Gordon C. Augason

Spectral irradiance calibration in the infrared. III - The influence of CO and SiO

We describe first efforts to establish a network of calibrated infrared spectra of «standard stars» suitable for calibration of at least low-resolution infrared spectrometers using ground-based, airborne, and satellite-borne broadband sensors. The focus of this paper is on the crucial 5-8 μm region, inaccessible from the ground, in K and M giants. In this region the fundamental bands of CO and SiO cause substantial departures from featureless pseudo-continua. These departures are, of course, well-known to stellar atmosphere theorists. However, they are still ignored by many astronomical infrared photometrists and spectroscopists who assume that these bright stars can be represent by blackbodies at their effective temperatures

A Determination of the Composition of the Venus Clouds from Aircraft Observations in the Near Infrared

Abstract We summarize the evidence showing that the first optical depth of the Venus cloud layer is composed of a water solution of sulfuric acid, including our earlier aircraft observations of Venus’ reflectivity in the 1–4 μm region obtained at a phase angle of 120° (Pollack et al.). Analyses of these aircraft results indicated that of all the proposed cloud candidates only a sulfuric acid solution with a concentration of 75% or more H2SO4, by weight was consistent with the observed 3 µm cloud feature. We present new aircraft observations of Venus obtained in the 1–4 µm region at a phase angle of 40° and in the 3–6 µm region at a phase angle of 136°. Comparing the two sets of observations in the 1–4 µm region, we find a striking phase effect: the reflectivity is much lower in the 3 µm region and there is a much more marked decline between 1.3 and 2.5 µm for the data obtained at the smaller phase angle. The observations made at the 40° phase angle are consistent with the theoretical behavior of a sulfuri...

Compensation for 6.5-K cryogenic distortion of a fused-quartz mirror by refiguring

A 46 cm diameter, lightweight, Amersil TO8E, fused-natural-quartz mirror with a single-arch cross section was tested at the NASA-Ames Research Center Cryogenic Optical Test Facility to measure its cryogenic distortion at 6.5 K. Then the mirror was refigured with the inverse of the measured cryogenic distortion to compensate for this figure defect. The mirror was retested at 6.5 K and found to have a significantly improved figure. The compensation for cryogenic distortion was not complete, but preliminary analysis indicates that the compensation was better than 0.25 waves P-V if edge effects are ignored. The feasibility of compensating for cryogenic distortion by refiguring has thus been verified.

Water vapor absorption spectra of the upper atmosphere (45–185 cm −1 )

The far ir nighttime absorption spectrum of the earths atmosphere above 14 km is determined from observations of the bright moon. The spectra were obtained using a Michelson interferometer attached to a 30-cm telescope aboard a high-altitude jet aircraft. Comparison with a single-layer model atmosphere implies a vertical column of 3.4 +/- 0.4 mum of percipitable water on 30 August 1971 and 2.4 +/- 0.3 mum of precipitable water on 6 January 1972.

Cryogenic distortion at 4.4 K of a 50-cm-diameter spherical beryllium mirror fabricated to reduce cryogenic distortion and hysteresis

A 50 cm diameter, beryllium mirror was fabricated and cryogenically tested as a joint project between NASA-Ames Research Center and Jet Propulsion Laboratory. The purpose of this project was to determine the cryogenic distortion and hysteresis of a large, state-of-the-art beryllium mirror when cooled to liquid helium temperatures. The mirror blank was HIPed from I-70 special beryllium and machined to a plano-concave sphere with a 200 cm radius of curvature. The blank was annealed, acid etched, and thermally cycled may times during machining, figuring, and polishing to reduce stress. The mirror was tested twice to liquid helium temperature using the Ames Research Center Cryogenic Test Facility. No hysteresis or temporal instability was measured in the two tests. The cryogenic distortion was 0.5 p-v. This distortion is comparable to fused silica and is the lowest for any beryllium mirror tested by this facility.

Cryogenic surface distortion and hysteresis of a 50-cm-diameter fused-silica mirror cooled to 77 K

A 50 cm diameter, lightweight, Amersil TO8E, fused-natural-quartz mirror with a single arch cross section was tested at the NASA/Ames Research Center Cryogenic Optics Test Facility to measure cryogenic distortion and hysteresis. The mirror was cooled to 77 K in four serial tests and the mirror figure was measured with a phase-measuring interferometer. On the basis of the repeatability of room temperature and cryogenic optical measurements, it was determined that the Single Arch Mirror had no measurable hysteresis and displayed repeatable cryogenic distortion. The Cryogenic Optics Test Facility, optical and thermal test methods, test results, and measurement accuracy are described.

Hydrogen Deficiency and Mass Loss from AGB Stars

The mechanism for mass loss from stars is not understood, but theoretical studies predict that it should accelerate once stars on the AGB begin thermal pulsations (TP) (Shoenberner, 1979). We have discoved that the stars on this part of the AGB are apparently hydrogen deficient which may result from the mass loss predicted by theory. Recent observations of non-Mira AGB stars (Goebel and Johnson, 1984, Augason et al, 1985) show that these stars have less flux at 1.6 microns (the H- opacity minimum) than do models with solar abundance. We interpret this missing flux as being due to a lack of H- resulting from a significant hydrogen deficiency. Johnson et al (1985) have computed opacity sampled models of cool, non-Mira carbon stars where hydrogen is replaced with helium and they find that it is necessary to reduce the hydrogen abundance of the models below 30% of the solar value to match the observations. Hydrogen deficiency is also supported by the nondetection of H2 in: non-Mira stars (Tsuji, 1983), S stars (Johnson, 1983) and carbon stars (Johnson et al, 1983).

Modelling the M-S-C Giants Spectral Sequence

We present pressure-dependent line-by-line LTE model atmospheres for cool red giants (Teff < 4000 K) in spherical geometry. The models are computed using the atmospheric code P H O E N I X , and they constitute an extension of the existing cool dwarf grids of Allard & Hauschildt (1995, ApJ, 445, 433) to the pressure regimes of red giants. The grid covers C/O ratios ranging from 0.27 to 1.02 with otherwise solar metallicity. We find our models comparable to those of Kurucz in regimes where the plane-parallel approximation is valid, with the exception of the predicted strength of TiO bands as expected from the use of different TiO opacity sources. Departures from LTE in the Ti I lines are investigated for some selected models across the grid, but onhr modest NLTE effects are found in the abundance of the important absorbei TiO. The models are used to construct a spectral sequence of M, S and C type giants for which both optical and infrared spectra are available. Colors of the combined dwarf and giant model grids are presented in the Wing eight-color system which reveals a clear separation of dwarf and giant stars, and of giants in carbon abundance and gravity, providing ideal grounds for the study of the chemical evolution of giants.

Absolute Spectrally Continuous Stellar Irradiance Calibration in the Infrared

We present first efforts to establish a network of absolutely calibrated continuous infrared spectra of standard stars across the l-35^m range in order to calibrate arbitrary broad and narrow passbands and low-resolution spectrometers from ground-based, airborne, balloon, and satellite-borne sensors. The value to photometry of such calibrated continuous spectra is that one can integrate arbitrary filters over the spectra and derive the stellar in-band flux, monochromatic flux density, and hence the magnitude, for any site. This work is based on new models of Sirius and Vega by Kurucz which were calculated by him, for the first time, with realistic stellar metallicities and a customized finely-gridded infrared wavelength scale. We have absolutely calibrated these two spectra and have calculated monochromatic flux densities for both stars, and isophotal wavelengths, for a number of infrared filters. Preliminarily, the current IRAS point source flux calibration is too high by 2, 6, 3, and 12% at 12, 25, 60, and 100/tm, respectively.

Thermal modeling of the NASA/Ames Research Center cryogenic optical test facility and a single-arch, fused-natural-quartz mirror

A thermal model of the dewar and optical system of the Cryogenic Optical Test Facility at NASA-Ames Research Center was developed using the computer codes SINDA and MONTE CARLO. The model was based on the geometry, boundary conditions, and physical properties of the test facility and was developed to investigate heat transfer mechanisms and temperatures in the facility and in test mirrors during cryogenic optical tests. A single-arch, fused-natural-quartz mirror was the first mirror whose thermal loads and temperature distributions were modeled. From the temperature distribution, the thermal gradients in the mirror were obtained. The model predicted that a small gradient should exist for the single arch mirror. This was later verified by the measurement of mirror temperatures. The temperatures, predicted by the model at various locations within the dewar, were in relatively good agreement with the measured temperatures. The model is applicable to both steady-state and transient cooldown operations.