The Evolution of Helium Star Plus Carbon-Oxygen White Dwarf Binary Systems and Implications for Diverse Stellar Transients and Hypervelocity Stars

Helium accretion induced explosions in CO white dwarfs (WDs) are considered promising candidates for a number of observed types of stellar transients, including supernovae (SNe) of Type Ia and Type Iax. However, a clear favorite outcome has not yet emerged. We explore the conditions of helium ignition in the white dwarf and the final fates of helium star-WD binaries as function of their initial orbital periods and component masses. We compute 274 model binary systems with the Binary Evolution Code (BEC), where both components are fully resolved. Stellar and orbital evolution is computed simultaneously, including mass and angular momentum transfer, tides, and gravitational wave emission, as well as differential rotation and internal hydrodynamic and magnetic angular momentum transport. We find that helium detonations are expected only in systems with the shortest initial orbital periods, and for initially massive white dwarfs (MWD > 1.0 MSun ) and lower mass donors (Mdonor < 0.8 MSun), with accumulated helium layers mostly exceeding 0.1 MSun. Upon detonation, these systems would release the donor as a hypervelocity pre-WD runaway star, for which we predict the expected range of kinematic and stellar properties. Systems with more massive donors or initial periods exceeding 1.5 h will likely undergo helium deflagrations after accumulating 0.1 - 0.001 MSun of helium. Helium ignition in the white dwarf is avoided in systems with helium donor stars below - 0.6 MSun, and lead to three distinctly different groups of double white dwarf systems. The size of the parameter space open to helium detonation corresponds to only about 3 % of the galactic SN Ia rate, and to 10 % of the SN Iax rate, while the predicted large amounts of helium (>0.1 MSun) in progenitors cannot easily be reconciled with observations of archetypical SN Ia. ...Comment: Accepted for publication in A&A, 28 pages, 16 figures, 6 table

Problem Assessment for China

One of the main purposes of the Food and Agriculture Program at IIASA is to build a series of national agricultural models, covering the relevant parts of the world's food production and consumptions. China, having 22% of the world's population and producing 13,22% of the agricultural production, cannot be neglected in this context. As the first step of investigations, a problem assessment has been prepared. One should stress that the agricultural development in China as discussed in this paper, can be drastically changed in the nearest future. According to the latest information, new agricultural price policies have been introduced, which may have decisive effects. The basic problem, however, of, whether China will be able to feed her population, is a more complex one and depends on a series of different factors. This is an attempt to give a short summary of them

Velocity limits in the thermonuclear supernova ejection scenario for hypervelocity stars and the origin of US 708

Hypervelocity stars (HVS) are a class of stars moving at high enough velocities to be gravitationally unbound from the Galaxy. Ejection from a close binary system in which one of the components undergoes a thermonuclear supernova (SN) has emerged as a promising candidate production mechanism for the least massive specimens of this class. This study presents a thorough theoretical analysis of candidate progenitor systems of thermonuclear SNe in the single degenerate helium donor scenario in the relevant parameter space leading to the ejection of HVS. The primary goal is investigation of the, previously unclear, characteristics of the velocity spectra of the ejected component. Presented are the results of 390 binary model sequences computed with the MESA framework, investigating the evolution of supernova progenitors composed of a helium-rich hot subdwarf and a accreting white dwarf. Results are then correlated with an idealized kinematic analysis of the observed object US 708. It is seen that the ejection velocity spectra reach a maximum in the range $0.19~M_\odot < M_{HVS} < 0.25~M_\odot$. Depending on the local Galactic potential, all donors below $0.4~\text{M}_\odot$ are expected to become HVS. This channel is able to account for runaway velocities up to $\sim1150~\text{km s}^{-1}$ with a Chandrasekhar mass accretor, exceeding $1200~\text{km s}^{-1}$ if super-Chandrasekhar mass detonations are taken into account. It is found that the previously assumed mass of $0.3~M_\odot$ for US 708, combined with more recently obtained proper motions, favor a sub-Chandrasekhar mass explosion with a terminal WD mass between $1.1~M_\odot$ and $1.2~M_\odot$. The presence of clear ejection velocity maxima provides constraints on the terminal state of a supernova progenitor. It is possible to discern certain types of explosion mechanisms from the inferred ejection velocities alone.Comment: 17 pages, 14 figures, accepted for publication by A&A, replacement due to typo in Fig. 1

Helium Accreting White Dwarfs as Progenitors of Explosive Stellar Transients

Accretion induced transients, which are thought to include the important, yet little understood, supernovae (SNe) of the Type Ia, are among the most luminous events observable in the cosmos. However, SNe Ia are only one of a number of distinct classes of transients that may be the result of accretion induced ignition of white dwarfs stars (WD). Other types include the somewhat recently identified SNe Iax, theoretically predicted SNe .Ia, classical novae or fast, faint, calcium-rich SNe. Theoretical predictions allow for the possibility of producing thermonuclear detonations at masses less than the Chandrasekhar mass in systems with low accretion rates, but the proposed mechanism (the double-detonation mechanism) sensitively depends on the conditions at the point of nuclear ignition, and thus on the evolution of the WD during the accretion phase, as well as physical mechanisms included in the calculations. This work discusses the viability of He-accreting WDs as candidate progenitors of SNe Ia and Iax and other transients from a number of different perspectives, presenting results of numerical simulations, focussing on the effects of previously neglected mechanisms, such as angular momentum diffusion through magnetic dynamo action (Tayler-Spruit mechanism) and the time variability of the mass transfer rate expected in physical binary systems. The constraints placed on progenitor systems by the evolutionary behavior of the non-degenerate companion is remarked upon, as is the expected evolutionary properties of the progenitor systems. It is shown that the physics included in the numerical framework have a significant impact on the obtainable predictions, that the double detonation scenario is statistically unable to account for a large majority of observed SNe Ia, but might, depending on the included physics, account for some of the observed peculiar transients. It is further shown, that inclusion of magnetic dynamos increase the amount of helium necessary to induce detonations, which reduces the viability of these systems as possible progenitors of SNe Ia and Iax, but suggests a possible link to faint, fast, Ca-rich SNe

On pressure robustness and independent determination of displacement and pressure in incompressible linear elasticity

We investigate the possibility to determine the divergence-free displacement $\mathbf{u}$ \emph{independently} from the pressure reaction $p$ for a class of boundary value problems in incompressible linear elasticity. If not possible, we investigate if it is possible to determine it \emph{pressure robustly}, i.e. pollution free from the pressure reaction. For convex domains there is but one variational boundary value problem among the investigated that allows the independent determination. It is the one with essential no-penetration conditions combined with homogeneous tangential traction conditions. Further, in most but not all investigated cases, the weakly divergence-free displacement can be computed pressure robustly provided the total body force is decomposed into its direct sum of divergence- and rotation-free components using a Helmholtz decomposition. The elasticity problem is solved using these components as separate right-hand sides. The total solution is obtained using the superposition principle. We employ a $(\mathbf{u},p)$ higher-order finite element formulation with discontinuous pressure elements. It is \emph{inf-sup} stable for polynomial degree $p\ge 2$ but not pressure robust by itself. We propose a three step procedure to solve the elasticity problem preceded by the Helmholtz decomposition of the total body force. The extra cost for the three-step procedure is essentially the cost for the Helmholtz decomposition of the assembled total body force, and the small cost of solving the elasticity problem with one extra right-hand side. The results are corroborated by theoretical derivations as well as numerical results.Comment: 31 pages, 31 references and 48 figure

Vertrauen eine "Conditio sine qua non" für Kooperationen?

Vertrauen hat Konjunktur. Seit etlichen Jahren ist es en vogue, Vertrauen einzufordern. – Politiker fordern das "Vertrauen" der Wähler und konstatieren konsterniert, dass sie das Vertrauen der Wähler verloren haben (Der Spiegel Nr. 26; 2004: 24, Müntefering: 'Uns ist das Vertrauen der Menschen verloren gegangen .....', um nur ein Beispiel zu nennen.). – Medien fordern "Vertrauen" in ihre Berichterstattung und berichtigen fehlerhafte Berichterstattung in den seltensten Fällen. – Unternehmen fordern von ihren Mitarbeitern "Vertrauen" in das Unternehmen und die Unternehmenspolitik der Geschäftsführung besonders nachdrücklich, wenn Umstrukturierungen, Lohnverzicht und Entlassungen geplant sind. – Teammitglieder sollen auch in virtuellen Teams einander vertrauen, – etc., etc., etc. .... Die Aufstellung lässt sich beliebig fortsetzen. Vertrauen ist offensichtlich etwas Wichtiges und Notwendiges, – zugleich scheint es etwas äußerst schwer Fassbares zu sein. In Politik, Medien und Unternehmen erlebt "Vertrauen" eine bemerkenswerte Renaissance, ebenso in Ökonomie und Soziologie. Nur die Psychologen betrachten die Aufgeregtheiten etwas verwundert. Die "Vertrauenskonzepte", mit denen die aktuellen Diskussionen geführt werden, stehen vorgeblich im Zentrum interdisziplinärer Forschung, die sich vor allem dadurch auszeichnet, dass jede Sozialwissenschaft (die Ökonomie wird in diesem Artikel zu den Sozialwissenschaften gezählt) ihre eigenen, sektoral zugeschnittenen Vertrauenskonzepte verfolgt und favorisiert. --