Showing papers by "Peter A. R. Ade published in 1996"

Cassini infrared Fourier spectroscopic investigation

Abstract: The composite infrared spectrometer (CIRS) is a remote sensing instrument to be flown on the Cassini orbiter. CIRS will retrieve vertical profiles of temperature and gas composition for the atmospheres of Titan and Saturn, from deep in their tropospheres to high in their stratospheres. CIRS will also retrieve information on the thermal properties and composition of Saturn's rings and Saturnian satellites. CIRS consists of a pair of Fourier Transform Spectrometers (FTSs) which together cover the spectral range from 10-1400 cm-1 with a spectral resolution up to 0.5 cm-1. The two interferometers share a 50 cm beryllium Cassegrain telescope. The far-infrared FTS is a polarizing interferometer covering the 10-600 cm-1 range with a pair of thermopile detectors, and a 3.9 mrad field of view. The mid-infrared FTS is a conventional Michelson interferometer covering 200-1400 cm-1 in two spectral bandpasses: 600-1100 cm- 1100-1400 cm(superscript -1 with a 1 by 10 photovoltaic HgCdTe array. Each pixel of the arrays has an approximate 0.3 mrad field of view. The HgCdTe arrays are cooled to approximately 80K with a passive radiative cooler.

The ISO Long-Wavelength Spectrometer

Abstract: The Long-Wavelength Spectrometer (LWS) is one of two complementary spectrometers aboard the European Space Agency's Infrared Space Observatory (ISO) (Kessler et al., 1996AA a companion paper (Swinyard et al, 1996) describes its performance and calibration.

First detection of the 56-mu m rotational line of hd in saturn's atmosphere

Abstract: The R(1) rotational line of HD at 5623 mu m has been detected for the first time on Saturn using the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO) The measurements were made using the LWS in Fabry-Perot mode in January 1996 during the ISO performance verification phase The measured spectrum has been compared with atmospheric models to determine the HD/H-2 abundance ratio The best model fit to the measured spectrum was obtained with a D/H ratio in hydrogen of 23x10(-5); D/H values between 15x10(-5) and 35x10(-5) are however also compatible with the data This result is intermediate between the saturnian value derived from ground-based observations of methane and its deuterated isotope, and the preliminary determination of the jovian D/H ratio measured by the mass spectrometer in the Galileo probe The initial Saturn measurements reported here will be repeated to improve the signal-to-noise ratio, and LWS observations of HD on Jupiter, Uranus and Neptune will also be made

ISO LWS measurement of the far-infrared spectrum of Saturn

Abstract: The spectrum of Saturn from 43 to 197μm 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 0.29μm from 43 to 90μm and 0.6μm from 90 to 197μm. 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 PH_3_ profile in which the tropospheric mixing ratio was increased to 7x10^-6^ and the cutoff due to photodissociation was lowered to 300mbar. 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.

100 mK bolometers for the submillimetre common-user bolometer array (scuba) I. Design and construction

Abstract: We describe the design and construction of bolometric detectors for SCUBA - the Submillimetre Common-User Bolometer Array for the James Clerk Maxwell Telescope in Hawaii. The instrument contains 131 individual detectors, in two arrays, optimized for the submillimetre atmospheric transmission windows. The detectors are cooled by dilution refrigeration to a temperature of 100 mK, so that the receiver performance will be limited by photon noise from the sky and telescope background in all wavebands. A future paper will describe the performance of the detectors with reference to typical data obtained during the laboratory commissioning period.

Transient response in doped germanium photoconductors under very low background operation

Abstract: Doped germanium photoconductors are the most sensitive detectors for astronomy in the wavelength range 40–240 μm. Under the extremely low background conditions encountered in cooled satellite instruments, these devices exhibit a number of transient effects, such as slow relaxation after a step change in illumination or bias, and spontaneous spiking at high signal levels. Such behavior can degrade the excellent instantaneous sensitivity of these detectors and create calibration uncertainties. These effects have been observed in the Ge:Be photoconductors and the stressed and unstressed Ge:Ga photoconductors in the Long Wavelength Spectrometer, one of the instruments on the Infrared Space Observatory. A systematic investigation of the transient response of the Long Wavelength Spectrometer detectors to a step change in illumination as a function of operating temperature, bias electric field, and illumination step size has been carried out to determine operating conditions that minimize the effects of this behavior. The transient effects appear to be due primarily to carrier sweep out, but they are not fully explained by existing models for transient response.

Thermal H_2_O emission from the Herbig-Haro flow HH 54.

Abstract: The first detection of thermal water emission from a Herbig-Haro object is presented. The observations were performed with the Lws (Long Wavelength Spectrograph) aboard Iso (Infrared Space Observatory). Besides H_2_O, rotational lines of CO are present in the spectrum of HH 54. These high-J CO lines are used to derive the physical model parameters of the Fir (far-infrared) molecular line emitting regions. This model fits simultaneously the observed OH and H_2_O spectra for an OH abundance X(OH)=10^-6^ and a water vapour abundance X(H_2_O)=10^-5^. At a distance of 250pc, the total CO, OH and H_2_O rotational line cooling rate is estimated to be 1.3x10^-2^Lsun_, which is comparable to the mechanical luminosity generated by the 10km/s shocks, suggesting that practically all of the cooling of the weak-shock regions is done by these three molecular species alone.

The ISO/LWS far infrared spectrum of IRC+10 216

Abstract: We present an ISO Long Wavelength Spectrometer (LWS) grating spectrum of the carbon-rich circumstellar envelope -CSE- of IRC+10216 between 43 and 197μm. The spectrum consists of strong dust emission plus a forest of emission lines from CO, HCN, H^13^CN and vibrationally excited HCN (ν_2_=1^1^,2^0,2^ and ν_1,3_=1). All the CO lines between J=14-13 and J=39-38 have been detected while lines of HCN with J_u_ as high as 48 have also been observed. The molecular emission arises from the warm and dense gas located in the innermost zone of the CSE. The CO and HCN emission can be easily explained if the vibrational and rotational temperatures are around 700-1500 K. We also report the tentative detection of the bending mode of the C_3_ molecule around 62cm^-1^ (157.2μm).

The ISO LWS grating spectrum of NGC 7027.

Abstract: We present a high signal-to-noise ISO Long Wavelength Spectrometer (LWS) grating spectrum of the planetary nebula NGC 7027 from 43-194μm. In total 40 emission lines have been detected, with 30 identified. From the ionized region, we observe fine-structure lines from [N II], [N III] and [O III]. The [O I] and [C II] fine-structure lines from the photodissociation region are the strongest features observed in this spectral region. Amongst the molecular lines, 11 pure rotation CO lines from J=14-13 up to J=24-23 have been detected. The most striking result, however, is the detection in this carbon-rich nebula of the o-H_2_O 179.53μm and the OH 119.3μm fundamental lines. Astrophysical implications are briefly discussed.

LWS-spectroscopy of Herbig Haro objects and molecular outflows in the Cha II dark cloud.

Abstract: We present the first far infrared spectra of the Herbig Haro objects HH 52-53-54 and of IRAS 12496-7650, all located in the nearby star forming region known as Chamaleon II dark cloud, obtained with the Long Wavelength Spectrometer (LWS) onboard the Infrared Space Observatory (ISO). The richest spectrum is found in HH 54, showing molecular transitions (CO with J_u_ from 19 to 14, water vapour mainly in its ortho form and OH) and low excitation fine structure lines ([OI]63, 145μm, [CII]158μm). In HH 52 and HH 53,only the [OI] and [CII] lines are detected. The LWS spectrum of IRAS 12496-7650 shows both fine structure and CO lines. The [CII]158μm line is ubiquitous in the region, as proved by its presence in all ISO pointings, including the raster scan maps. The fine structure lines are used to evaluate the physical parameters of the emitting regions. In particular, the mass loss rates of each outflow present in the region, are derived from the [OI]63μm line luminosity.

Testing of 100mK Bolometers for Space Applications

Abstract: Electrical and optical performance data are presented for a prototype 100 mK spider-web bolometer operating under very low photon backgrounds. These data are compared with the bolometer theory and are used to estimate the expected sensitivity of such a detector used for low background space astronomy. The results demonstrate that the sensitivity and speed of response requirements of the bolometer instruments proposed for these missions can be met by 100 mK spider-web bolometers using neutron transmutation doped germanium as the temperature sensitive element.

The ISO/LWS spectrum of the Egg nebula, AFGL 2688

Abstract: We present an ISO Long Wavelength Spectrometer (LWS) grating spectrum of the carbon-rich proto-planetary nebula AFGL 2688 between 43 and 194μm. The far-infrared spectrum of AFGL 2688 is dominated by strong rotational transitions of CO which are detected from J=14-13 up to J=23-22. The atomic fine structure lines [O I] and [C II] are not detected. This is consistent with the cool central star of AFGL 2688 not having yet photodissociated the molecular gas ejected during the AGB phase. The far-infrared CO emission in AFGL 2688 appears to originate in shocked dense gas at a temperature of ~400K.