Introduction to the Infrared Space Observatory (ISO)

The Infrared Space Observatory (ISO) is an astronomical satellite, which will operate at infrared wavelengths (2.5 to 200 microns) for a period of at least 18 months. Imaging, spectroscopic, photometric and polarimetric observations will be obtained by four scientific instruments in the focal plane of its 60-cm diameter, cryogenically-cooled telescope. Two-thirds of ISO's observing time will be available to the astronomical community. ISO is a fully approved and funded project of the European Space Agency (ESA) with a foreseen launch date of May 1993

Une description générale de la rupture dans les sols et son implication dans la stabilité des ouvrages hydrauliques

National audienceLa rupture des sols est classiquement décrite par le critère de Mohr-Coulomb correspondant à la condition limite de plasticité. Toutefois, les matériaux tels que les sols (présentant une règle d'écoulement non associée) peuvent être sujets à des modes de rupture pour des états de contrainte situés strictement à l'intérieur du critère de Mohr-Coulomb. Cela signifie que la vérification de la stabilité d'un ouvrage en sol s'appuyant uniquement sur un critère de rupture de type Mohr-Coulomb (soit sur la plastification du sol) ne permet pas, en général, de se prévenir de l'ensemble des modes de rupture pouvant se développer au sein du matériau constitutif. Dans cette communication, nous présentons un cadre unique permettant à la fois la description de la rupture plastique (survenant sur le critère de Mohr-Coulomb), et des ruptures se développant à l'intérieur du critère de Mohr-Coulomb. Nous indiquons comment détecter à l'aide du travail du second ordre les états de contrainte à partir desquels les ruptures sont susceptibles de se développer ; et nous donnons les conditions nécessaires et suffisantes (portant à la fois sur la direction de chargement et sur le mode de contrôle du volume de sol considéré) au développement effectif de ces ruptures. Enfin, l'implication de cette description généralisée de la rupture sur l'analyse de la stabilité des ouvrages hydrauliques en terre est abordée. L'ensemble de la discussion est appuyée sur des résultats d'essais de laboratoire et de simulations numériques

Macroscopic Quantum Tunnelling in Rotating Bose-Einstein Condensates

In this paper we investigate the macroscopic quantum tunnelling and the phase coherence property of the rotating Bose-Einstein condensates in both static and dynamic cases by using the mean field theory.Comment: 10 pages, 1 figure, submitted to Phys.Rev.

ISOCAM observations of the L1551 star formation region

The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud are presented. The aim of this survey is a search for new YSO (Young Stellar Object) candidates, using two broad-band filters centred at 6.7 and 14.3 micron. Although two regions close to the centre of L1551 had to be avoided due to saturation problems, 96 sources were detected in total (76 sources at 6.7 micron and 44 sources at 14.3 micron). Using the 24 sources detected in both filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were therefore classified as YSO candidates. Using additional observations in B, V, I, J, H and K obtained from the ground, most candidates detected at these wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and three additional YSO candidates were found. Prior to this survey only three YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau). This survey reveals 15 new YSO candidates, although several of these are uncertain due to their extended nature either in the mid-IR or in the optical/near-IR observations. Two of the sources with mid-IR excess are previously known YSOs, one is a brown dwarf MHO 5 and the other is the well known T Tauri star HH30, consisting of an outflow and an optically thick disk seen edge on.Comment: 14 Pages, 8 Figure

ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster

We present the results of the first extensive mid-infrared (IR) imaging survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main molecular cloud L1688, as well as L1689N and L1689S, have been completely surveyed for point sources at 6.7 and 14.3 micron. A total of 425 sources are detected including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the rho Ophiuchi central region. The stellar luminosity function of the complete sample of Class II YSOs is derived with a good accuracy down to L= 0.03 Lsun. A modeling of this lumino- sity function, using available pre-main sequence tracks and plausible star for- mation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the IMF of the embedded cluster. We estimate that the IMF in rho Ophiuchi is well described by a two-component power law with a low- mass index of -0.35+/-0.25, a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35), and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no evidence for a low-mass cutoff down to at least 0.06 Msun.Comment: A&A Document Class -- version 5.01, 27 pages, 10 figures v2: typos added including few changes in source numberin

Phenomenological interpretation of internal erosion in granular soils from a discrete fluid-solid numerical model

Internal erosion in granular soils may involve different steps: the detachment of solid particles from the granular skeleton under the action of water seepage; the transport of the detached particles carried with the water flow in the pore space; and eventually, for some erosion processes, such as suffusion, the possible reattachment of some transported particles to the solid skeleton of the soil, acting as a filter. The first part of this paper is devoted to the description and interpretation of the first step about the particle detachment. The analysis is mainly based on direct numerical simulations performed with a fully coupled discrete element-lattice Boltzmann method. Dynamics of the solid granular phase is represented thanks to the discrete element method in which each solid particle is explicitly described, whereas dynamics of the interstitial water flow is solved with the lattice Boltzmann method. Interactions between the solid phase and the fluid phase are handled at the particle scale avoiding the introduction in the model of some phenomenological constituents to deal with fluid-solid interactions. Numerical modellings of hole erosion can be interpreted similarly to laboratory hole erosion tests where the erosion rate is linearly related to the hydraulic shear stress. Further investigations from the numerical results suggest that the erosion rate for hole erosion in granular soil, can also be interpreted as a function of the water flow power according to a power law. The latter interpretation is applied to experimental data from suffusion tests on a cohesionless soil and glass bead mixtures. Here again, if change of erosion rate due to filtration is discarded, erosion rate is correctly described by the water seepage power according to a power law. Finally, a simple phenomenological model is suggested to describe the whole suffusion process, based on the previous results, to describe the particle detachment, and completed to take also into account the transport and filtration phases. Predictions of this model are compared with experimental results from suffusion tests on glass bead mixtures

Looking at the bright side of the ρ Ophiuchi dark cloud. Far infrared spectrophotometric observations of the ρ Oph cloud with the ISO-LWS

We present far infrared (45-195 μm) spectrophotometric observations with the ISO-LWS of the active star forming ρOph main cloud (L 1688). The [CII] 158 μm and [OI] 63 μm lines were detected at each of the 33 positions observed, whereas the [OI] 145 μm line was clearly seen toward twelve. The principal observational result is that the [CII] 158 μm line fluxes exhibit a clear correlation with projected distance from the dominant stellar source in the field (HD 147889). We interpret this in terms of PDR-type emission from the surface layers of the ρOph cloud. The observed [CII] 158 μm/[OI] 63 μm flux ratios are larger than unity everywhere. A comparison of the [CII] 158 μm line emission and the FIR dust continuum fluxes yields estimates of the efficiency at which the gas in the cloud converts stellar to [CII] 158 μm photons (χCII≳0.5%). We first develop an empirical model, which provides us with a three dimensional view of the far and bright side of the dark ρOph cloud, showing that the cloud surface towards the putative energy source is concave. This model also yields quantitative estimates of the incident flux of ultraviolet radiation (G0 ~ 101 - 102) and of the degree of clumpiness/texture of the cloud surface (filling of the 80" beam ~0.2). Subsequently, we use theoretical models of PDRS to derive the particle density, n(H), and the temperature structures, for Tgas and Tdust, in the surface layers of the ρOph cloud. Tgas is relatively low, ~60 K, but higher than Tdust ( ~30 K), and densities are generally found within the interval (1-3) 104 cm-3 . These PDR models are moderately successful in explaining the LWS observations. They correctly predict the [OI] 63 μm and [CII] 158 μm line intensities and the observed absence of any molecular line emission. The models do fail, however, to reproduce the observed small [OI] 63 μm/[OI] 145 μm ratios. We examine several possible explanations, but are unable to uniquely identify (or to disentangle) the cause(s) of this discrepancy. From pressure equilibrium arguments we infer that the total mass of the ρOph main cloud (2pc2) is ~2 500 M⊙, which implies that the star formation efficiency to date is ≲4%, significantly lower than previous estimates

Dissipative Chaos in Semiconductor Superlattices

We consider the motion of ballistic electrons in a miniband of a semiconductor superlattice (SSL) under the influence of an external, time-periodic electric field. We use the semi-classical balance-equation approach which incorporates elastic and inelastic scattering (as dissipation) and the self-consistent field generated by the electron motion. The coupling of electrons in the miniband to the self-consistent field produces a cooperative nonlinear oscillatory mode which, when interacting with the oscillatory external field and the intrinsic Bloch-type oscillatory mode, can lead to complicated dynamics, including dissipative chaos. For a range of values of the dissipation parameters we determine the regions in the amplitude-frequency plane of the external field in which chaos can occur. Our results suggest that for terahertz external fields of the amplitudes achieved by present-day free electron lasers, chaos may be observable in SSLs. We clarify the nature of this novel nonlinear dynamics in the superlattice-external field system by exploring analogies to the Dicke model of an ensemble of two-level atoms coupled with a resonant cavity field and to Josephson junctions.Comment: 33 pages, 8 figure