William M. Irvine

Infrared optical properties of water and ice spheres

Abstract The literature on the absorption coefficient and reflectivity of water and ice in the infrared is critically reviewed, and best values are chosen for the complex index of refraction for wavelengths 0.7 μ ⩽ λ ⩽ 200 μ. The Mie theory is then used to compute the single scattering albedo a, asymmetry factor 〈 cos θ〉, and normalized extinction cross section Qext for spheres of water and ice with radii 0.3, 1.0, 3.0, and 10.0 microns in this wavelength interval. Significant differences in the absorption spectrum between water and ice and among particles of different radii are illustrated. A useful approximate formula for a is given. The results are important for the study of radiative transfer in planetary cloud layers.

A Study of the Physics and Chemistry of TMC-1

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

Survey Observations of c-C2H4O and CH3CHO toward Massive Star-forming Regions

In order to clarify the formation mechanisms of ethylene oxide (cyclic-C2H4O, hereafter c-C2H4O) and its structural isomer acetaldehyde (CH3CHO), we carried out survey observations of these two molecules toward 20 massive star-forming regions and two dark clouds. CH3CHO and c-C2H4O were detected in 10 massive star-forming regions, and CH3CHO was also detected in five others. The column densities and the rotational temperatures were derived using the rotation diagram method. The column densities of these molecules were derived to be (0.1-3.3) × 1014 and (0.2-5.0) × 1014 cm-2 for c-C2H4O and CH3CHO, respectively. The fractional abundances with respect to H2 are X(c-C2H4O) = 4 × 10-11 to 6 × 10-10 and X(CH3CHO) = 7 × 10-12 to 3 × 10-9. We also detected several transitions of methanol (CH3OH), ethanol (C2H5OH), dimethyl ether [(CH3)2O], methyl formate (HCOOCH3), formic acid (HCOOH), vinyl cyanide (C2H3CN), and ethyl cyanide (C2H5CN). Comparing the abundances of the detected molecules with physical conditions of each source, we found that the abundances of most of the molecules except for c-C2H4O and CH3CHO increase along with the dust temperature of each source. On the other hand, the abundances of c-C2H4O and CH3CHO show little correlation with the dust temperature. The rotation temperatures of c-C2H4O, CH3CHO, and HCOOH are low (10-40 K) in all sources in spite of the fact that the gas kinetic temperature greatly varies from cloud to cloud. This may indicate that the line emission from each molecular species is excited in regions with different physical conditions. We performed pseudo-time-dependent chemical reaction simulations based on pure gas-phase reactions and found that the calculated abundances of observed molecules decreased when the gas kinetic temperature was raised. We investigated the relationship between the column density of C2H5OH and that of the C2H4O group (c-C2H4O + CH3CHO) because C2H5OH is believed to be a precursor of c-C2H4O and CH3CHO in the gas-phase chemistry scheme. If this hypothesis is correct, it is expected that the column density of C2H5OH is related to that of the C2H4O group. We found that the column density of the C2H4O group is high in sources where the column density of C2H5OH is high. This result is consistent with the above-mentioned hypothesis. We also investigated the relationships between the column densities of several organic species [CH3OH, C2H5OH, (CH3)2O, HCOOCH3, C2H3CN, and C2H5CN] and the luminosity-to-mass ratio, LIR/M, in OMC-1, W51A, and Sgr B2(N). We found that the column densities of these molecules are high in sources where LIR/M is high. Since LIR/M is believed to be a measure of the star formation rate per unit mass, it indicates that the column densities of these molecules become higher in sources where high star formation activity leads to a higher dust temperature. This strongly suggests that the formation of these molecules involves processes on the dust grains and subsequent sublimation to the gas phase, where they can be observed.

Monochromatic phase curves and albedos for the lunar disk.

Photoelectric observations of the entire lunar disk made in 1964-1965 over phase angles from 6 to 12 deg in nine narrow bands from 0.35 to 1.0 microns and in UBV are reviewed. Phase curves are presented as a function of wavelength. The results confirm a reddening with increasing phase angle found by previous investigators for particular areas.

Light scattering by randomly oriented crystals

The scattering phase function and the degree of linear polarization for small crystals oriented randomly in space have been computed using the geometric ray tracing theory and assuming that the crystals are homogeneous and isotropic. Calculations have been carried out for the main crystal geometries. Detection of halos from crystals other than hexagonal water ice is briefly discussed. The crystal size and shape parameters have also been averaged over some simple distributions in order to examine general light scattering properties of sharp-edged particles. A scalar physical optics correction has been developed for the geometric optics phase functions. Results can be applied to light scattering from regoliths and planetary rings, and possibly also to atmospheric halos. Retroreflecting crystals in the regolith would cause an opposition spike, a phenomenon observed for many bright satellites.

Molecular Abundance Variations Among and Within Cold, Dark Molecular Clouds

The latest table of molecular abundances in the cold, dark clouds TMC-1 and L134N is presented. Molecular abundance variations between TMC-1 and L134N, those within TMC-1 and L134N, and those among 49 dark cloud cores surveyed by Suzuki et al. (1991) are interpreted a.s an effect of chemical evolution.

Multiple Scattering in Planetary Atmospheres

Abstract Methods for solving radiative transfer problems within the extended visible spectrum in planetary atmospheres are reviewed for use by the nonspecialist. Emphasis is placed on rapid, approximate procedures for the determination of such quantities as the plane and spherical (Bond) albedo, surface illumination, absorbed energy, limb darkening, phase curve, and spectra. Precise numerical methods and analytical results are also discussed. Recent approaches to such complications as atmospheric inhomogeneity and reflection from a porous regolith are described briefly.

Scattering of light by stochastically rough particles

The single particle phase function and the linear polarization for large stochastically deformed spheres have been calculated by Monte Carlo simulation using the geometrical optics approximation. The radius vector of a particle is assumed to obey a bivariate lognormal distribution with three free parameters: mean radius, its standard deviation and the coherence length of the autocorrelation function. All reflections/refractions which include sufficient energy have been included. Real and imaginary parts of the refractive index can be varied without any restrictions. Results and comparisons with some earlier less general theories are presented. Applications of this theory to the photometric properties of atmosphereless bodies and interplanetary dust are discussed.

Multiple scattering by large particles

Numerical Neumann solution to scalar equation of transfer in homogeneous layer of optical thickness

A Three-Position Spectral Line Survey of Sagittarius B2 between 218 and 263 GHz. I. The Observational Data

We have surveyed the frequency band 218.30-263.55 GHz toward the core positions N and M and the quiescent cloud position NW in the Sgr B2 molecular cloud using the Swedish-ESO Submillimetre Telescope. In total 1730, 660, and 110 lines were detected in N, M, and NW, respectively, and 42 different molecular species were identified. The number of unidentified lines are 337, 51, and eight. Toward the N source, spectral line emission constitutes 22% of the total detected flux in the observed band, and complex organic molecules are the main contributors. Toward M, 14% of the broadband flux is caused by lines, and SO2 is here the dominant source of emission. NW is relatively poor in spectral lines and continuum. In this paper we present the spectra together with tables of suggested line identifications.