87 research outputs found
Processing of NaCI powders of controlled size and shape for the microstructural tailoring of aluminium foams
The plasticity size effect in replicated microcellular aluminium
Microcellular aluminium can be produced by replication in porous material with pores across a wide range of sizes but with otherwise identical structures. Compressive tests reveal a plasticity size effect, with samples showing higher strengths and higher rates of work hardening for smaller pore diameters. This size effect is shown to be dislocational, its main origin being dislocation emission during the composite stage of foam processing. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Interferometric imaging of carbon monoxide in comet C/1995 O1 (Hale-Bopp): evidence for a strong rotating jet
Observations of the CO J(1-0) 115 GHz and J(2-1) 230 GHz lines in comet
C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure
interferometer on 11 March, 1997. The observations were conducted in both
single-dish (ON-OFF) and interferometric modes with 0.13 km s-1 spectral
resolution. Images of CO emission with 1.7 to 3" angular resolution were
obtained. The ON-OFF and interferometric spectra show a velocity shift with
sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35
h. The peak position of the CO images moves perpendicularly to the spin axis
direction in the plane of the sky. This suggests the presence of a CO jet,
which is active night and day at about the same extent, and is spiralling with
nucleus rotation. The high quality of the data allows us to constrain the
characteristics of this CO jet. We have developed a 3-D model to interpret the
temporal evolution of CO spectra and maps. The CO coma is represented as the
combination of an isotropic distribution and a spiralling gas jet, both of
nucleus origin. Spectra and visibilities (the direct output of interferometric
data) analysis shows that the CO jet comprises ~40% the total CO production and
is located at a latitude ~20 degrees North on the nucleus surface. Our
inability to reproduce all observational characteristics shows that the real
structure of the CO coma is more complex than assumed, especially in the first
thousand kilometres from the nucleus. The presence of another moving CO
structure, faint but compact and possibly created by an outburst, is
identified.Comment: 20 pages, 26 figures. Accepted for publication in Astronomy &
Astrophysic
Detection of Cold Atomic Clouds in the Magellanic Bridge
We report a detection of cold atomic hydrogen in the Magellanic Bridge using
21-cm absorption spectroscopy toward the radio source B0312-770. With a column
density of N_HI=1.2E20 cm^-2, a maximum absorption optical depth of tau=0.10
and a maximum 21-cm emission brightness temperature of 1.4 K, this line of
sight yields a spin temperature, T_s, between 20 K and 40 K. H I 21-cm
absorption and emission spectroscopy toward 7 other low column density
sightlines on the periphery of the LMC and SMC reveal absorption toward one
additional background radio source behind the SMC with tau=0.03. The data have
typical sensitivities of sigma_tau=0.005 to 0.070 in absorption and
sigma_{T_B}=0.03 K in emission. These data demonstrate the presence of a cold
atomic phase which is probably accompanied by molecular condensations in the
tenuous interstellar medium of the Bridge region. Young OB stars observed in
the Magellanic Bridge could form "in situ" from these cold condensations rather
than migrate from regions of active star formation in the main body of the SMC.
The existence of cold condensations and star formation in the Magellanic Bridge
might be understood as a small scale version of the mechanism that produces
star formation in the tidal tails of interacting galaxies.Comment: 25 pages, uses AASTeX and psfig; Accepted for Publication in the
Astronomical Journa
Deuterated Water in Comet C/1996 B2 (Hyakutake) and its Implications for the Origin of Comets
The close approach to the Earth of comet C/1996 B2 (Hyakutake) in March 1996 allowed searches for minor volatile species outgassing from the nucleus. We report the detection of deuterated water (HDO) through its 1(sub 01)-0(sub 00) rotational transition at 464.925 GHz using the Caltech Submillimeter Observatory. We also present negative results of a sensitive research for the J(5-4) line of deuterated hydrogen cyanide (DCN) at 362.046 GHz. Simultaneous observations of two rotational lines of methanol together with HDO in the same spectrum allow us to determine the average gas temperature within the telescope beam to be 69 +/- 10 K. We are thus able to constrain the excitation conditions in the inner coma and determine reliably the HDO production rate as (1.20 +/- 0.28) x 10(exp 26)/s on March 23-24, 1996. Available IR, UV and radio measurements lead to a water production rate of (2.1 +/- 0.5) x 10(exp 29)/s at the time of our HDO observations. The resulting D/H ratio in cometary water is thus (29 +/- 10) x 10(exp -5) in good agreement with the values of (30.8(sub - 5.3, sup +3.8) (Balsiger et al. 1995) and (31.6 +/- 3.4) x 10(exp -5) (Eberhardt et al. 1995) determined in comet P/Halley from in situ ion mass spectra. The inferred 3 a upper limit for the D/H ratio in HCN is 1%. Deuterium abundance is a key parameter for studying the origin and the early evolution of the Solar System and of its individual bodies. Our HDO measurement confirms that, in cometary water, deuterium is enriched by a factor of at least 10 relative to the protosolar ratio, namely the D/H ratio in H2 in the primitive Solar Nebula which formed from the collapse of the protosolar cloud. This indicates that cometary water has preserved a major part of the high D/H ratio acquired in this protosolar cloud through ion-molecule isotopic exchanges or grain-surface reactions and was not re-equilibrated with H2 in the Solar Nebula. Scenarios of formation of comets consistent with these results are discussed
Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?
Planets less massive than about 10 MEarth are expected to have no massive
H-He atmosphere and a cometary composition (50% rocks, 50% water, by mass)
provided they formed beyond the snowline of protoplanetary disks. Due to inward
migration, such planets could be found at any distance between their formation
site and the star. If migration stops within the habitable zone, this will
produce a new kind of planets, called Ocean-Planets. Ocean-planets typically
consist in a silicate core, surrounded by a thick ice mantle, itself covered by
a 100 km deep ocean. The existence of ocean-planets raises important
astrobiological questions: Can life originate on such body, in the absence of
continent and ocean-silicate interfaces? What would be the nature of the
atmosphere and the geochemical cycles ?
In this work, we address the fate of Hot Ocean-Planets produced when
migration ends at a closer distance. In this case the liquid/gas interface can
disappear, and the hot H2O envelope is made of a supercritical fluid. Although
we do not expect these bodies to harbor life, their detection and
identification as water-rich planets would give us insight as to the abundance
of hot and, by extrapolation, cool Ocean-Planets.Comment: 47 pages, 6 Fugures, regular paper. Submitted to Icaru
Experimental investigation of pressure-drop characteristics across multi-layer porous metal structures
This study investigates the effect of airflow (in the range of 0–70 m s-1) on the pressure-drop characteristics for a novel multi-layered, nickel-based porous metal, as a function of thickness (affected by sectioning) and density (affected by compression). In addition to generating unique data for these materials, the study highlights the need for precise pinpointing of the different flow regimes (Darcy, Forchheimer and Turbulent) in order to enable accurate determination of the permeability (K) and form drag coefficient (C) defined by the Forchheimer equation and to understand the complex dependence of length-normalised pressure drop on sample thickness
The Main Belt Comets and ice in the Solar System
We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies
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