Chemical abundance study of two strongly s-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9

Context: This paper is part of a larger project in which we study the chemical abundances of extra-galactic post-AGB stars with the ultimate goal of improving our knowledge of the poorly understood AGB third dredge-up mixing processes and s-process nucleosynthesis. Aims: In this paper, we study two carefully selected post-AGB stars in the LMC. The combination of favourable atmospheric parameters for detailed abundance studies and their known distances make these objects ideal probes of the internal AGB third dredge-up and s-process nucleosynthesis in that they provide observational constraints for theoretical AGB models. Methods: We use high-resolution optical UVES spectra to determine accurate stellar parameters and perform detailed elemental abundance studies. Additionally, we use available photometric data to construct SEDs for reddening and luminosity determinations. We then estimate initial masses from theoretical post-AGB tracks. Results: Both stars show extreme s-process enrichment associated with relatively low C/O ratios of about 1.3. We could only derive upper limits of the lead (Pb) abundance which indicate no strong Pb overabundances with respect to other s-elements. Comparison with theoretical post-AGB evolutionary tracks in the HR-diagram reveals that both stars have low initial masses between 1.0 and 1.5 Msun. Conclusion: This study adds to the results obtained so far on a very limited number of s-process enriched post-AGB stars in the Magellanic Clouds. We find an increasing discrepancy between observed and predicted Pb abundances towards lower metallicities for all studied Magellanic Cloud post-AGB stars found so far, as well as moderate C/O ratios. We find that all s-process rich post-AGB stars in the LMC and SMC studied so far, cluster in the same region of the HR-diagram and are associated with low-mass stars with a low metallicity on average.Comment: 11 pages, 14 figure

Chemical abundance study of two strongly s-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9

Context: This paper is part of a larger project in which we study the chemical abundances of extra-galactic post-AGB stars with the ultimate goal of improving our knowledge of the poorly understood AGB third dredge-up mixing processes and s-process nucleosynthesis. Aims: In this paper, we study two carefully selected post-AGB stars in the LMC. The combination of favourable atmospheric parameters for detailed abundance studies and their known distances make these objects ideal probes of the internal AGB third dredge-up and s-process nucleosynthesis in that they provide observational constraints for theoretical AGB models. Methods: We use high-resolution optical UVES spectra to determine accurate stellar parameters and perform detailed elemental abundance studies. Additionally, we use available photometric data to construct SEDs for reddening and luminosity determinations. We then estimate initial masses from theoretical post-AGB tracks. Results: Both stars show extreme s-process enrichment associated with relatively low C/O ratios of about 1.3. We could only derive upper limits of the lead (Pb) abundance which indicate no strong Pb overabundances with respect to other s-elements. Comparison with theoretical post-AGB evolutionary tracks in the HR-diagram reveals that both stars have low initial masses between 1.0 and 1.5 Msun. Conclusion: This study adds to the results obtained so far on a very limited number of s-process enriched post-AGB stars in the Magellanic Clouds. We find an increasing discrepancy between observed and predicted Pb abundances towards lower metallicities for all studied Magellanic Cloud post-AGB stars found so far, as well as moderate C/O ratios. We find that all s-process rich post-AGB stars in the LMC and SMC studied so far, cluster in the same region of the HR-diagram and are associated with low-mass stars with a low metallicity on average.Comment: 11 pages, 14 figure

The lead discrepancy in intrinsically s-process enriched post-AGB stars in the Magellanic Clouds

Context: Our understanding of the s-process nucleosynthesis in asymptotic giant branch (AGB) stars is incomplete. AGB models predict, for example, large overabundances of lead (Pb) compared to other s-process elements in metal-poor low-mass AGB stars. This is indeed observed in some extrinsically enhanced metal-poor stars, but not in all. An extensive study of intrinsically s-process enriched objects is essential for improving our knowledge of the AGB third dredge-up and associated s-process nucleosynthesis. Aims: We compare the spectral abundance analysis of the SMC post-AGB star J004441.04-732136.4 with state-of-the-art AGB model predictions with a main focus on Pb. The low S/N in the Pb line region made the result of our previous study inconclusive. We acquired additional data covering the region of the strongest Pb line. Methods: By carefully complementing re-reduced previous data, with newly acquired UVES optical spectra, we improve the S/N of the spectrum around the strongest Pb line. Therefore, an upper limit for the Pb abundance is estimated from a merged weighted mean spectrum using synthetic spectral modeling. We then compare the abundance results from the combined spectra to predictions of tailored AGB evolutionary models from two independent evolution codes. In addition, we determine upper limits for Pb abundances for three previously studied LMC post-AGB objects. Results: Although theoretical predictions for J004441.04-732136.4 match the s-process distribution up to tungsten (W), the predicted very high Pb abundance is clearly not detected. The three additional LMC post-AGB stars show a similar lack of a very high Pb abundance. Conclusion: From our study, we conclude that none of these low-mass, low-metallicity post-AGB stars of the LMC and SMC are strong Pb producers. This conflicts with current theoretical predictions.Comment: 4 pages, 3 figure

Improved convergence and stability properties in a three-dimensional higher-order ice sheet model

We present a finite difference implementation of a three-dimensional higher-order ice sheet model. In comparison to a conventional centred difference discretisation it enhances both numerical stability and convergence. In order to achieve these benefits the discretisation of the governing force balance equation makes extensive use of information on staggered grid points. Using the same iterative solver, a centred difference discretisation that operates exclusively on the regular grid serves as a reference. The reprise of the ISMIP-HOM experiments indicates that both discretisations are capable of reproducing the higher-order model inter-comparison results. This setup allows a direct comparison of the two numerical implementations also with respect to their convergence behaviour. First and foremost, the new finite difference scheme facilitates convergence by a factor of up to 7 and 2.6 in average. In addition to this decrease in computational costs, the accuracy for the resultant velocity field can be chosen higher in the novel finite difference implementation. Changing the discretisation also prevents build-up of local field irregularites that occasionally cause divergence of the solution for the reference discretisation. <br><br> The improved behaviour makes the new discretisation more reliable for extensive application to real ice geometries. Higher accuracy and robust numerics are crucial in time dependent applications since numerical oscillations in the velocity field of subsequent time steps are attenuated and divergence of the solution is prevented

The orbits of subdwarf B + main-sequence binaries. I: The sdB+G0 system PG 1104+243

The predicted orbital period histogram of an sdB population is bimodal with a peak at short ( 250 days) periods. Observationally, there are many short-period sdB systems known, but only very few long-period sdB binaries are identified. As these predictions are based on poorly understood binary interaction processes, it is of prime importance to confront the predictions to observational data. In this contribution we aim to determine the absolute dimensions of the long-period sdB+MS binary system PG1104+243. High-resolution spectroscopy time-series were obtained with HERMES at the Mercator telescope at La Palma, and analyzed to obtain radial velocities of both components. Photometry from the literature was used to construct the spectral energy distribution (SED) of the binary. Atmosphere models were used to fit this SED and determine the surface gravity and temperature of both components. The gravitational redshift provided an independent confirmation of the surface gravity of the sdB component. An orbital period of 753 +- 3 d and a mass ratio of q = 0.637 +- 0.015 were found from the RV-curves. The sdB component has an effective temperature of Teff = 33500 +- 1200 K and a surface gravity of logg = 5.84 +- 0.08 dex, while the cool companion is found to be a G-type star with Teff = 5930 +- 160 K and logg = 4.29 +- 0.05 dex. Assuming a canonical mass of Msdb = 0.47 Msun, the MS component has a mass of 0.74 +- 0.07 Msun, and its Teff corresponds to what is expected for a terminal age main-sequence star with sub-solar metalicity. PG1104+243 is the first long-period sdB binary in which accurate physical parameters of both components could be determined, and the first sdB binary in which the gravitational redshift is measured. Furthermore, PG1104+243 is the first sdB+MS system that shows consistent evidence for being formed through stable Roche-lobe overflow.Comment: Accepted by A&A on 05-10-201

Partial survival and inelastic collapse for a randomly accelerated particle

We present an exact derivation of the survival probability of a randomly accelerated particle subject to partial absorption at the origin. We determine the persistence exponent and the amplitude associated to the decay of the survival probability at large times. For the problem of inelastic reflection at the origin, with coefficient of restitution $r$, we give a new derivation of the condition for inelastic collapse, $r<r_c=e^{-\pi/\sqrt{3}}$, and determine the persistence exponent exactly.Comment: 6 page

Unveiling the prehistoric landscape at Stonehenge through multi-receiver EMI

YesArchaeological research at Stonehenge (UK) is increasingly aimed at understanding the dynamic of the wider archaeological landscape. Through the application of state-of-the-art geophysical techniques, unprecedented insight is being gathered into the buried archaeological features of the area. However, applied survey techniques have rarely targeted natural soil variation, and the detailed knowledge of the palaeotopography is consequently less complete. In addition, metallic topsoil debris, scattered over different parts of the Stonehenge landscape, often impacts the interpretation of geophysical datasets. The research presented here demonstrates how a single multi-receiver electromagnetic induction (EMI) survey, conducted over a 22 ha area within the Stonehenge landscape, offers detailed insight into natural and anthropogenic soil variation at Stonehenge. The soil variations that were detected through recording the electrical and magnetic soil variability, shed light on the genesis of the landscape, and allow for a better definition of potential palaeoenvironmental and archaeological sampling locations. Based on the multi-layered dataset, a procedure was developed to remove the influence of topsoil metal from the survey data, which enabled a more straightforward identification of the detected archaeology. The results provide a robust basis for further geoarchaeological research, while potential to differentiate between modern soil disturbances and the underlying sub-surface variations can help in solving conservation and management issues. Through expanding this approach over the wider area, we aim at a fuller understanding of the human–landscape interactions that have shaped the Stonehenge landscape