The atmosphere of Io from Pioneer 10 radio occultation measurements

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10⁴ elcm^?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na⁺ and 495 K if N₂⁺ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10³ elcm^?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N₂^? and 131 K for Na⁺. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10¹¹?10¹² cm³, corresponding to a surface pressure of less than 10^?8 to 10^?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.     

Development of a three channel photometer for UPSO, Naini Tal

The design and performance of a portable three channel photometer installed at the Uttar Pradesh State Observatory (UPSO), Naini Tal is described. The photometer is modular and the whole unit can be disassembled as individual channels such that the system can also be used as a single channel or two channel photometer. The system also has provision to monitor a guide star. The instrument was put into operation since November 1999 on the 1m Sampurnanand telescope at UPSO, Naini Tal. Since then, it is used extensively for the ‘Survey of rapidly oscillating Ap (roAp) stars in the northern sky’ from UPSO. Observational results using this new photometer in its initial phase of operation are discussed. The advantage of having continuous sky measurement is demonstrated.     

Observations of GRS 1915+105 from the USA Experiment on ARGOS

We present first results from a monitoring campaign of GRS 1915+105 undertaken with the USA X-ray timing experiment on the ARGOS satellite. A variety of behaviour has been observed, ranging from low, steady X-ray emission to rapid quasi-periodic flaring on timescales of approximately 10–120 seconds.     

Precision timing measurements of PSR J1012+5307

We present results and applications of high-precision timing measurements of the binary millisecond pulsar J1012+5307. Combining our radio timing measurements with results based on optical observations, we derive complete 3D velocity information for this system. Correcting for Doppler effects, we derive the intrinsic spin parameters of this pulsar and a characteristic age of 8.6±1.9 Gyr . Our upper limit for the orbital eccentricity of only 8×10⁻⁷ (68 per cent confidence level) is the smallest ever measured for a binary system. We demonstrate that this makes the pulsar an ideal laboratory in which to test certain aspects of alternative theories of gravitation. Our precision measurements suggest deviations from a simple pulsar spin-down timing model, which are consistent with timing noise and the extrapolation of the known behaviour of slowly rotating pulsars.     

High-Resolution Near-Infrared Observations of NGC 1068

We present near-infrared observations of NGC 1068 obtained with the SHARP camera at the ESO 3.5 m telescope, and with SHARP II attached to the COME-ON+ adaptive optics system at the ESO 3.6 m telescope. From the SHARP observations we obtain a K band image of the stellar bar with 0.″4 resolution, and an upper limit to the size of the nuclear K band source of 0.″05 (3.5 pc). The adaptive optics observations are used to determine the position of the infrared nucleus with respect to the visible continuum. The centroid of the 5000 to 9000 Å continuum is displaced 0.″23 ± 0.″10 to the east and 0.″41 ± 0.″10 to the north of the K bank peak.     

Nuclear Aspects of the Solar Neutrino Problem

The present status of the nuclear reaction rates determining the solar neutrino flux is discussed. This includes the reaction rates for the two branching ratios of the three pp-chains involving the reactions 3He(3He,2p)4He and 3He(4He,γ)7Be for the first branching, and 7Be(e−, νe)7Li and 7Be(p, γ)8B for the second branching. Mainly we will concentrate on the basic nuclear reaction p + p → D + e+ + νe of the pp-chains. This reaction rate can only be determined using the theoretical methods. The present status of the application of the relativistic field theory model of the deuteron for this reaction will be discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

CAN WE OBSERVE THE TEMPERATURE INHOMOGENEITIES AND MAGNETIC FLUX TUBES AS THE FINE STRUCTURE ON THE EXTREME LIMB OF THE SUN?

Fine structure of brightness inhomogeneities with I r.m.s. = 2.9% was discovered on the extreme limb of the Sun on the best quality white-light photographs obtained at the Soviet Stratospheric Solar Observatory. The concept is that temperature inhomogeneities are responsible for the limb structure. The real value T r.m.s. = 109K is required to explain our observations. Another possible explanation is that small-scale magnetic flux tubes with the realistic filling factor f* = 1% are the source of the limb brightness fluctuations.     

Comet P/Shoemaker-Levy 9 Collision with Jupiter: A Model of G-Site Dust Composition

Scattering models of aerosol particles at the G-impact site (18 July1994) are presented for a number of likely compositional candidates. Two differing dust particle population distribution functions are taken, along with varying aerosol cloud densities, leading to differing optical depths. A number of models including graphite, amorphous carbon, astrophysical silicate, water ice and a number of organic compounds are discussed, but no single material provides a fully satisfactory fit to the published observations. A porous silicate/graphite composite is found to provide a good fit to the spectral data. This revised version was published online in July 2006 with corrections to the Cover Date.