Numerical simulation of the weak turbulence excited by a beam of electrons in the interplanetary plasma

The electron distribution functions measured at 1 AU in an electron stream passing the ISEE-3 spacecraft (Lin et al., 1981) are used as input data to a programme which simulates in a one-dimensional model the interaction between fast electrons and plasma waves (quasi-linear relaxation) together with the plasma wave scattering off the background ions. While the computed spectral energy density of the plasma waves excited in resonance with the streaming of electrons is below Zakharov's threshold of strong turbulence, it is sufficiently high to undergo a fast induced scattering off the background ions. The resulting spectrum is concentrated around the wave vector k = 0. Some simple analytic considerations show that this stage leads necessarily to the crossing of Zakharov's threshold and therefore this indirect excitation of strong turbulence seems to play an essential role in understanding the Langmuir turbulence generated by the motion of fast electron beams in the interplanetary plasma.     

Effect of alpha-damage annealing on apatite (U–Th)/He thermochronology

Recent data on He diffusion challenge the temperature sensitivity of apatite (U–Th)/He thermochronology: the damage induced by recoil of U and Th decay series during emission of α particles (α-recoil damage) has been proposed to modify He-diffusion properties through time. However, we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly-cooled or reheated basement rocks. To test this hypothesis, we developed a quantitative model including an explicit treatment of α-recoil damage, annealing, and their effect on He-diffusion kinetics, and calibrate it against literature data. Our model is based on two hypotheses: (1) helium is in equilibrium between an apatite crystal and its defects and (2) alpha-recoil damage annealing can be described analogously to fission-track annealing. This model has been embedded into a Monte Carlo simulation of helium production/ejection/diffusion and applied to data from the French Massif Central; a complex slowly-cooled terrain with burial reheating, where the thermal history has been constrained by previous fission-track (FT) data including FT length distributions. (U–Th)/He ages are close to the FT ages from the same samples and are generally reproducible among replicates, but some samples present He-age dispersion that is not correlated with crystal size. Our model reproduces the Massif Central data very well except for three samples where He ages are older than corresponding FT ages. We show that annealing of irradiation damage has an important impact on retentivity of helium and that the He content, [He] is only a rough approximation of the damage level. In particular our results show that independence of He ages on crystal sizes, in case of reheated samples, is a clear indication of the higher He retentivity induced by α-recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of (U–Th)/He ages in such a case. More generally a correlation with the crystal size can bring information on the thermal path only if the age of defects, well represented by the fission-track age, is available, due to the dependence of the partial retention zone on damages. Conversely, in case of rapid cooling or for samples having low U and Th contents, damage effects can be ignored without significant effects on He ages.     

Nitrogen and helium pollution in H II galaxies and AGNs

The large nitrogen abundance that is inferred in both narrow and (at high red-shift) broad-line regions of AGNs, if real, could be due to global effects of galactic chemical evolution or to local pollution of the ionized region by winds from massive stars. In the latter case, one might expect to find an associated excess of helium, similar to (but larger than) what has been found in some H II galaxies showing broad spectral features due to embedded Wolf-Rayet stars. However, no clear sign of any excess of helium is found in Koski's data on Seyfert 2's.     

东昆仑活动断裂带西大滩段断层气(Rn,CO2)的释放特征

通过在东昆仑活动断裂带西大滩段进行断层气测试,首次获取了该断裂带中Rn和CO2的释放量。在2004年开挖的2～3m深的探槽内,氡浓度可达20732Bq.m-3,氡发射率可达433mBq.m-2.s-1,远高于在地表的氡浓度505～2380Bq.m-3与氡发射率7～28.19mBq.m-2.s-1(地表氡发射率均值为14.7mBq.m-2.s-1,与世界平均值相当)。从而我们推断该断裂具有从上部第四系覆盖物到深部花岗岩之间的良好连通性。在地表CO2的析出率平均值为18.9g.m-2.d-1,与通常的背景值相当,在探槽中和距离断层1km的地方没有明显的空间变化,但是在断层北侧3km处的一个近乎直立的千枚岩小山上,CO2的析出率却很高,为421g.m-2.d-1,同时该处氡的发射率也高,达503mBq.m-2.s-1,因此,有必要在该断裂附近进行长期监测     

The chemical evolution of gas-rich dwarf galaxies

In an effort to understand the evolution of N, O and He abundances in gas-rich dwarf galaxies, we investigate the dispersion and mixing of supernova ejecta in relation to H  ii region evolution and develop a numerical model of chemical evolution based on a double-bursting mode of star formation (with an interval of the order of 3×10⁷ yr between bursts of a pair) which has been designed to account for the existence of significant scatter in the N/O–O/H relation.
The dependence of the abundances on gas fraction is explored on the basis of this and similar models, in combination with various hypotheses concerning inflow and selective and non-selective outflow. The gas fractions are uncertain within wide limits for blue compact galaxies, but are more well defined for some dwarf irregulars. Selective winds do not give a good fit to N/O, while closed models and models with non-selective winds with or without inflow are all found to be viable.     

Δ Y/Δ Z from fine structure in the main sequence based on Hipparcos parallaxes

The slope Δ Y /Δ Z is a quantity of interest in relation to stellar evolution, the initial mass function and the determination of the primordial helium abundance. In this paper we estimate Δ Y /Δ Z from fine structure in the main sequence of nearby stars from Hipparcos data for stars with Z  ≤ Z⊙. We find a value of about 3, which is consistent with what has been found in extragalactic H  II regions and with stellar models for suitable upper limits to the initial masses of supernovae according to the initial mass function slope adopted.     

Mid-Cretaceous uplift and erosion on the northern margin of the Ligurian Tethys deduced from thermal history reconstruction

The paleogeography during Early Cretaceous of the northern margin of the Ligurian Tethys is poorly constrained because of deformation and erosion during Pyrenean and Alpine orogenic phases. The present-day limit between Lower Cretaceous sediments in the South–East basin, located at the northwestern margin of the Ligurian Tethys, and basement rocks is the consequence of a protracted erosion history. Lower Cretaceous sediments observed today in the basin, even close to the present-day outcropping border, are characteristic of pelagic environments. A larger extent of a Lower Cretaceous cover on the basement must then be considered. This study focuses on the western part of this margin (the Causses basin), in the South of the Massif Central (France), using several thermochronometers and geothermometers to decipher the former extent of the sedimentary cover. Apatite fission track thermochronology on basement rocks surrounding the Causses basin suggests that these rocks cooled from temperatures higher than 110°C during the mid-Cretaceous. Average fluid inclusion homogenisation temperatures between 94°C and 108°C are recorded in calcite veins from outcropping Toarcian and Aalenian shales. In the shales, Tmax values, temperature obtained by Rock–Eval pyrolysis of organic matter, are in agreement with these elevated temperatures. Different explanations for these relatively high temperatures, which cannot be explained by the present-day sedimentary serie in the basin, have been tested using a 1D thermal modelling procedure (Genex). For a 95±10-mW/m² paleoflux, thick sedimentary deposits (2.5±0.3 km) including 1.3±0.3 km of Lower Cretaceous sediments cover the South of the Massif Central; these formations have been subsequently eroded from mid-Cretaceous time onwards. This study confirms that the South of the Massif Central was a site of marine sedimentation during the Early Cretaceous where a thick sedimentary sequence was once deposited.