.

We have made a detailed study of the gain and noise of a SIS heterodyne receiver at 345 GHz. As mixing element we use an array of two Nb-Al2O3-Nb SIS junctions in series. The array is operated in a waveguide mount with a backshort and an E-plane tuner. The best receiver noise temperature achieved is 140 K DSB. The embedding impedances were determined by fitting theory to the measured pumped curves. High-quality fits are obtained which constitute the first detailed test of the Tucker-theory at frequencies above 300 GHz. The impedances found by this method are in very good agreement with impedances measured in a scale model at 3.3 GHz. From these embedding impedances gain and noise of the mixer were calculated over a full bias range using the Tucker theory in the three-port low IF approximation. The calculated values are compared to mixer gain and noise as obtained from receiver measurements. The observed dependence of mixer gain and noise on bias voltage, pump power and embedding impedance is in good agreement with theory. However the absolute value of the measured gain is a factor .45 +/- 0.05 lower than calculated. The measured mixer noise is approximately two quanta, 38 +/- 10 K, higher than calculated. These discrepancies appear to be independent of the bias parameters of the mixer.

An array of four Nb-Al-Al2O3-Nb sis junctions in series is used as the mixing element in a waveguide mixer at 345 GHz. The area of one junction is 4-mu-m2 and its normal state resistance is about 12-OMEGA, yielding at 345 GHz, omega-RC approximately 5. The receiver noise temperature below 550 K is between 330 and 375 GHz with best results around 350 K measured with a hot- and cold-load calibration. The conversion loss calculated from the hot/cold measurements ranges from 10.5 to 12.5 dB. The mixer noise temperature is around 200 K, with an instantaneous bandwidth of 2 GHz.

The Short-Wavelength Spectrometer (SWS) is one of the four instruments on-board ESA's Infrared Space Observatory (ISO), launched on November 17, 1995. The spectrometer covers the wavelength range of 2.38 to 4.2 mu m with a spectral resolution ranging from 1000 to 2000. By inserting Fabry-Perot filters the resolution can be enhanced by a factor 20 for the wavelength range from 11.4 to 44.5 mu m. An overview is given of the instrument, its in-orbit calibration, performance, observing modes and off-line processing software.