John D. Fix

Pluto: New photometry and a determination of the axis of rotation☆

Abstract New photoelectric observations of Pluto, made in 1971, 1972, and 1973, show that the amplitude of the 6-day rotational light variation is larger (about 0.22 mag) and the mean magnitude is fainter ( V 0 = 15.1) than during earlier periods of observation. The change is interpreted as due to a large obliquity of the rotational axis (probably larger than 50°). The phase coefficient of Pluto is about 0.05 mag/deg at 1° solar phase angle.

Axisymmetric models of circumstellar dust shells

Luminosity constant models of axisymmetric circumstellar dust shells are calculated. The models consist of either donut-like or disklike distributions of silicate grains surrounding a 3000 K star. An iterative scheme is used to solve the radiative transfer problem taking into account both scattered and thermal radiation. Maps for models with varying degrees of asymmetry and optical thickness of the shell and for various inclinations of the line of sight to the axis of symmetry are presented. One-dimensional visibility amplitudes and angular sizes are computed for scans of the maps in two perpendicular directions. It is shown that one-dimensional visibility measurements at 10 and 20 microns may be used to distinguish between the two types of dust density distributions considered here. 30 refs.

Models of OH Maser Variations in U Herculis

Arecibo spectra of the main-line OH maser emission from U Her over more than a decade show variations of the OH emission over these timescales. These observations are combined with high spatial resolution VLBA maps to investigate the changes in the velocities of the maser components. Global properties of the dust shell, such as accelerations, variations in the pump, and shellwide magnetic field changes are examined as possibilities and eliminated. A solution to the problem involving plasma turbulence and the local magnetic field is introduced, and the relevant timescales of the turbulence are calculated. The turbulent velocity field causes variations on timescales that are too long (of order centuries), while the turbulent magnetic field causes variations on appropriate timescales of a few years. A line-of-sight model of the turbulence is developed and investigated. The complete exploration of this solution requires extensive theoretical and observational work. Possible avenues of investigation of the plasma turbulence model are presented.

Infrared spectroscopy of IRC + 10420

High resolution 1.6, 3.3 and 4.6 micron spectra were obtained on the F8 supergiant star IRC + 10420 on June 9-11, 1984, using the 4 m Mayall telescope and Fourier Transform Spectrometer at Kitt Peak. IRC + 10420 is distinguished by its hydrogen maser, unusual for stars of this type. The measured column densities of CO led to an estimated isotopic carbon ratio of 7-11, supporting a model of the object as a highly evolved star. The redshift is interpreted as due to scattering by outward-moving circumstellar dust grains. The IR spectrum of IRC + 10420 suggests that the object is a post-asymptotic branch star still surrounded by the thick dust shell it produced while it was much cooler.

AN INVESTIGATION OF THE ROTATIONAL PERIOD OF THE PLANET PLUTO

New photometry of Pluto was obtained to resolve the longstanding ambiguity in the synodic period of Pluto. Measurements obtained at Kitt Peak National Observatory on four consecutive nights in April 1973 show that the shorter period of 1d1819 is spurious and confirm the 6d3867 i 0.0003 period found by Hardie (1964). The times of mean light crossing on the rising branch yield an improved synodic period of 6d3874 i 0.0002 based on 20 years of observations. Key words: Pluto - photometry - rotational period

The distribution of maser emission in OH/IR stars

The 1612 MHz OH emission from five OH/IR stars has been mapped at three epochs over a 2.5 yr period of time. Although the stars were observed at very different phases in the radio light curve of each, there were no remarkable changes in the appearance of the maps. This probably implies that the properties of the masers do not range widely throughout a maser shell. The maps have been used to produce star-centered surface maps of the distribution of maser emission from each star. The surface maps generally are sparsely filled with OH emission and are dominated by relatively few (about 10) major clumps of emission. The presence of large regions of low intensity in the surface maps suggests that the number of individual emitting elements is relatively small or that there are a larger number of elements which are distributed in the shell in a distinctly nonrandom manner.

Measurements of the diffuse ultraviolet radiation

The imaging instrumentation on the Dynamics Explorer 1 satellite has been used to measure the intensity of the diffuse ultraviolet radiation on two great circles about the sky. It is found that the isotropic component of the diffuse ultraviolet radiation (possibly of extragalactic origin) has an intensity of 530 + or - 80 units (a unit is 1 photon per sq cm s A sr) at a wavelength of 150 nm. The Galactic component of the diffuse ultraviolet radiation has a dependence on Galactic latitude which requires strongly forward scattering particles if it is produced by dust above the Galactic plane. 24 refs.

1612 MHz OH maser emission from axisymmetric circumstellar envelopes : Miras

Radiative transfer calculations are performed using a modified form of the Sobolev approximation to determine the inversion of the 1612 MHz line of OH in axisymmetric circumstellar envelopes around Miras. The mass loss is assumed to be occurring in the form of a smooth wind. Line profiles and maps are presented for three models of varying degrees of asymmetry and for various orientations of the envelopes. It is concluded that the axisymmetric models can reproduce many of the features of observed profiles and maps which both the standard, spherically symmetric model and the discrete emission model cannot easily explain. The model profiles reproduce all of the general features seen in the line profiles of real sources.

Circular polarization in 1612 MHz OH maser emission from OH/IR stars

In order to study the incidence of circular polarization in the 1612 MHz OH emission from envelopes of OH/IR stars, the Arecibo radio telescope was used to obtain high-resolution, high signal-to-noise 1612 MHz spectra in both circular polarizations for several OH/IR stars. The method of Troland and Heiles (1982) was used to estimate the intensities of magnetic fields in the envelopes. The fields appear to have complex structures, and intensities on the order of 1-100 microG. The spectra are weakly polarized, probably as a result of weakly polarized emission from individual emitting elements. Integration over the Stokes parameter V yields nonzero results for three of the stars, suggesting that another polarizing mechanism (in addition to simple Zeeman splittings) is at work. 12 refs.

A MEASUREMENT OF THE RELATIVE REFLECTANCE OF PLUTO AT 0.86 MICRON.

We have examined Plutos near infrared reflectance by means of photoelectric photometry using a filter whose effective wavelength is 0.86 micron. The ratio of Plutos reflectance at 0.86 micron to that at 0.55 micron is 1.33 plus or minus 0.04. Our measurements, combined with the UBVR colors of Pluto as measured by Hardie, show that Plutos reflectance increases almost linearly with wavelength in the region between 0.36 micron and 0.86 micron. Plutos reflectance in the spectral region resembles the reflectance of the asteorid 5 Astrae as well as some stony-iron meteorites and iron meteorites having low nickel content.