Andrew T. Young

Revised optical air mass tables and approximation formula

We correct an error in a widely used air mass table by recalculating the values on the basis of the ISO Standard Atmosphere (1972) and revise its approximation formula.

Revised depolarization corrections for atmospheric extinction

Possible sources of errors in depolarization data are analyzed with special reference to the rotational Raman contribution to the optical depth. It is shown that the depolarization data now available are so accurate that their uncertainty affects the calculated Rayleigh optical depth by less than 1%, and consequently, the anomalously low extinctions occasionally reported must be due to systematic errors of measurement rather than errors in the Rayleigh optical depths.

Inelastic scattering in planetary atmospheres. I - The Ring effect, without aerosols

The contribution of inelastic molecular scattering (Rayleigh-Brillouin and rotational Raman scattering) to the filling-in of Fraunhofer lines in the light of the blue sky is studied. Aerosol fluorescence is shown to be negligible, and aerosol scattering is ignored. The angular and polarization dependences of the filling-in detail for single scattering are discussed. An approximate treatment of multiple scattering, using a backward Monte Carlo technique, makes it possible to investigate the effects of the ground albedo. As the molecular scatterings alone produce more line-filling than is observed, it seems likely that aerosols dilute the effect by contributing unaltered sunlight to the observed spectra.

Air mass and refraction

Approximate formulas are presented for relative optical air mass as a function of true, rather than refracted, zenith angle.

No sulfur flows on Io

Physical and chemical properties of elemental sulfur are incompatible with the suggestion that the colored flows associated with volcanoes on Io are quenched unstable allotropes of sulfur. Either the volcanic flows are not sulfur, or some mechanism other than quenching is required to produce colored forms of sulfur in them. The properties of sulfur are unsuited to the production and survival of colored unstable allotropes on Io. The color of this object is probably due to some other material, possibly iron compounds.

PRECISE AUTOMATIC DIFFERENTIAL STELLAR PHOTOMETRY

The factors limiting the precision of differential stellar photometry are reviewed. Errors due to variable atmospheric extinction can be reduced to below 0.001 mag at good sites by utilizing the speed of robotic telescopes. Existing photometric systems produce aliasing errors, which are several millimagnitudes in general but may be reduced to about a millimagnitude in special circumstances. Conventional differential photometry neglects several other important effects, which are discussed in detail. If all of these are properly handled, it appears possible to do differential photometry of variable stars with an overall precision of 0.001 mag with ground based robotic telescopes.

Temperature Effects in Photomultipliers and Astronomical Photometry

Temperature coefficients are reported for photomultiplier tube types used in astronomy. For 1% stability of gain and color response, temperature regulation of 1°C or better is generally required. This is nearly an order of magnitude better than what is usually achieved at the telescope, but careful use of a well-designed cold box should make 1°C temperature stability possible. For maximum stability and reproducibility, ordinary blue-sensitive tubes should be avoided at wavelengths longer than 5000 A, and trialkali cathodes should not be used beyond 6500 A.

RECENT IMPROVEMENTS TO A VERSION OF THE WILSON-DEVINNEY PROGRAM

We summarize recent improvements to a version of the Wilson-Devinney program that is widely used for the analysis of eclipsing binary data, and we describe the new WD95 program. WD95 contains the University of Calgary version of the Wilson-Devinney code, which supports the use of the Kurucz atmo- sphere models; it provides options to use multiple epoch data and multiwavelength synoptic passbands. The WD95 program contains an improved input/output interface, simplex algorithms for initial searches and tests, and versions of Wilson-Devinney DC and LC programs and options to switch to automatic di†erential corrections or a damped least-squares solver using normal equations that are modi-ed as per the Levenberg-Marquardt scheme. This paper describes some tests of the damped least-squares solver with simulated data. Subject headings: binaries: eclipsing E methods: numerical

An improved Venus cloud model

Abstract A simple radiative-transfer theory that allows for the change in the absorptions of sulfur and carbon dioxide with depth in the atmosphere of Venus can account simultaneously for (1) the spectral reflectance of Venus; (2) the wavelength dependence of contrast in uv cloud features; (3) the CO 2 line profile; (4) the change in slope of the curve of growth from the 7820- to the 10488-A CO 2 bands; and (5) the rotational temperature near 246°K found for all CO 2 bands. The model cloud consists of 1-μm sulfuric-acid particles, which are well mixed between about 64 km and the 49-km cloud base found by Veneras 9 and 10, plus an overlapping cloud of much larger sulfur particles that extends down to the 35-km cloud base found by Venera 8. The mixing ratios (by number of molecules) below about 64 km are: H 2 O, 2 × 10 −4 ; H 2 SO 4 , 10 −5 ; and sulfur, 10 −4 . Although the cloud contains an order of magnitude more sulfur than sulfuric acid, the sulfur particles are an order of magnitude larger, and so have only about 1% of the number density of the acid droplets. The “black-white” radiative-transfer model assumes perfectly conservative scattering above the level (which depends on wavelength) where an absorber becomes “black” due to the local temperature and pressure. So-called homogeneous scattering models are inherently self-contradictory, and are inapplicable to planetary atmospheres; the vertical inhomogeneity is an essential feature that must be modeled correctly. The pressure of CO 2 line formation is about half the pressure in the region where uv markings occur.

Venus cloud microphysics

Abstract Because sulfuric acid does not wet sulfur, composite drops in the atmosphere of Venus cannot have sulfur “cores,” but must instead have sulfur coats. Both components then communicate with the vapor phase. Drops that are fully coated with sulfur are immune to coalescence; this sets a limit to growth that may explain “Mode 3” particles. The sulfur coating is probably responsible for the anomalously low refractive indices derived from entry-probe nephelometer data. There appears to be about an order of magnitude less elemental sulfur than sulfuric acid in the clouds.