26Al and the partial ionization of the solar nebula

Calculation of the ionization state and consequent magnetic Reynolds number for the solar nebula shows that the presence of²⁶Al will result in strong coupling of the gas to magnetic fields. In the absence of²⁶Al,⁴⁰K will still result in substantial ionization, but the degree of magnetic coupling is much more model dependent.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.also Department of Astronomy.     

Phase coverage of Venus during the 1975 apparition: Diurnal variations in equatorial infrared brightness

Day-night contrasts in the equatorial infrared (8- to 14-ωm) flux from Venus were observed in digital images obtained during the 1975 apparition. A systematic increase in 8- to 14-ωm flux associated with the emergence of the atmosphere from daylight into evening was observed and is shown to exhibit significant day-to-day variability. The data also show that the daytime side of the morning terminator is warmer than the dark side, but this night-day asymmetry is systematically smaller than the day-night brightening seen in the Venus evening.     

Properties of the clouds of Venus,as inferred from airborne observations of its near-infrared reflectivity spectrum

We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H_2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H₂SO₄ and H₂O contained in the cloud material both equal approximately 2× 10^?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10^?4 and 10^?2.     

Global fracture patterns of a despun planet: Application to Mercury

We have determined the global fracture patterns resulting from combinations of stresses due to tidal despinning and contraction or expansion. We find that Mercury's lineament pattern is consistent with a history of despinning and contraction. According to our model, the observed tectonic pattern implies that the despinning process reached completion before the planet ceased contracting. Our model predicts a stress due to contraction which is up to 1.8 times the maximum despinning stress on Mercury. The maximum contractional stress could be as large as 4 times the maximum despinning stress if the oldest fractures on the planet are N-S thrust faults in the equatorial region.     

Coherence analysis of granular intensity

A high resolution spectrogram of the Mg b₂ line from the quiet Sun disc centre is subjected to a coherence analysis. We find that the coherence between intensity fluctuations in the continuum and the wings of the line breaks down at a distance = 0.35 Å from line centre. From this and the r.m.s. intensity contrast as a function of we are led to the following simple model of temperature fluctuation T in the solar photosphere: A lower part (below 50 km, or ₅₀₀₀ > 0.25) with strongly inward increasing T and an upper part (above 50 km) with constant T = 75 K. The two parts are supposed to fluctuate incoherently.Mitteilungen aus dem Kiepenheuer Institut Nr. 166.     

Asteroid rotation: III. 1978 Observations

Photoelectric observations of 32 asteroids observed from Table Mountain Observatory during the second half of 1978 are reported. Rotation periods were obtained for most objects. Absolute magnitudes and phase functions were not determined for any of these asteroids. The geometric mean rotation period of the 32 asteroids observed is 14.2 ± 1.6 hr, as compared to 9.38 ± 0.35 hr for 182 asteroids analyzed in Paper I (A. W. Harris and J. A. Burns, 1979, Icarus 40, 115–144). We attribute this difference to an observational selection effect which favors detection of fast rotation, as discussed in Paper I. If this is true, then the present sample contains the reverse bias, since it is complete in that a period (in some cases very approximate) was obtained for each object observed, but fast rotators are underrepresented due to prior discovery of their rotation properties.     

A high-sensitivity search for extraterrestrial intelligence at γ18 cm

We have conducted a targeted high-sensitivity search for narrow-band signals near γ18 cm using the 91-m radiotelescope of the National Radio Astronomy Observatory. The search included 201 nearby solar-type stars and achieved a frequency resolution of 5.5 Hz over a 1.4-MHz bandwidth. This high spectral Mark I VLBI recording terminal in conjunction with the CDC 7600 computational facility at the NASA-Ames Research Center. This is the first high-resolution search for narrow-band signals in this wavelength regime. To date it is the most sensitive search per unit observing time of any search strategy which does not postulate a unique magic frequency. Our data show no evidence for narrow-band signals due to extraterrestrial intelligence at a 12-σ upper limit on signal strength of 1.1 × 10^?23 W m^?2.     

Extragalactic sources with very asymmetric radio structure: VLA and MERLIN observations

As part of our study to understand the nature of extragalactic radio sources which are very asymmetric in the surface brightness of the two lobes, often with radio emission on only one side of the nucleus, we have observed a large number of them with high angular resolution and good surface brightness sensitivity at radio frequencies. In this paper we present VLA and MERLIN observations of 15 such sources. We discuss their observed structures and spectra, and possible explanations for their morphologies. We report evidence of a possible correlation between the hot-spot brightness ratio and the degree of core prominence, used as a Statistical measure of source orientation, suggesting that relativistic beaming of the hot-spot emission does play a significant role in the observed brightness asymmetry. To explain the apparently one-sided sources within the relativistic beaming framework, the velocities required are in the range of 0.2 to 0.8c. We discuss the possibility that the lobe which is seen to the south of the jet in 3C273 is the counter-lobe seen in projection. We also draw-attention to a number of one-sided sources with very weak cores, and discuss their possible nature.     

The influence of interstellar medium on subsonic stellar motions

It is not a trivial problem to imagine how a spherical high-pressure balloon with supersonic gas jets leaving from pores densely distributed on its surface can be influenced by an ambient gas flow. The relative motion of such a balloon can be controlled by a corresponding rearrangement of the gas outflow into an aspherical configuration. A similar problem is connected with stars driving a supersonic stellar wind and moving relative to the interstellar medium. As we shall show, the adapted circumstellar flow leads to an upwind-downwind pressure asymmetry balancing the momentum loss that is braking such stars. The opposite process — i.e., acceleration — may occur if luminous stars are closely associated and their wind systems interfere with each other. This should lead to a mutual repulsion.