Multidimensional hydrodynamic simulations of the hydrogen injection flash

The injection of hydrogen into the convection shell powered by helium burning during the core helium flash is commonly encountered during the evolution of metal-free and extremely metal-poor low-mass stars. With specifically designed multidimensional hydrodynamic simulations, we aim to prove that an entropy barrier is no obstacle for the growth of the helium-burning shell convection zone in the helium core of a metal-rich Pop I star, i.e. convection can penetrate into the hydrogen-rich layers for these stars, too. We further study whether this is also possible in one-dimensional stellar evolutionary calculations. Our hydrodynamical simulations show that the helium-burning shell convection zone in the helium core moves across the entropy barrier and reaches the hydrogen-rich layers. This leads to mixing of protons into the hotter layers of the core and to a rapid increase of the nuclear energy production at the upper edge of the helium-burning convection shell - the hydrogen injection flash. As a result a second convection zone appears in the hydrogen-rich layers. Contrary to 1D models, the entropy barrier separating the two convective shells from each other is largely permeable to chemical transport when allowing for multidimensional flow, and consequently, hydrogen is continuously mixed deep into the helium core. We find it difficult to achieve such a behavior in one-dimensional stellar evolutionary calculations.Comment: 8 pages, 8 figures - accepted for publication in Astronomy and Astrophysics. Animations related to the manuscript can be downloaded from http://www-astro.ulb.ac.be/~mocak/index.php/Main/AnimationsHeFlas

A new stellar mixing process operating below shell convection zones following off-center ignition

During most stages of stellar evolution the nuclear burning of lighter to heavier elements results in a radial composition profile which is stabilizing against buoyant acceleration, with light material residing above heavier material. However, under some circumstances, such as off-center ignition, the composition profile resulting from nuclear burning can be destabilizing, and characterized by an outwardly increasing mean molecular weight. The potential for instabilities under these circumstances, and the consequences that they may have on stellar structural evolution, remain largely unexplored. In this paper we study the development and evolution of instabilities associated with unstable composition gradients in regions which are initially stable according to linear Schwarzschild and Ledoux criteria. In particular, we explore the mixing taking place under various conditions with multi-dimensional hydrodynamic convection models based on stellar evolutionary calculations of the core helium flash in a 1.25 \Msun star, the core carbon flash in a 9.3\,\Msun star, and of oxygen shell burning in a star with a mass of 23\,\Msun. The results of our simulations reveal a mixing process associated with regions having outwardly increasing mean molecular weight that reside below convection zones. The mixing is not due to overshooting from the convection zone, nor is it due directly to thermohaline mixing which operates on a timescale several orders of magnitude larger than the simulated flows. Instead, the mixing appears to be due to the presence of a wave field induced in the stable layers residing beneath the convection zone which enhances the mixing rate by many orders of magnitude and allows a thermohaline type mixing process to operate on a dynamical, rather than thermal, timescale. We discuss our results in terms of related laboratory phenomena and associated theoretical developments.Comment: accepted for publication in Astrophysical Journal, 9 pages, 8 figure

Hydrodynamic simulations of shell convection in stellar cores

Shell convection driven by nuclear burning in a stellar core is a common hydrodynamic event in the evolution of many types of stars. We encounter and simulate this convection (i) in the helium core of a low-mass red giant during core helium flash leading to a dredge-down of protons across an entropy barrier, (ii) in a carbon-oxygen core of an intermediate-mass star during core carbon flash, and (iii) in the oxygen and carbon burning shell above the silicon-sulfur rich core of a massive star prior to supernova explosion. Our results, which were obtained with the hydrodynamics code HERAKLES, suggest that both entropy gradients and entropy barriers are less important for stellar structure than commonly assumed. Our simulations further reveal a new dynamic mixing process operating below the base of shell convection zones.Comment: 8 pages, 3 figures .. submitted to a proceedings of conference about "Red Giants as Probes of the Structure and Evolution of the Milky Way" which has taken place between 15-17 November 2010 in Rom

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars II. s-process nucleosynthesis during the core He flash

Models of primordial and hyper-metal-poor stars with masses similar to the Sun experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star of 1 solar mass and metallicity [Fe/H] = -6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2,366 reactions and treats mixing and burning simultaneously. The mixing and burning of protons into the hot convective core leads to the production of 13C, which then burns via the 13C(alpha,n)16O reaction releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later mixed to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance. Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of ~4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If true then this puts constraints on the primordial initial mass function.Comment: Accepted for publication on Astronomy & Astrophysics Letter

Stimulating poverty alleviation by developing tourism in marginalised Roma communities: a case study of the central Spiš region (Slovakia)

The main objective of this study is to demonstrate that opportunities to support the poor marginalised Roma communities (MRCs) by creating opportunities for their active participation in tourism development have been under-ultilised to date. In particular, the application of a pro-poor tourism (PPT) approach is an advantage that allows the cultural specificities of the MRCs to be exploited for their own development. A risk factor for successful implementation is the considerable differentiation within Roma communities. This study analyses the approaches of 65 municipalities of the Central Spiš region to the development of PPT as an option for poverty reduction in the MRCs in Slovakia. Based on the analysis of the local preconditions for tourism development, the specific potential for the development of Roma culture and the traditional economic activities, human resources, and the degree of segregation, we show that there is no universal solution that would be applicable in all communities. The limited possibilities to obtain relevant data determined the analysis options we used. Therefore, we used a mixed-method approach, in which we subjected the quantitative data to qualitative analysis based on interviews with the mayors of the communities being studied. Four assumptions were verified in the aforementioned way: (1) the MRCs in villages in the immediate vicinity of developed tourist centres show a higher level of activity in relation to their own development; (2) Roma who are active in small-scale farming will also be more active in maintaining other traditional craft activities, typical for Roma communities in Slovakia, forming the basis for the development of their own PPT product; (3) the activity of NGO’s focusing on the development of the local MRC’s is not a guarantee for the development of PPT activities in the villages; and finally that (4) the high level of segregation of marginalised Roma communities is a major obstacle for the implementation of PPT. This paper concludes with a typology of municipalities in relation to the preconditions for the development of PPT, which can be considered as a contribution to the discussion on the possibilities of a differentiated approach to PPT development strategies at the level of municipalities with the numerous MRC’s

Towards a new generation of multi-dimensional stellar evolution models: development of an implicit hydrodynamic code

This paper describes the first steps of development of a new multidimensional time implicit code devoted to the study of hydrodynamical processes in stellar interiors. The code solves the hydrodynamical equations in spherical geometry and is based on the finite volume method. Radiation transport is taken into account within the diffusion approximation. Realistic equation of state and opacities are implemented, allowing the study of a wide range of problems characteristic of stellar interiors. We describe in details the numerical method and various standard tests performed to validate the method. We present preliminary results devoted to the description of stellar convection. We first perform a local simulation of convection in the surface layers of a A-type star model. This simulation is used to test the ability of the code to address stellar conditions and to validate our results, since they can be compared to similar previous simulations based on explicit codes. We then present a global simulation of turbulent convective motions in a cold giant envelope, covering 80% in radius of the stellar structure. Although our implicit scheme is unconditionally stable, we show that in practice there is a limitation on the time step which prevent the flow to move over several cells during a time step. Nevertheless, in the cold giant model we reach a hydro CFL number of 100. We also show that we are able to address flows with a wide range of Mach numbers (10^-3 < Ms< 0.5), which is impossible with an anelastic approach. Our first developments are meant to demonstrate that the use of an implicit scheme applied to a stellar evolution context is perfectly thinkable and to provide useful guidelines to optimise the development of an implicit multi-D hydrodynamical code.Comment: 21 pages, 18 figures, accepted for publication in A&

Presupernova Structure of Massive Stars

Issues concerning the structure and evolution of core collapse progenitor stars are discussed with an emphasis on interior evolution. We describe a program designed to investigate the transport and mixing processes associated with stellar turbulence, arguably the greatest source of uncertainty in progenitor structure, besides mass loss, at the time of core collapse. An effort to use precision observations of stellar parameters to constrain theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics and Space Science, submitted for peer review: 7 pages, 3 figure

3D Simulations and MLT. I. Renzini’s Critique

Renzini wrote an influential critique of “overshooting” in mixing-length theory (MLT), as used in stellar evolution codes, and concluded that three-dimensional fluid dynamical simulations were needed. Such simulations are now well tested. Implicit large eddy simulations connect large-scale stellar flow to a turbulent cascade at the grid scale, and allow the simulation of turbulent boundary layers, with essentially no assumptions regarding flow except the number of computational cells. Buoyant driving balances turbulent dissipation for weak stratification, as in MLT, but with the dissipation length replacing the mixing length. The turbulent kinetic energy in our computational domain shows steady pulses after 30 turnovers, with no discernible diminution; these are caused by the necessary lag in turbulent dissipation behind acceleration. Interactions between coherent turbulent structures give multi-modal behavior, which drives intermittency and fluctuations. These cause mixing, which may justify use of the instability criterion of Schwarzschild rather than the Ledoux. Chaotic shear flow of turning material at convective boundaries causes instabilities that generate waves and sculpt the composition gradients and boundary layer structures. The flow is not anelastic; wave generation is necessary at boundaries. A self-consistent approach to boundary layers can remove the need for ad hoc procedures of “convective overshooting” and “semi-convection.” In Paper II, we quantify the adequacy of our numerical resolution in a novel way, determine the length scale of dissipation—the “mixing length”—without astronomical calibration, quantify agreement with the four-fifths law of Kolmogorov for weak stratification, and deal with strong stratification

Recent Advances in Modeling Stellar Interiors

Advances in stellar interior modeling are being driven by new data from large-scale surveys and high-precision photometric and spectroscopic observations. Here we focus on single stars in normal evolutionary phases; we will not discuss the many advances in modeling star formation, interacting binaries, supernovae, or neutron stars. We review briefly: 1) updates to input physics of stellar models; 2) progress in two and three-dimensional evolution and hydrodynamic models; 3) insights from oscillation data used to infer stellar interior structure and validate model predictions (asteroseismology). We close by highlighting a few outstanding problems, e.g., the driving mechanisms for hybrid gamma Dor/delta Sct star pulsations, the cause of giant eruptions seen in luminous blue variables such as eta Car and P Cyg, and the solar abundance problem.Comment: Proceedings for invited talk at conference High Energy Density Laboratory Astrophysics 2010, Caltech, March 2010, submitted for special issue of Astrophysics and Space Science; 7 pages; 5 figure

Gravitational settling in pulsating subdwarf B stars and their progenitors

Diffusion of atoms can be important during quiescent phases of stellar evolution. Particularly in the very thin inert envelopes of subdwarf B stars, diffusive movements will considerably change the envelope structure and the surface abundances on a short timescale. Also, the subdwarfs will inherit the effects of diffusion in their direct progenitors, namely giants near the tip of the red giant branch. This will influence the global evolution and the pulsational properties of subdwarf B stars. We investigate the impact of gravitational settling, thermal diffusion and concentration diffusion on the evolution and pulsations of subdwarf B stars. Our diffusive stellar models are compared with models evolved without diffusion. We constructed subdwarf B models with a mass of 0.465 Msun from a 1 and 3 Msun ZAMS progenitor. The low mass star ignited helium in an energetic flash, while the intermediate mass star started helium fusion gently. For each progenitor type we computed series with and without atomic diffusion. Atomic diffusion in red giants causes the helium core mass at the onset of helium ignition to be larger. We find an increase of 0.0015 Msun for the 1 Msun model and 0.0036 Msun for the 3 Msun model. The effects on the red giant surface abundances are small after the first dredge up. The evolutionary tracks of the diffusive subdwarf B models are shifted to lower surface gravities and effective temperatures due to outward diffusion of hydrogen. This affects both the frequencies of the excited modes and the overall frequency spectrum. Especially the structure and pulsations of the post-non-degenerate sdB star are drastically altered, proving that atomic diffusion cannot be ignored in these stars.Comment: 10 pages, 6 figures, accepted for publication in A&