The Dergaon (H5) chondrite: Fall,classification, petrological and chemical characteristics,cosmogenic effects,and noble gas records

Abstract— A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ～10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ～340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. ²⁶Al and ²²Na activities as well as the ²²Na/²⁶Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low ⁶⁰Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ～106 cm^?2 in samples from the largest fragment to approximately (4–9) × 105 cm^?2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ～20 cm for the Dergaon meteorite.     

Tidal effects on temperature front in the Yellow Sea

Temperature front (TF) is one of the important features in the Yellow Sea, which forms in spring, thrives in summer, and fades in autumn as thermocline declines. TF intensity ⋎S _T ⋎ is defined to describe the distribution of TF. Based on the MASNUM wave-tide-circulation coupled model, temperature distribution in the Yellow Sea was simulated with and without tidal effects. Along 36°N, distribution of TF from the simulated results are compared with the observations, and a quantitative analysis is introduced to evaluate the tidal effects on the forming and maintaining processes of the TF. Tidal mixing and the circulation structure adapting to it are the main causes of the TF. Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No. 49736190).     

Infrared spectrophotometry of Comet IRAS-Araki-Alcock (1983d): a bare nucleus revealed?

Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ～ 10⁵ g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ～ 10^?5 g/cm²/sec.     

Contrasting the chemical evolution of the Milky Way and Andromeda

The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass.     

CATASTROPHIC ECO-ENVIRONMENTAL CHANGE IN THE SONGNEN PLAIN, NORTHEASTERN CHINA SINCE 1900S

Although the Songnen Plain in the northeastern China was developed relatively late in the temperate zone of the world, its eco-environment has changed greatly. This paper analyzes the changes of land cover and the rates and trends ofdesertification during the past 100 years in the Songnen Plain. According to the macroscopic analysis, we find that the eco-environment in the plain has reached to the threshold of catastrophic change since the 1950s. The Thorn Needle Catastrophic Model was used to determine and validate this conclusion. Human activities, including large-scale construction projects, such as huge dams and dikes, and excessive grazing were the primary factors contributing to regional eco-environmental catastrophe. And irrational reclamation of the wilderness also affected the eco-environmental change. The results reveal the complex human-land interactions.     

The intracluster iron distribution around 4C+55.16

We report on the metal distribution in the intracluster medium around the radio galaxy 4C+55.16     observed with the Chandra X-ray Observatory . The radial metallicity profile shows a dramatic change at 10 arcsec (∼50 kpc) in radius from half solar to twice solar at inner radii. Also found was a plume-like feature located at ∼3 arcsec to the south-west of the centre of the galaxy, which is mostly accounted for by a strong enhancement of iron L emission. The X-ray spectrum of the plume is characterized by the metal abundance pattern of Type Ia supernovae (SNeIa), i.e. large ratios of Fe to α elements, with the iron metallicity being unusually high at     solar (90 per cent error). How the plume has been formed is not entirely clear. The inhomogeneous iron distribution suggested in this cluster provides important clues to understanding the metal enrichment process of the cluster medium.