Comptes Rendus de l'Académie des Sciences - Series IV - Physics-Astrophysics最新文献

Widely regarded as pathological variable stars – with erratic photometric and spectroscopic behavior of unknown physical origin – 20 years ago, T Tauri stars (TTSs) turned out in the last decade to be promising laboratories for observing the formation of solar systems such as ours. This is because circumstellar, presumably protoplanetary disks were found to surround a large fraction of them. While evidence for disks was primarily indirect until 1995, recent high resolution imaging confirmed earlier claims that the infrared (IR) and ultraviolet (UV) excesses seen in the spectral energy distribution (SED) of these stars were due to disk emission. The activity displayed by young stellar objects at all wavelengths is due to the interaction between the circumstellar disk and the magnetized star and to non-stationary accretion/ejection phenomena. In this review, we briefly summarize properties of these young solar-type stars and describe their circumstellar disks in some detail, focusing on current optical, infrared and millimeter high angular resolution observations that now allow us to resolve the disks.

Optical diagnostics can provide meaningful information in diversity of flows. Among various optical techniques, imaging has allowed the determination of flow structure and dynamics. Up till now, qualitative images were supplemented by quantitative single-point measurements. The development of Planar Laser-Induced Fluorescence (PLIF) offers the possibility to perform non-intrusive, instantaneous imaging of flowfields. While PLIF is extensively used, our main goal is to retrieve the fluid properties quantitatively using this method. In practical environments, the complexity of the experiments and the difficulty of data interpretation remains a constant challenge. Recent developments performed at Onera will be presented in which PLIF is used for trace species concentration measurements and for temperature mapping in turbulent flows. Comments and perspectives will also be given.

The Cavity Ringdown Laser Absorption Spectroscopy (CRLAS or CRDS) technique has acquired a enviable audience in the spectroscopy community during the past decade. Based on a high-Q optical cavity, it largely bypasses the advantages of multipass absorption cells, offering ppm range sensitivities or better, and emulates rapid developments of the experimental configurations. The basic idea consists of measuring the intracavity electromagnetic field time behavior which reflects the cavity optical properties and medium losses. This article is divided in three main parts. The first one is devoted to the description of the CRLAS technique, including: (i) a brief formalism about the principles of an empty high-Q cavity (Fabry–Perot) coupled to an incoming electromagnetic field and (ii) the absorption model allowing one to deal with absorbing species inserted inside the cavity. The second part succinctly reviews and compares some of the usual highly sensitive spectroscopy techniques and the main applications of the CRLAS technique are presented. The last part of the paper reports the recent results obtained at the laboratory concerning the NO₂ molecular species excited by a CW single mode laser source and under slit jet expansion conditions. Two energy ranges are primarily investigated, firstly the region around 800 nm in which three kinds of behaviors are identified: Doppler-limited linear absorption, Doppler-free two-photon absorption and saturation absorption. Secondly, by using radiation at 397 nm, the lowest photodissociation threshold of NO₂ is interrogated in order to address the unimolecular reaction processes.

In this paper, we present some preliminary results on a new terahertz-Time Domain Spectroscopy (THz-TDS) experiment. After a general introduction about the interest of the THz range, we particularly discuss some advantages of this technique for combustion applications and compare it with the classical linear and nonlinear optical techniques. The set-up developed in our laboratory is described and some results on NO displayed.

Infrared spectroscopy is a powerful tool for precise measurements of atmospheric trace species concentrations through the use of characteristic spectral signatures of the different molecular species and their associated vibration–rotation bands in the mid- or near-infrared. Different methods based on quantitative spectroscopy permit tropospheric or stratospheric measurements: in situ long path absorption, atmospheric absorption/emission by Fourier transform spectroscopy with high spectral resolution instruments on the ground, airborne, balloon-borne or satellite-borne.

We review several numerical approaches which have been developed during the last 10 years in order to perform more realistic radiative modelling of solar and stellar atmospheres. However, we shall restrict ourselves to the same ‘family’ of method, the ones which are derived from the so-called Accelerated Lambda-Iteration technique.

The nuclear pore complex sits as the gateway between the genomic, nuclear environment and the primarily enzymatic realm of the cytoplasm. Large channels traversing the two membranes of the nuclear envelope, the nuclear pores govern the passage of specific molecules between these two major compartments of the cell. Its ability to limit passage to specific molecules, and furthermore to pump them against a gradient in concentration, raises intriguing physical questions. This article reviews basic aspects of the structure and operation of this biochemical pump, whose thermodynamic cycle differs from that of conventional machines.

We review recent work on the physical properties of model fluid membranes in nonequilibrium situations resembling those encountered in the living cell and contrast their properties with those of the more familiar membranes at thermal equilibrium. We survey models for the effect of (i) active pumps and (ii) active fission–fusion processes encountered in intracellular trafficking on the stability and fluctuations of fluid membranes. Our purpose is twofold: to highlight the exciting nonequilibrium phenomena that arise in biological systems, and to show how some crucial features of living systems, namely dissipative energy uptake and directed motion, can fruitfully be incorporated into physical models of biological interest.