Asteroseismic classification of stellar populations among 13000 red giants observed by Kepler

Of the more than 150000 targets followed by the Kepler Mission, about 10% were selected as red giants. Due to their high scientific value, in particular for Galaxy population studies and stellar structure and evolution, their Kepler light curves were made public in late 2011. More than 13000 (over 85%) of these stars show intrinsic flux variability caused by solar-like oscillations making them ideal for large scale asteroseismic investigations. We automatically extracted individual frequencies and measured the period spacings of the dipole modes in nearly every red giant. These measurements naturally classify the stars into various populations, such as the red giant branch, the low-mass (M/Msol 1.8) secondary clump. The period spacings also reveal that a large fraction of the stars show rotationally induced frequency splittings. This sample of stars will undoubtedly provide an extremely valuable source for studying the stellar population in the direction of the Kepler field, in particular when combined with complementary spectroscopic surveys.Comment: 6 page, 5 figures, accepted by ApJ

MESA and NuGrid simulations of classical novae: CO and ONe nova nucleosynthesis

Classical novae are the result of thermonuclear flashes of hydrogen accreted by CO or ONe white dwarfs, leading eventually to the dynamic ejection of the surface layers. These are observationally known to be enriched in heavy elements, such as C, O and Ne that must originate in layers below the H-flash convection zone. Building on our previous work, we now present stellar evolution simulations of ONe novae and provide a comprehensive comparison of our models with published ones. Some of our models include exponential convective boundary mixing to account for the observed enrichment of the nova ejecta even when accreted material has a solar abundance distribution. Our models produce maximum temperature evolution profiles and nucleosynthesis yields in good agreement with models that generate enriched ejecta by assuming that the accreted material was pre-mixed. We confirm for ONe novae the result we reported previously, i.e.\ we found that $^3$He could be produced {\it in situ} in solar-composition envelopes accreted with slow rates (\dot{M} < 10^{-10}\,M_\odot/\mbox{yr}) by cold ($T_{\rm WD} < 10^7$ K) CO WDs, and that convection was triggered by $^3$He burning before the nova outburst in that case. In addition, we now find that the interplay between the $^3$He production and destruction in the solar-composition envelope accreted with an intermediate rate, e.g.\ \dot{M} = 10^{-10}\,M_\odot/\mbox{yr}, by the $1.15\,M_\odot$ ONe WD with a relatively high initial central temperature, e.g.\ $T_{\rm WD} = 15\times 10^6$ K, leads to the formation of a thick radiative buffer zone that separates the bottom of the convective envelope from the WD surface. (Abridged)Comment: 19 pages, 23 figures, 2 tables, accepted to publication by MNRA

H ingestion into He-burning convection zones in super-AGB stellar models as a potential site for intermediate neutron-density nucleosynthesis

We investigate the evolution of super-AGB (SAGB) thermal pulse (TP) stars for a range of metallicities (Z) and explore the effect of convective boundary mixing (CBM). With decreasing metallicity and evolution along the TP phase, the He-shell flash and the third dredge-up (TDU) occur closer together in time. After some time (depending upon the CBM parametrization), efficient TDU begins while the pulse-driven convection zone (PDCZ) is still present, causing a convective exchange of material between the PDCZ and the convective envelope. This results in the ingestion of protons into the convective He-burning pulse. Even small amounts of CBM encourage the interaction of the convection zones leading to transport of protons from the convective envelope into the He layer. H-burning luminosities exceed 10⁹ (in some cases 10¹⁰) L⊙. We also calculate models of dredge-out in the most massive SAGB stars and show that the dredge-out phenomenon is another likely site of convective-reactive H-¹²C combustion. We discuss the substantial uncertainties of stellar evolution models under these conditions. Nevertheless, the simulations suggest that in the convective-reactive H-combustion regime of H ingestion the star may encounter conditions for the intermediate neutron capture process (i-process). We speculate that some CEMP-s/r stars could originate in i-process conditions in the H ingestion phases of low-Z SAGB stars. This scenario would however suggest a very low electron-capture supernova rate from SAGB stars. We also simulate potential outbursts triggered by such H ingestion events, present their light curves and briefly discuss their transient properties

Identifying persuasive public health messages to change community knowledge and attitudes about bulimia nervosa

Addressing stigma through social marketing campaigns has the potential to enhance currently low rates of treatment seeking and improve the well-being of individuals with the eating disorder bulimia nervosa. This study aimed to evaluate the persuasiveness of health messages designed to reduce stigma and improve mental health literacy about this disorder. A community sample of 1,936 adults (48.2% male, 51.8% female) from Victoria, Australia, provided (a) self-report information on knowledge and stigma about bulimia nervosa and (b) ratings of the persuasiveness of 9 brief health messages on dimensions of convincingness and likelihood of changing attitudes. Messages were rated moderately to very convincing and a little to moderately likely to change attitudes toward bulimia nervosa. The most persuasive messages were those that emphasized that bulimia nervosa is a serious mental illness and is not attributable to personal failings. Higher ratings of convincingness were associated with being female, with having more knowledge about bulimia nervosa, and with lower levels of stigma about bulimia nervosa. Higher ratings for likelihood of changing attitudes were associated with being female and with ratings of the convincingness of the corresponding message. This study provides direction for persuasive content to be included in social marketing campaigns to reduce stigma toward bulimia nervosa.10 page(s

Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions

We update the capabilities of the software instrument Modules for Experiments in Stellar Astrophysics (MESA) and enhance its ease of use and availability. Our new approach to locating convective boundaries is consistent with the physics of convection, and yields reliable values of the convective core mass during both hydrogen and helium burning phases. Stars with $M<8\,{\rm M_\odot}$ become white dwarfs and cool to the point where the electrons are degenerate and the ions are strongly coupled, a realm now available to study with MESA due to improved treatments of element diffusion, latent heat release, and blending of equations of state. Studies of the final fates of massive stars are extended in MESA by our addition of an approximate Riemann solver that captures shocks and conserves energy to high accuracy during dynamic epochs. We also introduce a 1D capability for modeling the effects of Rayleigh-Taylor instabilities that, in combination with the coupling to a public version of the STELLA radiation transfer instrument, creates new avenues for exploring Type II supernovae properties. These capabilities are exhibited with exploratory models of pair-instability supernova, pulsational pair-instability supernova, and the formation of stellar mass black holes. The applicability of MESA is now widened by the capability of importing multi-dimensional hydrodynamic models into MESA. We close by introducing software modules for handling floating point exceptions and stellar model optimization, and four new software tools -- MESAWeb, MESA-Docker, pyMESA, and mesastar.org -- to enhance MESA's education and research impact.Comment: 64 pages, 61 figures; Accepted to AAS Journal

Conservative Initial Mapping For Multidimensional Simulations of Stellar Explosions

Mapping one-dimensional stellar profiles onto multidimensional grids as initial conditions for hydrodynamics calculations can lead to numerical artifacts, one of the most severe of which is the violation of conservation laws for physical quantities such as energy and mass. Here we introduce a numerical scheme for mapping one-dimensional spherically-symmetric data onto multidimensional meshes so that these physical quantities are conserved. We validate our scheme by porting a realistic 1D Lagrangian stellar profile to the new multidimensional Eulerian hydro code CASTRO. Our results show that all important features in the profiles are reproduced on the new grid and that conservation laws are enforced at all resolutions after mapping.Comment: 7 pages, 5 figures, Proceeding for Conference on Computational Physics (CCP 2011

Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars

We find that low-degree low-order g-modes become unstable in metal-poor low-mass stars due to the $\varepsilon$-mechanism of the pp-chain. Since the outer convection zone of these stars is limited only to the very outer layers, the uncertainty in the treatment of convection does not affect the result significantly. The decrease in metallicity leads to decrease in opacity and hence increase in luminosity of a star. This makes the star compact and results in decrease in the density contrast, which is favorable to the $\varepsilon$-mechanism instability. We find also instability for high order g-modes of metal-poor low-mass stars by the convective blocking mechanism. Since the effective temperature and the luminosity of metal-poor stars are significantly higher than those of Pop I stars, the stars showing $\gamma$ Dor-type pulsation are substantially less massive than in the case of Pop I stars. We demonstrate that those modes are unstable for about $1\,M_\odot$ stars in the metal-poor case.Comment: 4 pages, 4 figures, To be published in Astrophysics and Space Science Proceedings series (ASSP). Proceedings of the "20th Stellar Pulsation Conference Series: Impact of new instrumentation and new insights in stellar pulsations", 5-9 September 2011, Granada, Spai

Does the group leader matter? The impact of monitoring activities and social ties of group leaders on the repayment performance of groupbased lending Eritrea

This paper analyzes whether the effects of monitoring and social ties of the group leader and other group members on repayment performance of groups differ, using data from an extensive questionnaire held in Eritrea among participants of 102 groups. We hypothesize that the monitoring activities and social ties of the group leader have a stronger positive impact on the repayment performance of groups. The results show that social ties of the group leader do have a positive effect on repayment performance of groups, whereas this is not true for social ties of other group members. We do not find evidence for the hypothesis that monitoring activities of the group leader have a stronger positive impact on group repayment performance. All variables measuring monitoring activities, either of the group leader or the other group members, are found to be statistically insignificant.