128 research outputs found
Search for surface magnetic fields in Mira stars. First detection in chi Cyg
In order to complete the knowledge of the magnetic field and of its influence
during the transition from Asymptotic Giant Branch to Planetary Nebulae stages,
we have undertaken a search for magnetic fields at the surface of Mira stars.
We used spectropolarimetric observations, collected with the Narval instrument
at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman
signature in the visible part of the spectrum. We present the first
spectropolarimetric observations of the S-type Mira star chi Cyg, performed
around its maximum light. We have detected a polarimetric signal in the Stokes
V spectra and we have established its Zeeman origin. We claim that it is likely
to be related to a weak magnetic field present at the photospheric level and in
the lower part of the stellar atmosphere. We have estimated the strength of its
longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the
variation of the magnetic field strength across the circumstellar envelope of
chi Cyg. This is the first detection of a weak magnetic field at the stellar
surface of a Mira star and we discuss its origin in the framework of shock
waves periodically propagating throughout the atmosphere of these radially
pulsating stars. At the date of our observations of chi Cyg, the shock wave
reaches its maximum intensity, and it is likely that the shock amplifies a weak
stellar magnetic field during its passage through the atmosphere. Without such
an amplification by the shock, the magnetic field strength would have been too
low to be detected. For the first time, we also report strong Stokes Q and U
signatures (linear polarization) centered onto the zero velocity (i.e., at the
shock front position). They seem to indicate that the radial direction would be
favored by the shock during its propagation throughout the atmosphere.Comment: 9 pages, 4 figures accepted by Astronomy and Astrophysics (21
November 2013
Corrosion par metal dusting d'alliages austénitiques, modélisation cinétique et mécanismes
Le metal dusting est un type de corrosion catastrophique des alliages Ă base de fer, de nickel ou de cobalt. Il se caractĂ©rise par une dĂ©gradation de ces matĂ©riaux en une fine poussiĂšre de particules mĂ©talliques et de carbone graphitique, appelĂ©e « coke », pouvant Ă©galement contenir des carbures et des oxydes. Ce phĂ©nomĂšne a lieu lorsque le mĂ©lange gazeux est sursaturĂ© en carbone (ac>>1), Ă des tempĂ©ratures comprises entre 400°C et 800°C. Cinq alliages commerciaux austĂ©nitiques (800HT, HR120, Inconel 625, Inconel 690 et Inconel 693) et deux matĂ©riaux modĂšles fabriquĂ©s par SPS (NiFeCr et NiFeCr+Cu) sont testĂ©s dans deux environnements de metal dusting Ă 570°C. Le premier test est effectuĂ© sous pression atmosphĂ©rique dans un mĂ©lange CO-H2-H2O, le second dans une atmosphĂšre CO-H2-CO2-CH4-H2O sous 21 bars de pression. La premiĂšre composition est ajustĂ©e pour obtenir une activitĂ© en carbone et une pression partielle en dioxygĂšne proches de celles de lâenvironnement sous haute pression. AprĂšs plus de 14 000 h heures dâexposition, lâalliage 625 nâest pas dĂ©gradĂ©. Il prĂ©sente une prĂ©cipitation dâaiguilles de Îłââ-Ni3Nb, le niobium migrant vers la surface suite Ă lâappauvrissement en chrome par oxydation. Le matĂ©riau NiFeCr+Cu prĂ©sente une Ă©volution microstructurale proche, le cuivre formant une couche continue Ă lâinterface mĂ©tal/oxyde. Le cuivre Ă©tant non-catalytique pour la formation de carbone, sa sĂ©grĂ©gation en surface amĂ©liore la rĂ©sistance du matĂ©riau. Lâalliage 690 prĂ©sente une carburation homogĂšne sur toute la surface qui nâĂ©volue pas dans le temps. Lâalliage 693 prĂ©sente au contraire une carburation trĂšs importante, de plus en plus profonde avec la durĂ©e dâexposition. Celle-ci sâexplique par la formation dâune couche continue dâalumine de transition, mĂ©tastable. Sa transformation en alumine α, stable, sâaccompagne dâune contraction de la maille, fissurant la couche dâoxyde. LâatmosphĂšre accĂšde alors directement Ă la surface mĂ©tallique, carburant lâalliage. La bonne tenue de cet alliage, malgrĂ© la fissuration de lâoxyde, sâexplique par sa forte teneur en chrome et par la faible cinĂ©tique de la transformation allotropique Ă 570°C. Les alliages 800HT, HR120 et NiFeCr sont corrodĂ©s par piqĂ»ration. Pour lâalliage 800HT, celle-ci est simulĂ©e en surface par un modĂšle de germination-croissance dĂ©pendant du temps dâincubation des piqĂ»res, de leur croissance et de la densitĂ© de piqĂ»res. La prise en compte du volume des piqĂ»res pour reproduire les pertes de masses enregistrĂ©es est concluante sous haute pression mais pas Ă pression atmosphĂ©rique. Cela met en exergue lâinfluence de la gĂ©omĂ©trie de lâĂ©chantillon (les Ă©chantillons testĂ©s Ă pression atmosphĂ©riques Ă©tant trĂšs attaquĂ©s sur les bords), et donc lâintĂ©rĂȘt dâĂ©tudier la piqĂ»ration. Sous pression atmosphĂ©rique, la croissance latĂ©rale des piqĂ»res se fait par oxydation des carbures tandis que la croissance en profondeur se fait par un mĂ©canisme de graphitisation accĂ©lĂ©rĂ©e, lorsque le flux de carbone est suffisamment grand devant le flux dâoxygĂšne. La graphitisation accĂ©lĂ©rĂ©e nâa lieu quâen fond de fissure du fait du faible renouvellement de lâatmosphĂšre. Les fissures se forment lors du cyclage thermique effectuĂ© toutes les 500 h pour caractĂ©riser les Ă©chantillons. Cela conduit Ă un faciĂšs de corrosion constituĂ© dâune oxydation interne fine et discontinue exposant directement Ă lâatmosphĂšre lâalliage carburĂ©, qui est alors graphitisĂ©. Il en rĂ©sulte lâapparition dâune succession dâanneaux de corrosion, un sur deux croissant en profondeur. La morphologie issue du mĂ©canisme de graphitisation favorisĂ©e est visible sur toute la circonfĂ©rence des piqĂ»res formĂ©es sous haute pression. Le mĂȘme mĂ©canisme a donc lieu, globalement cette fois, le flux de carbone Ă©tant suffisamment grand devant le flux dâoxygĂšne dĂšs lâintroduction dans le banc de corrosion. Les morphologies observĂ©es sont donc liĂ©es aux conditions expĂ©rimentales (tempĂ©rature, atmosphĂšre, dĂ©bit) et Ă la procĂ©dure de suivi (retraits)
Relevance of other parameters than carbon activity in defining the severity of a metal dusting environment
Two metal dusting experiments were carried out at 570 °C on 800HT and HR120 alloys, for more than 6000 h. The tests were designed to run at different total pressures and gas velocities but similar carbon activities and oxygen partial pressures. For a given alloy, shorter average incubation times and larger mass losses were observed at high pressure. For both tests, HR120 alloy underwent greater mass losses and exhibited a higher pit density. For nearly all samples, pit densities greatly differed between both sides of the specimens. Therefore, the carbon and oxygen activities alone are not sufficient to evaluate the aggressiveness of a metal dusting environment. Greater degradation was the result of the association of a higher gas velocity with a higher total pressure and a finer alloy grain size
Mechanism of metal dusting corrosion by pitting of a chromia-forming alloy at atmospheric pressure and low gas velocity
FeNiCr samples (800HT) were exposed at 570 °C, 1 bar to a 47.25CO-47.25H2-5.5H2O atmosphere (ac = 33) flowing at 18 Όm/s. Pitting corrosion was observed. Pits showed a flattened morphology and a constant pit diameter growth rate. Corrosion rings appeared successively at the surface during pit growth. A four-step mechanism is proposed which includes internal oxidation of carbides, graphitisation and localised enhanced graphitisation. Gas velocity and thermal cycling play key roles in pit morphology. Thermal cycling induces circular cracks. Low gas velocity induces the gas to evolve in crevices, due to local oxygen consumption
Modelling of the kinetics of pitting corrosion by metal dusting
Commercial 800HT alloy was exposed to 49.1%H2â12.8%COâ3.1%CO2â1.6CH4â33.4%H2O gas at 21bars and 570°C up to 5000h. Metal dusting attack by pitting was observed. The kinetics parameters were identified to be the incubation time, pit density and individual pit growth rate. These parameters were introduced in a nucleation-growth model to simulate the pitted surface area kinetics. This model was then extended to the volume considering several geometrical hypotheses. Considering only surface coalescence of the pits without their volume coalescence allowed to correctly reproduce the experimental mass loss kinetics. An even simpler conservative model was proposed for an easy lifetime modelling
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