4,802 research outputs found
Jet launching and field advection in quasi-Keplerian discs
The fact that self-confined jets are observed around black holes, neutron
stars and young forming stars points to a jet launching mechanism independent
of the nature of the central object, namely the surrounding accretion disc. The
properties of Jet Emitting Discs (JEDs) are briefly reviewed. It is argued
that, within an alpha prescription for the turbulence (anomalous viscosity and
diffusivity), the steady-state problem has been solved. Conditions for
launching jets are very stringent and require a large scale magnetic field
close to equipartition with the total (gas and radiation) pressure. The
total power feeding the jets decreases with the disc thickness: fat ADAF-like
structures with cannot drive super-Alfv\'enic jets. However, there
exist also hot, optically thin JED solutions that would be observationally very
similar to ADAFs.
Finally, it is argued that variations in the large scale magnetic field
is the second parameter required to explain hysteresis cycles seen in LMXBs
(the first one would be ).Comment: 4 pages, 1 figure, proceedings of IAU 275 "Jets at all scales"
(Gustavo E. Romero, Rashid A. Sunyaev and Tomaso Belloni, eds
Application of CrAlN coatings on carbide substrates in routing of MDF
This study deals with the development of Chromium Aluminium Nitride (CrAlN) hard coatings (by varying the nitrogen content in the plasma, the target bias voltage, the working pressure and the deposition time) and their characterization by physical and mechanical techniques (XRD, nanoindentation, Youngās modulus, stress, scratch-test, composition, etc.) in order to determine the optimal deposition conditions to apply the coatings to carbide substrates. Moreover, in order to improve the adhesion of the same optimal hard coatings and as it was efficient with Diamond Like Carbon (DLC) coatings, we tried to modify the carbide inserts by chemical attacks with Murakamiās agent before machining. The coated tools (with and without Murakamiās attack) were then tested in routing of Medium Density Fibreboard (MDF). A comparison of the abrasion and shock resistance of the conventional tools with the treated ones, was made. The efficiency of Murakamiās agent was also studied. It was obvious that the coated carbide tools had greater tool life than the untreated ones. Besides, Murakamiās attack was not optimized or not a solution to improve the adhesion of nitride coatings on carbide inserts.Regional Council of Burgundy ISOROY Franc
The influence of collimation on the appearance of relativistic jets
The question of the collimation of relativistic jets is the subject of a
lively debate in the community. We numerically compute the apparent velocity
and the Doppler factor of a non homokinetic jet using different velocity
profile, to study the effect of collimation on the appearance of relativistic
jets (apparent velocity and Doppler factor). We argue that if the motion is
relativistic, the high superluminal velocities are possible only if the
geometrical collimation is smaller than the relativistic beaming angle
. In the opposite case, the apparent image will be dominated by
the part of the jet traveling directly towards the observer resulting in a
smaller apparent velocity. Furthermore, getting rid of the homokinetic
hypothesis yields a complex relation between the observing angle and the
Doppler factor, resulting in important consequences for the numerical
computation of AGN population and unification scheme model.Comment: 4 pages, 4 figures. To appear in Proceedings of IAU Symposium 275
"Jets at all Scales", 13-17 September 2010, Buenos Aires, Argentin
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