Maarten Reyniers

A study of the s-process in the carbon-rich post-AGB stars IRAS 06530–0213 and IRAS 08143–4406 on the basis of VLT-UVES spectra

In an effort to extend the still limited sample of s-process enriched post-AGB stars, high-resolution, high signal- to-noise VLT+UVES spectra of the optical counterparts of the infrared sources IRAS 06530-0213 and IRAS 08143-4406 were analysed. The objects are moderately metal deficient by (Fe/H) = −0. 5a nd−0.4 respectively, carbon-rich and, above all, heavily s-process enhanced with a (ls/Fe) of 1.8 and 1.5 respectively. Especially the spectrum of IRAS 06530-0213 is dominated by transitions of s-process species, and therefore resembling the spectrum of IRAS 05341+0852, the most s-process enriched object known so far. The two objects are chemically very similar to the 21 µm objects discussed in Van Winckel & Reyniers (2000). A homogeneous comparison with the results of these objects reveals that the relation between the third dredge- up efficiency and the neutron nucleosynthesis efficiency found for the 21 µm objects, is further strengthened. On the other hand, a detailed comparison with the predictions of the latest AGB models indicates that the observed spread in nucleosynthesis efficiency is certainly intrinsic, and proves that different 13 C pockets are needed for stars with comparable mass and metallicity

Long term photometric monitoring with the Mercator telescope - Frequencies and mode identification of variable O-B stars

Aims. We selected a large sample of O-B stars that were considered as (candidate) slowly pulsating B, beta Cep, and Maia stars after the analysis of their hipparcos data. We analysed our new seven passband geneva data collected for these stars during the first three years of scientific operations of the mercator telescope. We performed a frequency analysis for 28 targets with more than 50 high-quality measurements to improve their variability classification. For the pulsating stars, we tried both to identify the modes and to search for rotationally split modes. Methods: We searched for frequencies in all the geneva passbands and colours by using two independent frequency analysis methods and we applied a 3.6 S/N-level criterion to locate the significant peaks in the periodograms. The modes were identified by applying the method of photometric amplitudes for which we calculated a large, homogeneous grid of equilibrium models to perform a pulsational stability analysis. When both the radius and the projected rotational velocity of an object are known, we determined a lower limit for the rotation frequency to estimate the expected frequency spacings in rotationally split pulsation modes. Results: We detected 61 frequencies, among which 33 are new. We classified 21 objects as pulsating variables (7 new confirmed pulsating stars, including 2 hybrid beta Cep/SPB stars), 6 as non-pulsating variables (binaries or spotted stars), and 1 as photometrically constant. All the Maia candidates were reclassified into other variability classes. We performed mode identification for the pulsating variables for the first time. The most probable l value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes, respectively, including only 4 unambiguous identifications. For 7 stars we cannot rule out that some of the observed frequencies belong to the same rotationally split mode. For 4 targets we may begin to resolve close frequency multiplets. Based on observations collected with the p7 photometer attached to the Flemish 1.2-m mercator telescope situated at the Roque de los Muchachos observatory on La Palma (Spain). Section [see full text], including Figs. is only available in electronic form at http://www.aanda.org, and Tables 2 and 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/243

The circumbinary disc around the J-type C-star IRAS 18006-3213

Context: In the generally accepted, but poorly documented model, silicate J-type C-stars are binary objects for which the silicate emission originates from a circumbinary or a circumcompanion disc. Aims: We aim at testing this hypothesis by a thorough spectral and spatial observational study of one object: IRAS 18006-3213 . Methods: We obtained, analysed and modeled high spatial resolution interferometric VLTI/MIDI observations on multiple baselines ranging from 45 m to 100 m. Results: All observations resolved the object and show the very compact nature of the N-band emission (~30 mas). In addition, the highest spatial resolution data show a significant differential phase jump around 8.3 mum. This demonstrates the asymmetric nature of the N-band emission. Moreover, the single telescope N-band spectrum shows the signature of highly processed silicate grains. These data are used to confirm the model on silicate J-type C-stars for IRAS 18006-3213 . We show that the most favourable model of the dust geometry is a stable circumbinary disc around the system, seen under an intermediate inclination. Conclusions: The data presented on the silicate J-type C-star IRAS 18006-3213 provide evidence that the oxygen rich dust is trapped in a circumbinary disc. The formation of this disc is probably linked to the binary nature of the central star. Based on observations collected at the European Southern Observatory, Chile. Program ID: 073.D-610(A).

Cerium: The lithium substitute in post-AGB stars ?

In this letter we present an alternative identification for the line detected in the spectra of s-process enriched low-mass post-AGB stars around 6708 A and which was interpreted in the literature as due to Li. Newly released line lists of lanthanide species reveal, however, the likely identification of the line to be due to a Ceii transition. We argue that this identification is consistent with the Ce abundance of all the objects discussed in the literature and conclude that in none of the low-mass s-process enriched post-AGB stars there is indication for Li-production.

HD 172481: A super lithium-rich metal-deficient post-AGB binary with a red AGB companion

We present in this paper a study on the peculiar supergiant HD 172481. Its spectral type (F2Ia), high galactic latitude ( b = -10 fraction of degree37), circumstellar dust, high radial velocity and moderate metal deficiency ([Fe/H] = -0.55) confirm the post-AGB character of this object. A detailed chemical analysis shows slight but real s -process overabundances, however no CNO-enhancement was detected. Furthermore, the spectral energy distribution and the TiO bands in the red part of the spectrum reveal a red luminous companion. The luminosity ratio of the hot F type component and this cool M type companion

AGB nucleosynthesis in the Large Magellanic Cloud. Detailed abundance analysis of the RV Tauri star MACHO 47.2496.8

L_{\rm F}/L_{\rm M}

V453 Oph: a s-process enriched, but carbon-deficient RV Tauri star of low intrinsic metallicity

is derived for a reddening of

Photometric multi-site campaign on the open cluster NGC 884 - I. Detection of the variable stars

E(B-V)=0.44

IRAS 08281-4850 and IRAS 14325-6428: two A-type post-AGB stars with s-process enrichment

(

Stellar population synthesis of post-AGB stars: the s-process in MACHO 47.2496.8

L_{\rm F}/L_{\rm M} \sim 1.8