Circumstellar shells and mass loss rates: Clues to the evolution of S stars

It is the purpose of this paper to rediscuss the circumstellar properties of S stars and to put these properties in perspective with our current understanding of the evolutionary status of S stars, in particular the intrinsic/extrinsic dichotomy. Accordingly, an extensive data set probing the circumstellar environment of S stars (IRAS flux densities, maser emission, CO rotational lines) has been collected and critically evaluated. This data set combines new observations (9 stars have been observed in the CO J=2-1 line and 3 in the CO J=3-2 line, with four new detections) with existing material (all CO and maser observations of S stars published in the literature). The IRAS flux densities of S stars have been re-evaluated by co-adding the individual scans, in order to better handle the intrinsic variability of these stars in the IRAS bands, and possible contamination by Galactic cirrus. Mass loss rates or upper limits have been derived for all S stars observed in the CO rotational lines, and range from < 2 10^{-8} Msun y^{-1} for extrinsic S stars to 10^{-5} Msun y^{-1}. These mass-loss rates correlate well with the K - [12] color index, which probes the dust loss rate, provided that the mass loss rate be larger than 10^{-8} Msun~y^{-1}. Small mass-loss rates are found for extrinsic S stars, consistent with their not being so evolved (RGB or Early-AGB) as the Tc-rich S stars. This result does not support the claim often made in relation with symbiotic stars that binarity strongly enhances the mass-loss rate.Comment: Astronomy & Astrophysics Suppl., 40 pages, 22 figures, 6 tables (LaTeX). Also available at: http://astro.ulb.ac.be/Htm/ps.ht

Envelope tomography of long-period variable stars: I. The Schwarzschild mechanism and the Balmer emission lines

This paper is the first one in a series devoted to the study of the dynamics of the atmospheres of long-period variable stars. Results from a two-month-long monitoring of the Mira variables RT Cyg and X Oph around maximum light with the ELODIE spectrograph at the Haute-Provence Observatory are presented. The monitoring covers phases 0.80 to 1.16 for RT Cyg and phases 0.83 to 1.04 for X Oph. The cross-correlation profile of the spectrum of RT Cyg with a K0 III mask confirms that the absorption lines of RT Cyg in the optical domain appear double around maximum light. No line doubling was found in the optical spectrum of X Oph around maximum light, indicating that this feature is not common to all long-period variables. This paper also presents the application to RT Cyg of a new tomographic technique deriving the velocity field across the atmosphere by cross-correlating the optical spectrum with numerical masks constructed from synthetic spectra and probing layers of increasing depths. This technique reveals that both the temporal evolution of the line doubling, and its variation with depth in the atmosphere of RT Cyg, are consistent with the ``Schwarzschild scenario''. This scenario relates the temporal evolution of the red and blue peaks of the double absorption lines to the progression of a shock wave in the atmosphere. The temporal evolution of the Balmer Halpha, H beta, Hgamma and Hdelta emission lines around maximum light is also presented for RT Cyg and X Oph. The velocity variations of Halpha and of the absorption lines are discussed in the framework of two competing models for the formation of Balmer emission lines in long-period variable stars.Comment: 11 pages, 8 figures, Latex, accepted for publication in Astronomy and Astrophysics main journal. Also available at http://www-astro.ulb.ac.be/Html/ps.htm

Modelling the evolution and nucleosynthesis of carbon-enhanced metal-poor stars

We present the results of binary population simulations of carbon-enhanced metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers of the origin of CEMP stars, and conclude that the observed paucity of very nitrogen-rich stars puts strong constraints on possible modifications of the initial mass function at low metallicity. The large number fraction of CEMP stars may instead require much more efficient dredge-up from low-metallicity asymptotic giant branch stars.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU Symposium 252 "The Art of Modelling Stars in the 21st Century", April 6-11, 2008, Sanya, Chin

FUSE Measurements of Interstellar Fluorine

The source of fluorine is not well understood, although core-collapse supernovae, Wolf-Rayet stars, and asymptotic giant branch stars have been suggested. A search for evidence of the nu process during Type II supernovae is presented. Absorption from interstellar F I is seen in spectra of HD 208440 and HD 209339A acquired with the Far Ultraviolet Spectroscopic Explorer. In order to extract the column density for F I from the line at 954 A, absorption from H2 has to be modeled and then removed. Our analysis indicates that for H2 column densities less than about 3 x 10^20 cm^-2, the amount of F I can be determined from lambda 954. For these two sight lines, there is no clear indication for enhanced F abundances resulting from the nu process in a region shaped by past supernovae.Comment: 17 pages, 4 figures, accepted for publication in Ap

CH in stellar atmospheres: an extensive linelist

The advent of high-resolution spectrographs and detailed stellar atmosphere modelling has strengthened the need for accurate molecular data. Carbon-enhanced metal-poor (CEMP) stars spectra are interesting objects with which to study transitions from the CH molecule. We combine programs for spectral analysis of molecules and stellar-radiative transfer codes to build an extensive CH linelist, including predissociation broadening as well as newly identified levels. We show examples of strong predissociation CH lines in CEMP stars, and we stress the important role played by the CH features in the Bond-Neff feature depressing the spectra of barium stars by as much as 0.2 magnitudes in the $\lambda=$3000 -- 5500 \AA\ range. Because of the extreme thermodynamic conditions prevailing in stellar atmospheres (compared to the laboratory), molecular transitions with high energy levels can be observed. Stellar spectra can thus be used to constrain and improve molecular data.Comment: 33pages, 15 figures, accepted in A&A external data available at http://www.astro.ulb.ac.be/~spectrotools

Evidence for water-mediated mechanisms in coral-algal interactions

Although many coral reefs have shifted from coral-to-algal dominance, the consequence of such a transition for coral algal interactions and their underlying mechanisms remain poorly understood. At the microscale, it is unclear how diffusive boundary layers (DBLs) and surface oxygen concentrations at the coral algal interface vary with algal competitors and competitiveness. Using field observations and microsensor measurements in a flow chamber, we show that coral (massive Porites) interfaces with thick turf algae, macro algae, and cyanobacteria, which are successful competitors against coral in the field, are characterized by a thick DBL and hypoxia at night. In contrast, coral interfaces with crustose coralline algae, conspecifics, and thin turf algae, which are poorer competitors, have a thin DBL and low hypoxia at night. Furthermore, DBL thickness and hypoxia at the interface with turf decreased with increasing flow speed, but not when thick turf was upstream. Our results support the importance of water-mediated transport mechanisms in coral algal interactions. Shifts towards algal dominance, particularly dense assemblages, may lead to thicker DBLs, higher hypoxia, and higher concentrations of harmful metabolites and pathogens along coral borders, which in turn may facilitate algal overgrowth of live corals. These effects may be mediated by flow speed and orientation

Recent turbidite deposition in the eastern Atlantic: Early diagenesis and biotic recovery

An interface core taken in Capbreton canyon shows a succession of sedimentary facies interpreted as classical Bouma turbiditic sequences. Activities of 234Th and 210Pb suggest that the deposition of the most recent turbidite was triggered by the violent storm that affected the Atlantic coast of southern France on the 27th of December 1999, about four months before the sampling of the core. This turbidite allows us to study the ongoing diagenesis of the new sediment layer and of the previous sediment-water interface, which has been buried and only slightly eroded. A study of benthic foraminiferal populations informs us about the rate of benthic ecosystem recovery after such a major ecosystem disturbance event. The composition of the benthic foraminiferal fauna suggests that the benthic ecosystem in Capbreton canyon remains in an early stage of colonization. The rare agglutinant taxon Technitella melo appears to be the first colonizing species. It is suggested that Technitella melo is advantaged by the food-impoverished conditions in the days following turbidite deposition. Almost all of the turbidite layer and the previous oxic sediment-water interface contain reduced dissolved metal species and were anoxic. The buried interface contains Fe- and Mn-oxides inherited from its recent oxic past. The reduction of manganese oxides was in progress at the time of core collection. The reduced Mn remained trapped in the sediment as Mn-containing carbonates. Iron-oxides did not undergo significant reductive dissolution. The top of the newly deposited turbidite formed an oxic layer, which was rapidly enriched in metal-oxides. The enrichment of manganese oxides was mostly due to the oxidation of dissolved Mn2+, which diffused from below. The enrichment of iron oxides is explained both by the oxidation of the upward flux of dissolved Fe2+, and by the input of detrital iron oxide after, or as a result of the turbidite deposition

Benthic foraminifera as bio-indicators of drill cutting disposal in tropical east Atlantic outer shelf environments

We present a study of benthic foraminiferal faunas from the outer continental shelf off Congo (tropical West Africa), with the aim to determine the impact of the discharge of oily drill cuttings on the sea floor environment, to judge the regenerating capacity of the benthic ecosystem, and to investigate the possibility to develop an environmental monitoring method for open marine continental shelf environments, based on benthic foraminifera. We studied the spatial distribution and microhabitats of living and dead foraminiferal faunas, sampled in April 2003, 4 years after the end of disposal activities, in the upper 3 cm of the sediment at 9 stations (about 180 m depth) offshore Congo, that were subject to various degrees of pollution by oily cuttings from 1993 until 1999. Our results describe a zonation of foraminiferal faunas in the 750 m around the former disposal sites. At the immediate vicinity of the discharge points (within 70 m), faunas are characterized by low foraminiferal densities. Faunas between 70 m and 250 m of the disposal sites have very high foraminiferal densities, with high percentages (about 80%) of opportunistic taxa such as Bulimina aculeata, Bulimina marginata, Textularia sagittula, Trifarina bradyi and Bolivina spp. Between 250 and 750 m from the disposal site, foraminiferal densities decrease, and the percentages of opportunistic species are lower (40–60% of indicator species). These results show that 4 years after the cessation of oily cutting disposal, strong environmental impact is limited to the 250 m around the disposal sites. We used these data to develop a quantitative pollution index, values of which are strongly correlated to distance to the disposal site. This foraminiferal index offers the possibility to quantify the impact of anthropogenic eutrophication in continental shelf environments, but its validity must be tested in other continental shelf environments

The Unusual S Star Binary HD 191589

Recently, we discovered with International Ultraviolet Explorer (IUE) an F0-F2 IV-V companion to the T(sub c)-deficient S star HD 191589. If the magnitude difference is (delta)V=3.7, as indicated by several arguments, and E(B-V) = 0.0, we obtain a value of M(sub v)= - 1.5 +/- 0.4 for the Peculiar Red Giant (PRG), too faint for it to be a thermally-pulsing asymptotic giant branch star. According to the binary mass-transfer hypothesis for T(sub c)-deficient PRG's, a white dwarf must be the source of the s-process enhancement of the current primary star, but it cannot be seen because of the presence of the secondary. If such is the case, the F-star companion may also have been contaminated by s-process material. High-dispersion IUE observations indicate an enhancement of Zr II in the photosphere of the F-star as well. Thus, HD 191589 is likely a triple system, where what was once the most massive component of the system has polluted both of its companions with s-process material. One of these is the current S star, while the other is the companion still near the main sequence