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

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

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

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

Unstable Helium Shell Burning on Accreting White Dwarfs

AM Canum Venaticorum (AM CVn) binaries consist of a degenerate helium donor and a helium, C/O, or O/Ne WD accretor, with accretion rates of Mdot = 1e-13 - 1e-5 Msol/yr. For accretion rates < 1e-6 Msol/yr, the accreted helium ignites unstably, resulting in a helium flash. As the donor mass and Mdot decrease, the ignition mass increases and eventually becomes larger than the donor mass, yielding a "last-flash" ignition mass of < 0.1 Msol. Bildsten et al. (2007) predicted that the largest outbursts of these systems will lead to dynamical burning and thermonuclear supernovae. In this paper, we study the evolution of the He-burning shells in more detail. We calculate maximum achievable temperatures as well as the minimum envelope masses that achieve dynamical burning conditions, finding that AM CVn systems with accretors > 0.8 Msol will undergo dynamical burning. Triple-alpha reactions during the hydrostatic evolution set a lower limit to the 12C mass fraction of 0.001 - 0.05 when dynamical burning occurs, but core dredge-up may yield 12C, 16O, and/or 20Ne mass fractions of ~ 0.1. Accreted 14N will likely remain 14N during the accretion and convective phases, but regardless of 14N's fate, the neutron-to-proton ratio at the beginning of convection is fixed until the onset of dynamical burning. During explosive burning, the 14N will undergo 14N(a,g)18F(a,p)21Ne, liberating a proton for the subsequent 12C(p,g)13N(a,p)16O reaction, which bypasses the relatively slow alpha-capture onto 12C. Future hydrodynamic simulations must include these isotopes, as the additional reactions will reduce the Zel'dovich-von Neumann-Doring (ZND) length, making the propagation of the detonation wave more likely.Comment: Accepted for publication in The Astrophysical Journal, 10 pages, 8 figures; v2: Minor changes to correct typos and clarify conten

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&

Dynamics in a stellar convective layer and at its boundary: Comparison of five 3D hydrodynamics codes

This is the final version. Available from EDP Sciences via the DOI in this recordOur ability to predict the structure and evolution of stars is in part limited by complex, 3D hydrodynamic processes such as convective boundary mixing. Hydrodynamic simulations help us understand the dynamics of stellar convection and convective boundaries. However, the codes used to compute such simulations are usually tested on extremely simple problems and the reliability and reproducibility of their predictions for turbulent flows is unclear. We define a test problem involving turbulent convection in a plane-parallel box, which leads to mass entrainment from, and internal-wave generation in, a stably stratified layer. We compare the outputs from the codes FLASH, MUSIC, PPMSTAR, PROMPI, and SLH, which have been widely employed to study hydrodynamic problems in stellar interiors. The convection is dominated by the largest scales that fit into the simulation box. All time-averaged profiles of velocity components, fluctuation amplitudes, and fluxes of enthalpy and kinetic energy are within ≲3σ of the mean of all simulations on a given grid (1283 and 2563 grid cells), where σ describes the statistical variation due to the flow’s time dependence. They also agree well with a 5123 reference run. The 1283 and 2563 simulations agree within 9% and 4%, respectively, on the total mass entrained into the convective layer. The entrainment rate appears to be set by the amount of energy that can be converted to work in our setup and details of the small-scale flows in the boundary layer seem to be largely irrelevant. Our results lend credence to hydrodynamic simulations of flows in stellar interiors. We provide in electronic form all outputs of our simulations as well as all information needed to reproduce or extend our study.Science and Technology Facilities Council (STFC)European Research Council (ERC

Classification of selected Slovak variety wines.

During a three years period, the content of the selected volatile substances has been detected in the Slovak varietal wines, Welschriesling, Grüner Veltliner and Müller Thurgau. The acquired data were achieved by means of different multivariation methods for the purpose of finding a combination of volatile substances that would enable a classification of the tested variety wines. A proper classification and finding of authentic Slovak variety wines was possible due to the lineal and quadratic discriminant analysis.During a three years period, the content of the selected volatile substances has been detected in the Slovak varietal wines, Welschriesling, Grüner Veltliner and Müller Thurgau. The acquired data were achieved by means of different multivariation methods for the purpose of finding a combination of volatile substances that would enable a classification of the tested variety wines. A proper classification and finding of authentic Slovak variety wines was possible due to the lineal and quadratic discriminant analysis