Photometric properties of outer planetary satellites

We present optical broadband photometry for the satellites J6, J7, J8, S7, S9, U3, U4, N1, and polarimetry for J6, obtained between 1970 and 1979. The outer Jovian satellites resemble C-type asteroids; J6 has a rotational lightcurve with period ～9.5 hr. The satellites beyond Jupiter also show C-like colors with the exception of S7 Hyperion. S9 Phoebe has a rotational lightcurve with period near either 11.25 or 21.1 hr. For U4 and N1 there is evidence for a lightcurve synchronous with the orbital revolution. The seven brighter Saturnian satellites show a regular relation between the ultraviolet dropoff and distance to the planet, probably related with differences in the rock component on their surfaces.     

On the astrometric detection of neighboring planetary systems,II

Extensive testing suggests that astrometric techniques can be employed to detect and study virtually any planetary system that may exist within 40 light years (12.5 parsec) of the Sun. Following the conclusion of Paper I [G. Gatewood, Icarus27 (1976), 1–12], the astrometric group at the Allegheny Observatory began an intensive survey of 20 nearby stars to detect the nonlinear variations in their motion that planetary systems would induce. Several tests conducted to further our understanding of the limitations of this survey indicated that the photographic detector itself is responsible for the majority of the random error. A new photoelectric detector has been designed and a simplified prototype of it successfully tested. The new detector is expected to be able to utilize virtually all of the astrometric information transmitted through the Earth's atmosphere. This is sufficient to determine relative positions to within an accuracy of approximately 1 milliarcsec/hr. Such precisions exceed the design capabilities of the best existing astrometric telescopes, thus a feasibility study has been conducted for the design of an improved instrument. The study concludes that a new ground-based telescope and the new detector combined should be able to study stars as faint as the 17th magnitude with an annual accuracy of a few tenths of a milliarcsecond. However, to obtain the ultimate accuracy possible from current technology, we must place an astrometric system above the Earth's atmosphere. A space-borne instrument utilizing the new detector would in theory have sufficient accuracy to detect any Earth-like planet orbiting any of the several hundred stars nearest the Sun.     

Rare opportunity for information on the upper atmosphere of Saturn

We report on taking, successfully, the rare opportunity to monitor photoelectrically the eclipse of Saturn's largest satellite (Titan) and present a light curve. Comparing this light curve with similar ones obtained for Jovian satellites we deduce the Saturnian stratosphere to be relatively clear, at least at the latitude (25° S) probed.     

A numerical study of aerosol growth in Titan's atmosphere

We present a simple model for the formation and growth of photochemical aerosols in the atmosphere of Titan. We show that, in general, an optically thick layer of particles in the size range required by models of Titan cannot be obtained at pressures less than about 2 mbar. Since the thin model of Titan's atmosphere requires that the inversion not extend below pressures of 0.11 mbar (D. M. Hunten and J. J. Caldwell, 1978, preprint), it seems to be ruled out by the calculations.     

The eruptive evolution of the Galilean satellites: Implications for the ancient magnetic field of Jupiter

The hypothesis considering the Jupiter-Sun system as a limiting case of a close binary star implies the initial relative ice abundances in all the Galilean satellites to be essentially equal. The satellites move in the Jovian magnetosphere; thus the unipolar current flowing through their bodies subjected their ices to volumetric electrolysis. Explosions of the electrolysis products resulted in a loss of ices. While Callisto did not explode at all, Ganymede exploded once, Europa twice, and Io two or three times. An analysis of the magnetic field changes needed to create the modern ice abundances in the satellite shows:

1. the initial field of Jupiter was ～10² times stronger when compared with the present-day field, and
2. the field had to decrease exponentially with τ_2| ≈ (0.6?1), which means its relic nature.

     

Population pressure and land use change: from tree crops to sawah in Northwestern Kalimantan,Indonesia

The author describes the role of population pressure in the current transition from tree crops to wet rice cultivation (sawah) in Northwestern Kalimantan, Indonesia. He contends that this change is occurring "because the senescent trees no longer produce enough commodities to exchange for rice to feed an enlarged population." The historical background is described, policies of the Indonesian government are discussed, and data are presented on the rate of population growth     

Systematic reductions of 19th century planetary observations

Systematic reductions of nineteenth century observations to the system of the FK4 are discussed. Reductions made on a nightly basis are described and compared with the results obtained through the use of conventional tables. The series of observations made at the Paris Observatory from 1837 to 1881 was used to compare the two methods, and a combined system of 24 000 FK4, FK4 Sup and AGK 3R positions and proper motions provided the reference stars. The results show that for Uranus the mean error of a single observation in right ascension is ±1..33 when tables are used for the reductions, and ±1.12 when nightly reductions are made, while in declination the corresponding mean errors are ±0.88 and ±0.80. The observations of Neptune show an even greater difference between the two methods; the mean errors for the tabular and nightly reductions are ±1.57 and ±1.09 in right ascension and ±0.88 and ±0.75 in declination. Secular rates in the (0–C)'s of Uranus of –0.029/year in right ascension and ±0.030/year in declination are present when the observations are reduced with tables. These rates are reduced to –0.007/year and +0.015/year, respectively, when nightly reductions are made.Presented at the Symposium Star Catalogues, Positional Astronomy and Celestial Mechanics, held in honor of Paul Herget at the U.S. Naval Observatory, Washington, November 30, 1978.     

The temperature of interstellar iron grains

In order for catalytic reactions to occur in interstellar dust clouds, it is necessary for the temperature of the grains to be about an order of magnitude hotter than usually calculated for grains of dielectric materials. However, transition metal (e.g., iron) grains should be fairly abundant, and because they absorb strongly in the visible and ultraviolet regions of the interstellar radiation field, they have equilibrium temperatures 133 K. This was determined by a new method which utilizes Semi-Empirical free electron theory parameters in the infrared, coupled with a reiteration scheme which takes into consideration the change of conductivity of the iron with temperature.     

A polarimetric investigation of the eclipsing binary XY ursae majoris

On three nights in February 1976 we carried out polarimetric measurements, in V, of the short periodic eclipsing binary XY UMa, covering a complete cycle. The results are as follows:

1. Within all phase intervals the linear polarization does not exceed 0.1%.
2. In the phase range 0 ^p .95–1 ^p .35 the scatter of the Stokes parametersQ andU is about twice that within the phase interval 0 ^p .35–0 ^p .95.
3. A periodogram analysis of these data revealed a period of 21000 s, which is equal to half the orbital periodP _o=0^d.47899 within 1.5%.

From these we derive the conclusions that no circumstellar envelope can be made responsible for the observed long-term changes of the light curve and system brightness, supporting the earlier spectroscopic finding. The different scatter of the Stokes parameters at different phase intervals and theP _o/2 periodicity are in favor of the star spot model for XY UMa proposed by one of the authors (E. G.).