scholarly journals Metal Abundances in Hot White Dwarfs: Predictions of the Diffusion Theory

1979 ◽  
Vol 53 ◽  
pp. 61-65
Author(s):  
Gérard Vauclair ◽  
Sylvie Vauclair
Keyword(s):  
White Dwarfs ◽  
Diffusion Processes ◽  
Diffusion Theory ◽  
Short Note ◽  
Theoretical Predictions ◽  
Metal Abundances ◽  
Radiative Acceleration

In the course of an exploration of the diffusion processes in white dwarfs, the role of the radiative acceleration has been investigated in detail and found to be quite important in hot white dwarfs (Vauclair, Vauclair and Greenstein, 1979; hereafter referred to as V2G). In view of the increasing interest accorded to hot white dwarfs which become accessible to observations in the ultraviolet owing to satellites like IUE, it appeared quite relevant to obtain theoretical predictions of the effect of the diffusion of metals in hot white dwarfs. This short note is a summary of the results presented in more detail elsewhere (V2G).

scholarly journals Diffusion and Metal Abundances in Hot White Dwarfs Gerard Vauclair

1989 ◽  
Vol 114 ◽  
pp. 176-187 ◽  
Author(s):  
Gérard Vauclair
Keyword(s):  
Time Scales ◽  
White Dwarf ◽  
White Dwarfs ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
Metal Abundances ◽  
Cool White Dwarfs ◽  
First Time ◽  
Radiative Acceleration

While the efficiency of gravitational settling to produce chemically pure atmospheres in white dwarf stars was outlined for the first time 30 years ago (Schatzman 1958), the competing role of the radiation flux in the hot white dwarfs was considered only 10 years ago (Fontaine and Michaud 1979; Vauclair, Vauclair and Greenstein 1979). At that time, there was more motivation to understand how metals could reappear in the long lived cool non DA white dwarfs, where diffusion time scales are shorter by orders of magnitude than evolutionary time scales. Various processes were invoked to help restore some metal content in the white dwarf atmospheres: convection mixing and dredge up, accretion of interstellar matter. In cool white dwarfs, the radiative acceleration is negligeable in the diffusion process; this is not the case at the hot end of the sequence where radiation may balance gravity. The short lived hot white dwarfs just started to become exciting with the contemporary discoveries that i) some show metallic lines in their spectra, both hydrogen rich and hydrogen poor; ii) some of these are pulsating. In the following years, the number of hot white dwarfs revealing trace abundance of metals has increased, mainly owing to IUE observations.

scholarly journals Metal Abundances in Cool White Dwarfs and the Diffusion Theory

1979 ◽  
Vol 53 ◽  
pp. 165-178
Author(s):  
Gérard Vauclair
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Convection Zone ◽  
Diffusion Theory ◽  
Short Review ◽  
Convective Mixing ◽  
Substantial Progress ◽  
Metal Abundances ◽  
Cool White Dwarfs ◽  
Made In

In this theoretical review about cool white dwarfs, I will restrict myself to the problem of the metallic content in white dwarf outer layers. The first section will be a short review of what we know about the metal abundances. The hottest presently known white dwarf showing metal in its spectrum is the DB GD 40 (Te = 15 000 K). This temperature will be considered here as the hot boundary of the “cool” white dwarfs. Many efforts have been recently devoted to the understanding of these metal abundances. Section 2 will be a summary of recent calculations of diffusion time scales in both hydrogen and helium white dwarfs. It will be seen that diffusion is so efficient in white dwarf conditions that the convection zone which develops in the envelope as the effective temperature decreases along the cooling sequence is never deep enough to bring back to the surface the metals which had previously diffused downwards. A discussion of the carbon white dwarfs, also called λ 4670 stars, will be presented in section 3. Recent calculations show that the convective mixing between a helium envelope and a carbon core would produce λ 4670 composition for only very special conditions and for this reason we believe that this is an improbable explanation for this type of white dwarfs. We clearly need another physical mechanism to compete with diffusion and to maintain an observable amount of metals in some cool white dwarf atmospheres. We discuss in section 4 the competition between diffusion and accretion. This seems a very promising mechanism in spite of the fact that considerable improvements are still needed in the theory of accretion. Substantial progress has to be made in this direction. A few problems related to this model are invoked in the conclusion.

Effect of cluster-gradient architecture of nanostructured topocomposites on features of tribointeraction with heterophased material

2020 ◽  
pp. 130-135
Author(s):  
D.N. Korotaev ◽  
K.N. Poleshchenko ◽  
E.N. Eremin ◽  
E.E. Tarasov
Keyword(s):  
Wear Resistance ◽  
Titanium Alloy ◽  
Energy Dissipation ◽  
Phase Structure ◽  
Diffusion Processes ◽  
Structural Phase ◽  
High Wear Resistance ◽  
Wear Characteristics ◽  
And Diffusion

The wear resistance and wear characteristics of cluster-gradient architecture (CGA) nanostructured topocomposites are studied. The specifics of tribocontact interaction under microcutting conditions is considered. The reasons for retention of high wear resistance of this class of nanostructured topocomposites are studied. The mechanisms of energy dissipation from the tribocontact zone, due to the nanogeometry and the structural-phase structure of CGA topocomposites are analyzed. The role of triboactivated deformation and diffusion processes in providing increased wear resistance of carbide-based topocomposites is shown. They are tested under the conditions of blade processing of heat-resistant titanium alloy.

An Experimental Investigation of Auditors' Liability: Implications for Social Welfare and Exploration of Deviations from Theoretical Predictions

2000 ◽  
Vol 75 (4) ◽  
pp. 429-451 ◽  
Author(s):  
Ronald R. King ◽  
Rachel Schwartz
Keyword(s):  
Experimental Investigation ◽  
Social Welfare ◽  
Strict Liability ◽  
Liability Rule ◽  
Optimal Response ◽  
Legal Regimes ◽  
Theoretical Predictions ◽  
Best Responses

This paper reports the results of an experiment designed to investigate how legal regimes affect social welfare. We investigate four legal regimes, each consisting of a liability rule (strict or negligence) and a damage measure (out-of-pocket or independent-of-investment). The results of the experiment are for the most part consistent with the qualitative predictions of Schwartz's (1997) model; however, subjects' actual choices deviate from the point predictions of the model. We explore whether these deviations arise because: (1) subjects form faulty anticipations of their counterparts' actions and/or (2) subjects do not choose the optimal responses given their anticipations. We find that subjects behave differently under the four regimes in terms of anticipation errors and departures from best responses. For example, subjects playing the role of auditors anticipate investments most accurately under the regime with strict liability combined with out-of-pocket damages, but are least likely to choose the optimal response given their anticipations. This finding implies that noneconomic factors likely play a role in determining subjects' choices.

scholarly journals A Stochastic Kinetic Type Reactions Model for COVID-19

Mathematics ◽  
2021 ◽  
Vol 9 (11) ◽  
pp. 1221
Author(s):  
Giorgio Sonnino ◽  
Fernando Mora ◽  
Pasquale Nardone
Keyword(s):  
Real Data ◽  
Stochastic Noise ◽  
Mathematical Basis ◽  
Sis Model ◽  
Theoretical Predictions ◽  
Coronavirus Infection ◽  
Massive Distribution ◽  
Second Wave ◽  
Stochastic Sis Model

We propose two stochastic models for the Coronavirus pandemic. The statistical properties of the models, in particular the correlation functions and the probability density functions, were duly computed. Our models take into account the adoption of lockdown measures as well as the crucial role of hospitals and health care institutes. To accomplish this work we adopt a kinetic-type reaction approach where the modelling of the lockdown measures is obtained by introducing a new mathematical basis and the intensity of the stochastic noise is derived by statistical mechanics. We analysed two scenarios: the stochastic SIS-model (Susceptible ⇒ Infectious ⇒ Susceptible) and the stochastic SIS-model integrated with the action of the hospitals; both models take into account the lockdown measures. We show that, for the case of the stochastic SIS-model, once the lockdown measures are removed, the Coronavirus infection will start growing again. However, the combined contributions of lockdown measures with the action of hospitals and health institutes is able to contain and even to dampen the spread of the SARS-CoV-2 epidemic. This result may be used during a period of time when the massive distribution of vaccines in a given population is not yet feasible. We analysed data for USA and France. In the case of USA, we analysed the following situations: USA is subjected to the first wave of infection by Coronavirus and USA is in the second wave of SARS-CoV-2 infection. The agreement between theoretical predictions and real data confirms the validity of our approach.

scholarly journals The Possible Role of Radiative Acceleration in Supporting Extended Atmospheres in Be Stars

1976 ◽  
Vol 70 ◽  
pp. 377-382 ◽  
Author(s):  
R. L. Kurucz ◽  
R. E. Schild
Keyword(s):  
Effective Temperature ◽  
Detailed Calculation ◽  
Be Stars ◽  
Be Star ◽  
Hα Emission ◽  
Two Peaks ◽  
Radiative Acceleration ◽  
Emission Strength ◽  
Peaked Function

A detailed calculation of the radiative acceleration in B-type stars shows it to be a double-peaked function of effective temperature at small optical depths. The two peaks are shown to coincide approximately with peaks in the distribution of mean Hα emission strength as a function of B - V color in Be stars. These facts suggest that radiation may play an important role in the support of the Be star extended atmosphere.

On the Role of Diffusion in the Oxidation of Fe, Ni, Co and the Fe-Sn Alloy by Calcium and Sodium Sulfates

2006 ◽  
Vol 258-260 ◽  
pp. 63-67
Author(s):  
V.M. Chumarev ◽  
V.P. Maryevich ◽  
V.A. Shashmurin
Keyword(s):  
Co Oxidation ◽  
Chemical Reaction ◽  
Diffusion Processes ◽  
Transport Processes ◽  
Diffusion Transport ◽  
Sodium Sulfates ◽  
Product Forms ◽  
Kinetics Of ◽  
Dominant Part

Diffusion processes play a dominant part in the macro kinetics of Fe, Ni and Co oxidation by calcium and sodium sulfates. Here, the reaction product forms a compact covering which spatially divides the reagents on the surface in the same way as in the oxidation and sulfidization of metals by oxygen and sulfur. Therefore, it is possible to assume in advance that interaction of metals with calcium and sodium sulfates will be determined not by the actual chemical reaction properly but by the diffusion transport processes.

Diffusion Mechanisms in Sige Alloys

1999 ◽  
Vol 568 ◽  
Author(s):  
Arthur F.W. Willoughby ◽  
Janet M. Bonar ◽  
Andrew D.N. Paine
Keyword(s):  
Diffusion Processes ◽  
Dopant Diffusion ◽  
Elemental Silicon ◽  
Self Diffusion ◽  
Si Substrates ◽  
Marker Layer ◽  
Diffusion Mechanisms ◽  
Silicon And Germanium ◽  
Charged Defects

ABSTRACTInterest in diffusion processes in SiGe alloys arises from their potential in HBT's, HFET's, and optoelectronics devices, where migration over distances as small as a few nanometres can be significant. Successful modelling of these processes requires a much improved understanding of the mechanisms of self- and dopant diffusion in the alloy, although recent progress has been made. It is the purpose of this review to set this in the context of diffusion processes in elemental silicon and germanium, and to identify how this can help to elucidate behaviour in the alloy. Firstly, self diffusion processes are reviewed, from general agreement that self-diffusion in germanium is dominated by neutral and acceptor vacancies, to the position in silicon which is still uncertain. Germanium diffusion in silicon, however, appears to be via both vacancy and interstitial processes, and in the bulk alloy there is evidence for a change in dominant mechanism at around 35 percent germanium. Next, a review of dopant diffusion begins with Sb, which appears to diffuse in germanium by a mechanism similar to self-diffusion, and in silicon via monovacancies also, from marker layer evidence. In SiGe, the effects of composition and strain in epitaxial layers on Si substrates are also consistent with diffusion via vacancies, but questions still remain on the role of charged defects. The use of Sb to monitor vacancy effects such as grown-in defects by low temperature MBE, are discussed. Lastly, progress in assessing the role of vacancies and interstitials in the diffusion of boron is reviewed, which is dominated by interstitials in silicon-rich alloys, but appears to change to domination by vacancies at around 40 percent germanium, although studies in pure germanium are greatly needed.

Prediction of Ball Motion in High-Speed Thrust-Loaded Ball Bearings

1976 ◽  
Vol 98 (3) ◽  
pp. 463-469 ◽  
Author(s):  
C. R. Gentle ◽  
R. J. Boness
Keyword(s):  
Computer Program ◽  
High Speed ◽  
Ball Bearing ◽  
Viscoelastic Behavior ◽  
Specific Situation ◽  
Fluid Models ◽  
Ball Bearings ◽  
Theoretical Predictions ◽  
Ball Motion

This paper describes the development of a computer program used to analyze completely the motion of a ball in a high-speed, thrust-loaded ball bearing. Particular emphasis is paid to the role of the lubricant in governing the forces and moments acting on each ball. Expressions for these forces due to the rolling and sliding of the ball are derived in the light of the latest fluid models, and estimates are also made of the cage forces applicable in this specific situation. It is found that only when lubricant viscoelastic behavior is considered do the theoretical predictions agree with existing experimental evidence.

Role of respiratory heat exchange in production of exercise-induced asthma

1979 ◽  
Vol 46 (3) ◽  
pp. 467-475 ◽  
Author(s):  
E. C. Deal ◽  
E. R. McFadden ◽  
R. H. Ingram ◽  
R. H. Strauss ◽  
J. J. Jaeger
Keyword(s):  
Heat Exchange ◽  
Thermal Load ◽  
Tracheobronchial Tree ◽  
Total Heat Flux ◽  
Random Fashion ◽  
Theoretical Predictions ◽  
Per Se ◽  
Exercise Induced ◽  
Respiratory Heat Exchange

We have hypothesized that it is the total heat flux in the tracheobronchial tree during exercise that determines the degree of postexertional obstruction in asthma, and have developed quanititative expressions that relate these two events. We tested this hypothesis by comparing the observed responses to exercise, while our subjects inhaled dry air at various temperatures ranging from subzero to 80 degrees C in a random fashion, to those that we predicted would occur based upon calculations of respiratory heat exchange. We further determined if heat could be transferred from the inspired air to the mucosa so as to offset evaporative losses from the airways. The observed responses fell as air temperature was increased from -11 to +37 degrees C and exactly matched theoretical predictions. Above 37 degrees C, the observed response exceeded predictions, indicating that it was not possible to provide sufficient heat per se in the air to offset the vaporization of water. However, when small amounts of water vapor were added to the inspirate at high temperatures, bronchospasm was virtually abolished and the response again closely matched theoretical expectations. We conclude that the magnitude of exercise-induced asthma is directly proportional to the thermal load placed on the airways and that this reaction is quantifiable in terms of respiratory heat exchange.

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