Hans Van Winckel

A mid-infrared imaging catalogue of post-AGB stars ?

Post-asymptotic giant branch (post-AGB) stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the AGB towards the planetary nebula stage. There is growing evidence that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VLT spectrometer and imager for the mid-infrared (VISIR)/VLT, T-Recs/Gemini-South and Michelle/Gemini-North. We found that all the proto-planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non-resolved. The resolved targets can be divided into two categories. (i) The nebulae with a dense central core, that are either bipolar and multipolar and (ii) the nebulae with no central core, with an elliptical morphology. The dense central torus observed likely hosts binary systems which triggered fast outflows that shaped the nebulae.

A mid-infrared imaging catalogue of post-asymptotic giant branch stars★

Post-asymptotic giant branch (post-AGB) stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the AGB towards the planetary nebula stage. There is growing evidence that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VLT spectrometer and imager for the mid-infrared (VISIR)/VLT, T-Recs/Gemini-South and Michelle/Gemini-North. We found that all the proto-planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non-resolved. The resolved targets can be divided into two categories. (i) The nebulae with a dense central core, that are either bipolar and multipolar and (ii) the nebulae with no central core, with an elliptical morphology. The dense central torus observed likely hosts binary systems which triggered fast outflows that shaped the nebulae.

Post-AGB stars in the SMC as tracers of stellar evolution: the extreme s-process enrichment of the 21 μm star J004441.04-732136.4

Context. This paper is part of a larger project in which we want to focus on the still poorly understood asymptotic giant branch (AGB) third dredge-up processes and associated s-process nucleosynthesis. Aims. We confront accurate spectral abundance analyses of post-AGB stars in both the Magellanic Clouds, to state-of-the-art AGB model predictions. With this comparison we aim at improving our understanding of the 3rd dredge-up phenomena and their dependencies on initial mass and metallicity. Methods. Because of the well constrained distance with respect to Galactic post-AGB stars, we choose an extra-galactic post-AGB star for this contribution, namely the only known 21 μm object of the Small Magellanic Cloud (SMC): J004441.04-732136.4. We used optical UVES spectra to perform an accurate spectral abundance analysis. With photometric data of multiple catalogues we construct a spectral energy distribution (SED) and perform a variability analysis. The results are then compared to predictions of tailored theoretical chemical AGB evolutionary models for which we used two evolution codes. Results. Spectral abundance results reveal J004441.04-732136.4 to be one of the most s-process enriched objects found up to date, while the photospheric C/O ratio of 1.9 ± 0.7, shows the star is only modestly C-rich. J004441.04-732136.4 also displays a low [Fe/H] = −1.34 ± 0.32, which is significantly lower than the mean metallicity of the SMC. From the SED, a luminosity of 7600 ± 200 L is found, together with E(B − V) = 0.64 ± 0.02. According to evolutionary post-AGB tracks, the initial mass should be ≈1.3 M . The photometric variability shows a clear period of 97.6 ± 0.3 days. The detected C/O as well as the high s-process overabundances (e.g. [Y/Fe] = 2.15, [La/Fe] = 2.84) are hard to reconcile with the predictions. The chemical models also predict a high Pb abundance, which is not compatible with the detected spectrum, and a very high 12C/13C, which is not yet constrained by observations. The predictions are only marginally dependent on the evolution codes used. Conclusions. By virtue of their spectral types, favourable bolometric corrections as well as their constrained distances, post-AGB stars in external galaxies offer unprecedented tests to AGB nucleosynthesis and dredge-up predictions. We focus here on one object J004441.04-732136.4, which is the only known 21 μm source of the SMC. We show that our theoretical predictions match the s-process distribution, but fail in reproducing the detected high overabundances and predict a high Pb abundance which is not detected. Additionally, there remain serious problems in explaining the observed pulsational properties of this source.

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.

Binary central stars of planetary nebulae with long orbits: the radial velocity orbit of BD+33°2642 (PN G052.7+50.7) and the orbital motion of HD 112313 (PN LoTr5)

Aims. We study the impact of binary interaction processes on the evolution of low- and intermediate-mass stars using long-term monitoring of their radial velocity. Here we report on our results on the central stars of two planetary nebulae (PNe): the well-studied spectrophotometric standard BD+33 2642 (central star of PNG 052.7+50.7) and HD 112313 (central star of PN LoTr5), the optical light of which is dominated by a rapidly rotating G star. Methods. The high-resolution spectra were cross-correlated with carefully selected masks of spectral lines. The individual masks were optimised for the spectral signatures of the dominant contributor of the optical light. Results. We report on the first detection of orbital motion in these two objects. For BD+33 2642 we sampled 1.5 cycles of the 1105 24 day orbital period. For HD 112313 a full period is not yet covered, despite our 1807 days of monitoring. The radial-velocity amplitude shows that it is unlikely that the orbital plane is co-planar with the one defined by the nebular waist of the bipolar nebula. To our knowledge these are the first detections of orbits in PNe that are in a range from several weeks to a few years. Conclusions. The orbital properties and chemical composition of BD+33 2642 are similar to what is found in post-AGB binaries with circumbinary discs. The latter are probably progenitors of these PNe. For LoTr5 the Ba-rich central star and the long orbital period are similar to the Ba star giants, which hence serve as natural progeny. In contrast to the central star in LoTr5, normal Ba stars are slow rotators. The orbits of these systems have a low probability of occurrence according to recent population synthesis calculations.

Testing eccentricity pumping mechanisms to model eccentric long-period sdB binaries with MESA

Context. Hot subdwarf-B stars in long-period binaries are found to be on eccentric orbits, even though current binary-evolution theory predicts those objects to be circularised before the onset of Roche-lobe overflow (RLOF). Aims. We aim to find binary-evolution mechanisms that can explain these eccentric long-period orbits, and reproduce the currently observed period-eccentricity diagram. Methods. Three di erent processes are considered; tidally-enhanced wind mass-loss, phase-dependent RLOF on eccentric orbits and the interaction between a circumbinary disk and the binary. The binary module of the stellar-evolution code MESA (Modules for Experiments in Stellar Astrophysics) is extended to include the eccentricity-pumping processes. The e ects of di erent input parameters on the final period and eccentricity of a binary-evolution model are tested with MESA. Results. The end products of models with only tidally-enhanced wind mass-loss can indeed be eccentric, but these models need to lose too much mass, and invariably end up with a helium white dwarf that is too light to ignite helium. Within the tested parameter space, no sdBs in eccentric systems are formed. Phase-dependent RLOF can reintroduce eccentricity during RLOF, and could help to populate the short-period part of the period-eccentricity diagram. When phase-dependent RLOF is combined with eccentricity pumping via a circumbinary disk, the higher eccentricities can be reached as well. A remaining problem is that these models favour a distribution of higher eccentricities at lower periods, while the observed systems show the opposite. Conclusions. The models presented here are potentially capable of explaining the period-eccentricity distribution of long-period sdB binaries, but further theoretical work on the physical mechanisms is necessary.

Detailed abundance study of four s-process enriched post-AGB stars in the Large Magellanic Cloud

Context. The photospheric abundances of evolved solar-type stars of different metallicities serve as probes into stellar evolution theory. Aims. Stellar photospheres of post-asymptotic giant branch (post-AGB) stars bear witness to the internal chemical enrichment processes, integrated over their entire stellar evolution. He re we study post-AGB stars in the Large Magellanic Cloud (LMC). With their known distances, these rare objects are ideal tracers of AGB nucleosynthesis and dredge-up phenomena. Methods. We used the UVES spectrograph mounted on the Very Large Telescope (VLT) at the European Southern Observatory (ESO), to obtain high-resolution spectra with high signal- to-noise (S/N) of a sample of four post-AGB stars. The objects display a spectral energy distribution (SED) that indicates the pres ence of circumstellar dust. We perform a detailed abundance analysis on the basis of these spectra. Results. All objects are C-rich, and strongly enhanced in s-process elements. We deduced abundances of heavy s-process elements for all stars in the sample, and even found an indication of the presence of Hg in the spectrum of one object. The metallicity of all stars except J053253.51-695915.1 is considerably lower than the average value that is observed for the LMC. The derived luminosities show that we witness the late evolution of low-mass stars with initial masses close to 1 M⊙. An exception is J053253.51-695915.1 and we argue that this object is likely a binary. Conclusions. We confirmed the correlation between the effi ciency of the third-dredge up and the neutron exposure that is detected in Galactic post-AGB stars. The non-existence of a correlation between metallicity and neutron irradiation is also confirmed and expanded to smaller metallicities. We confirm the status of 2 1µm stars as post-Carbon stars. Current theoretical AGB models overestimate the observed C/O ratios and fail to reproduce the variety of s-process abundance patterns that is observed in otherwise very similar objects. Similar results have recently been found f or a post-AGB star in the Small Magellanic Cloud (SMC).

Establishing binarity amongst Galactic RV Tauri stars with a disc

This study is a contribution in comprehending the role of binarity upon late stages of stellar evolution. We determine the binary status of six Galactic RV Tauri stars, namely DY Ori, EP Lyr, HP Lyr, IRAS 17038-4815, IRAS 09144-4933 and TW Cam, which are surrounded by a dusty disc. We also place them on the HR diagram, thereby establishing their evolutionary nature. All the six Galactic RV Tauri stars included in this study are binaries with orbital periods ranging between

Mercator and the P7-2000 photometer

\sim

STUDIES OF VARIABILITY IN PROTO-PLANETARY NEBULAE. II. LIGHT AND VELOCITY CURVE ANALYSES OF IRAS 22272+5435 AND 22223+4327

650 and 1700 days and with eccentricities between 0.2 and 0.6. The mass functions range between 0.08 to 0.55 M