Convective–reactive nucleosynthesis of K, Sc, Cl and p-process isotopes in O–C shell mergers

© 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. We address the deficiency of odd-Z elements P, Cl, K and Sc in Galactic chemical evolution models through an investigation of the nucleosynthesis of interacting convective O and C shells in massive stars. 3D hydrodynamic simulations of O-shell convection with moderate C-ingestion rates show no dramatic deviation from spherical symmetry. We derive a spherically averaged diffusion coefficient for 1D nucleosynthesis simulations, which show that such convective-reactive ingestion events can be a production site for P, Cl, K and Sc. An entrainment rate of 10-3M⊙s-1features overproduction factors OPs≈ 7. Full O-C shell mergers in our 1D stellar evolution massive star models have overproduction factors OPm> 1 dex but for such cases 3D hydrodynamic simulations suggest deviations from spherical symmetry. γ - process species can be produced with overproduction factors of OPm> 1 dex, for example, for130, 132Ba. Using the uncertain prediction of the 15M⊙, Z = 0.02 massive star model (OPm≈ 15) as representative for merger or entrainment convective-reactive events involving O- and C-burning shells, and assume that such events occur in more than 50 per cent of all stars, our chemical evolution models reproduce the observed Galactic trends of the odd-Z elements

The Role of Organizational Capacity in Student-Athlete Development

In-depth interviews were conducted with the life skills coordinators of 9 of 21 institutions identified as being “dedicated” to service (Andrassy & Bruening, 2011). As a result of service being one portion of CHAMPS/Life Skills programming, we expanded our investigation to include all aspects of this student development program. In particular, we focused our inquiry on organizational capacity and its role in student involvement. Findings indicate these ‘dedicated’ athletic departments were characterized by strong organizational capacity for engaging student-athletes in meaningful service efforts. The critical role of coaches and mutual values among internal stakeholders emerged as the primary strengths of department’s human resources capacity. Despite the limited financial capacity, departments were able to creatively secure some funding for development programs. The ability to leverage external relationships, an organizational culture promoting participative decision-making and student-athlete development, and on-going efforts to improve service and life skills opportunities for student-athletes indicated strong structural capacity

Shedding light on the elusive role of endothelial cells in cytomegalovirus dissemination.

Cytomegalovirus (CMV) is frequently transmitted by solid organ transplantation and is associated with graft failure. By forming the boundary between circulation and organ parenchyma, endothelial cells (EC) are suited for bidirectional virus spread from and to the transplant. We applied Cre/loxP-mediated green-fluorescence-tagging of EC-derived murine CMV (MCMV) to quantify the role of infected EC in transplantation-associated CMV dissemination in the mouse model. Both EC- and non-EC-derived virus originating from infected Tie2-cre(+) heart and kidney transplants were readily transmitted to MCMV-naïve recipients by primary viremia. In contrast, when a Tie2-cre(+) transplant was infected by primary viremia in an infected recipient, the recombined EC-derived virus poorly spread to recipient tissues. Similarly, in reverse direction, EC-derived virus from infected Tie2-cre(+) recipient tissues poorly spread to the transplant. These data contradict any privileged role of EC in CMV dissemination and challenge an indiscriminate applicability of the primary and secondary viremia concept of virus dissemination

Turbulent dynamo action and its effects on the mixing at the convective boundary of an idealized oxygen-burning shell

Convection is one of the most important mixing processes in stellar interiors. Hydrodynamic mass entrainment can bring fresh fuel from neighboring stable layers into a convection zone, modifying the structure and evolution of the star. Under some conditions, strong magnetic fields can be sustained by the action of a turbulent dynamo, adding another layer of complexity and possibly altering the dynamics in the convection zone and at its boundaries. In this study, we used our fully compressible Seven-League Hydro code to run detailed and highly resolved three-dimensional magnetohydrodynamic simulations of turbulent convection, dynamo amplification, and convective boundary mixing in a simplified setup whose stratification is similar to that of an oxygen-burning shell in a star with an initial mass of $25\ M_\odot$. We find that the random stretching of magnetic field lines by fluid motions in the inertial range of the turbulent spectrum (i.e., a small-scale dynamo) naturally amplifies the seed field by several orders of magnitude in a few convective turnover timescales. During the subsequent saturated regime, the magnetic-to-kinetic energy ratio inside the convective shell reaches values as high as $0.33$, and the average magnetic field strength is ${\sim}10^{10}\,\mathrm{G}$. Such strong fields efficiently suppress shear instabilities, which feed the turbulent cascade of kinetic energy, on a wide range of spatial scales. The resulting convective flows are characterized by thread-like structures that extend over a large fraction of the convective shell. The reduced flow speeds and the presence of magnetic fields with strengths up to $60\%$ of the equipartition value at the upper convective boundary diminish the rate of mass entrainment from the stable layer by ${\approx}\,20\%$ as compared to the purely hydrodynamic case

Towards a self-consistent model of the convective core boundary in upper-main-sequence stars

There is strong observational evidence that convective cores of intermediate-mass and massive main-sequence stars are substantially larger than standard stellar-evolution models predict. However, it is unclear what physical processes cause this phenomenon or how to predict the extent and stratification of stellar convective boundary layers. Convective penetration is a thermal-time-scale process that is likely to be particularly relevant during the slow evolution on the main sequence. We use our low-Mach-number Seven-League Hydro (SLH) code to study this process in 2.5D and 3D geometries. Starting with a chemically homogeneous model of a $15$ M$_\odot$ zero-age main-sequence star, we construct a series of simulations with the luminosity increased and opacity decreased by the same factor ranging from $10^3$ to $10^6$. After reaching thermal equilibrium, all of our models show a clear penetration layer. Its thickness becomes statistically constant in time and it is shown to converge upon grid refinement. As the luminosity is decreased, the penetration layer becomes nearly adiabatic with a steep transition to a radiative stratification. This structure corresponds to the adiabatic ,,step overshoot'' model often employed in stellar-evolution calculations. The thickness of the penetration layer slowly decreases with decreasing luminosity. Depending on how we extrapolate our 3D data to the actual luminosity of the initial stellar model, we obtain penetration distances ranging from $0.09$ to $0.44$ pressure scale heights, which are broadly compatible with observations.Comment: 10 pages, 12 figures, submitted to A&

Performance of high-order Godunov-type methods in simulations of astrophysical low Mach number flows

High-order Godunov methods for gas dynamics have become a standard tool for simulating different classes of astrophysical flows. Their accuracy is mostly determined by the spatial interpolant used to reconstruct the pair of Riemann states at cell interfaces and by the Riemann solver that computes the interface fluxes. In most Godunov-type methods, these two steps can be treated independently, so that many different schemes can in principle be built from the same numerical framework. In this work, we use our fully compressible Seven-League Hydro (SLH) code to test the accuracy of six reconstruction methods and three approximate Riemann solvers on two- and three-dimensional (2D and 3D) problems involving subsonic flows only. We consider Mach numbers in the range from $10^{-3}$ to $10^{-1}$ in a well-posed, 2D, Kelvin--Helmholtz instability problem and a 3D turbulent convection zone that excites internal gravity waves in an overlying stable layer. We find that (i) there is a spread of almost four orders of magnitude in computational cost per fixed accuracy between the methods tested in this study, with the most performant method being a combination of a "low-dissipation" Riemann solver and a sextic reconstruction scheme, (ii) the low-dissipation solver always outperforms conventional Riemann solvers on a fixed grid when the reconstruction scheme is kept the same, (iii) in simulations of turbulent flows, increasing the order of spatial reconstruction reduces the characteristic dissipation length scale achieved on a given grid even if the overall scheme is only second order accurate, (iv) reconstruction methods based on slope-limiting techniques tend to generate artificial, high-frequency acoustic waves during the evolution of the flow, (v) unlimited reconstruction methods introduce oscillations in the thermal stratification near the convective boundary, where the entropy gradient is steep

Carbon sources of Antarctic nematodes as revealed by natural carbon isotope ratios and a pulse-chase experiment

δ13C of nematode communities in 27 sites was analyzed, spanning a large depth range (from 130 to 2,021 m) in five Antarctic regions, and compared to isotopic signatures of sediment organic matter. Sediment organic matter δ13C ranged from −24.4 to −21.9‰ without significant differences between regions, substrate types or depths. Nematode δ13C showed a larger range, from −34.6 to −19.3‰, and was more depleted than sediment organic matter typically by 1‰ and by up to 3‰ in silty substrata. These, and the isotopically heavy meiofauna at some stations, suggest substantial selectivity of some meiofauna for specific components of the sedimenting plankton. However, 13C-depletion in lipids and a potential contribution of chemoautotrophic carbon in the diet of the abundant genus Sabatieria may confound this interpretation. Carbon sources for Antarctic nematodes were also explored by means of an experiment in which the fate of a fresh pulse of labile carbon to the benthos was followed. This organic carbon was remineralized at a rate (11–20 mg C m−2 day−1) comparable to mineralization rates in continental slope sediments. There was no lag between sedimentation and mineralization; uptake by nematodes, however, did show such a lag. Nematodes contributed negligibly to benthic carbon mineralization

Fully compressible simulations of waves and core convection in main-sequence stars

Context. Recent, nonlinear simulations of wave generation and propagation in full-star models have been carried out in the anelastic approximation using spectral methods. Although it makes long time steps possible, this approach excludes the physics of sound waves completely and rather high artificial viscosity and thermal diffusivity are needed for numerical stability. Direct comparison with observations is thus limited. Aims. We explore the capabilities of our compressible multidimensional hydrodynamics code SLH to simulate stellar oscillations. Methods. We compare some fundamental properties of internal gravity and pressure waves in 2D SLH simulations to linear wave theory using two test cases: (1) an interval gravity wave packet in the Boussinesq limit and (2) a realistic $3\mathrm{M}_\odot$ stellar model with a convective core and a radiative envelope. Oscillation properties of the stellar model are also discussed in the context of observations. Results. Our tests show that specialized low-Mach techniques are necessary when simulating oscillations in stellar interiors. Basic properties of internal gravity and pressure waves in our simulations are in good agreement with linear wave theory. As compared to anelastic simulations of the same stellar model, we can follow internal gravity waves of much lower frequencies. The temporal frequency spectra of velocity and temperature are flat and compatible with observed spectra of massive stars. Conclusion. The low-Mach compressible approach to hydrodynamical simulations of stellar oscillations is promising. Our simulations are less dissipative and require less luminosity boosting than comparable spectral simulations. The fully-compressible approach allows the coupling of gravity and pressure waves to be studied too.Comment: Accepted for publication in A&