The 1999 Quadrantids and the lunar Na atmosphere

Enhancements of the Na emission and temperature from the lunar atmosphere were reported during the Leonid meteor showers of 1995, 1997 and 1998. Here we report a search for similar enhancement during the 1999 Quadrantids, which have the highest mass flux of any of the major streams. No enhancements were detected. We suggest that different chemical–physical properties of the Leonid and Quadrantid streams may be responsible for the difference.     

The ejection velocity of meteoroids from cometary nuclei deduced from observations of meteor shower outbursts

The ejection velocities of meteoroids belonging to the Leonid and Perseid meteoroid streams are deduced from the observed differences between the longitude of the ascending node of the outburst meteoroids and that of the parent comet. The difference is very sensitive to the true anomaly of the ejection point, as well as the ejection velocity, and probable values for both are discussed.     

On the onset of thermal metastabilities in the solar core

For infinitesimal, homologous perturbations, stability analysis has found the solar radiative interior thermally stable. It is considered for the first time here whether stability is preserved when finite amplitude nonhomologous perturbations are present. We argue that local heated regions may develop in the solar core due to magnetic instabilities. Simple numerical estimations are derived for the timescales of the decay of these events and, when heated bubbles are generated that rise towards the surface, of their rising motion. These estimations suggest that the solar core is in a metastable state. For more detailed analysis, we developed a numerical code to solve the differential equation system. Our calculations determined the conditions of metastability and the evolution of timescales. We obtained two principal results. One of them shows that small amplitude heating events (with energy surplus Q_o < 10²⁶ ergs) contribute to subtle but long-lifetime heat waves and give the solar interior a persistently oscillating character. Interestingly, the slow decay of heat waves may make their accumulation possible and so their overlapping may contribute to the development of an intermittent, individual, local process of bubble generation, which may also be generated directly by stronger (Q_o > 10²⁶ ergs) heating events. Our second principal result is that for heated regions with ΔT/T ≥ 10⁻⁴ and radius 10⁵–10⁶cm, the generated bubbles may travel distances larger than their linear size. We point out to some possible observable consequences of the obtained results.     

Evidence for transverse spread in Leonid meteors

We report here evidence for significant transverse spread of the light production region in bright Leonid meteors. One Leonid meteor has an apparent spread in the light production region of about 600 m perpendicular to the flight path for the meteor, that transverse spread persisting for at least 0.3 s. We have also detected short-duration, jet-like features emanating from a bright Leonid meteor recorded in 1998. These jet-like features have maximum spatial dimensions up to 1.9 km. While we cannot definitively rule out instrumental artefacts as a cause for these jet-like features, they may be evidence of motion contributing to the observed spatial spread in the light production region.     

MODELING THE SPORADIC METEOROID BACKGROUND CLOUD

The orbital distributions of dust particles in interplanetary space are revised in the ESA meteoroid model to incorporate more observational data and to comply with the constraints due to the long-term particle dynamics under the planetary gravity and Poynting–Robertson effect. Infrared observations of the zodiacal cloud by the COBE Earth-bound observatory, flux measurements by the dust detectors on board Galileo and Ulysses spacecraft, and the crater size distributions on lunar rock samples retrieved by the Apollo missions are fused into a single model. Within the model, the orbital distributions are expanded into a sum of contributions due to a number of known sources, including the asteroid belt with the emphasis on the prominent families Themis, Koronis, Eos and Veritas, as well as comets on Jupiter-encountering orbits. An attempt to incorporate the meteor orbit database acquired by the Advanced Meteor Orbit Radar at Christchurch is also discussed. Work was done during D. Galligan’s stay at the University of Canterbury.     

Elemental Abundances in Leonid and Perseid Meteoroids

High dispersion photographic spectra of three Leonid and five Perseid meteors are used to derive relative abundances of nine chemical elements in the radiating meteoric vapors and in the meteoroids. Al and Ca were found to be incompletely evaporated in the main spectral component at 5000 K but completely evaporated in the second component at 10,000 K. Si lines are present in both components which enhances the reliability of determination of the Si abundance. The composition of the meteoroids was found to be more similar to comet Halley than to chondritic meteoroids. Fe, Cr, and Mn are depleted and Si, Na, and H are enhanced relative to Mg in comparison with CI chondrites.     

恒星尘埃的实验室研究--实验天体物理学

原始球粒陨石含有来自恒星的微小固体颗粒(微米级),这些尘埃的同位素组成与太阳系物质截然不同,它们是目前唯一能直接获得的恒星固体样品．已发现的恒星尘埃有金刚石、石墨、碳化硅、刚玉、尖晶石、氮化物、和硅酸盐等,它们的母体恒星包括红巨星,AGB恒星、新星和超新星．对恒星尘埃的研究,使得更深入地了解星系的化学演化历史、恒星内部的核反应和湍流机制、恒星大气中尘埃的形成、星际介质物理现象等．恒星尘埃把天体物理领域延伸到了微观世界,它有机地结合了地球化学实验技术和天体物理理论,开辟了一门崭新的天文学分支实验天体物理学．     

Meteoric Activities of the Last Millennium

We have analyzed the meteor records in the chronicles of the east Asian countries, especially the Korean records. Our results show that the seasonal variation of sporadic meteors has persisted at least for the last two millennia. We also observed the prominent showers such as the Perseids and the Leonids, which are formed by Halley-type comets. We obtained the regression rate of nodal points for the Leonids to be approximately 1.52± 0.04 days per century.     

DYNAMICAL RELATION OF METEORIDS TO COMETS AND ASTEROIDS

We tested four criteria used for discrimination between asteroidal and cometary type of orbits: Whipple criterion K, Kresak criterion Pe, Tisserand invariant T and aphelion distance Q. To estimate their reliability, all criteria were applied to classify the 2225 orbits of NEAs and 582 orbits of comets, for several epochs spanning the time interval of 40 thousands years. The Q-criterion produced the smallest number of exceptions and has shown the best stability. The biggest number of exceptions and the biggest variations are obtained for the K-criterion. We applied the Q-criterion to classify meteor orbits from the IAU Meteor Data Center and the video meteor orbits available on the Web sites. Among the sporadic radar orbits, as well as among the mean orbits of meteor streams a strong preponderance of asteroid-type orbits was observed. In case of the photographic and video meteors a weak preponderance of cometary and asteroidal orbits was found, respectively.