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Obscured by their circumstellar dusty envelopes post-AGB stars emit a large fraction of their energy in the infrared and thus, infrared sky surveys like IRAS were essential for discoveries of post-AGBs in the past. Now, with the AKARI infrared sky survey we can extend our knowledge about the late stages of stellar evolution. The long-term goal of our work is to define new photometric criteria to distinguish new post-AGB candidates from the AKARI data.
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There is ample evidence for strong magnetic fields in the envelopes of (Post-)Asymptotic Giant Branch (AGB) stars as well as supergiant stars. The origin and role of these fields are still unclear. This paper updates the current status of magnetic field observations around AGB, post-AGB stars and describes their possible role during these stages of evolution. The discovery of magnetically aligned dust around a supergiant star is also highlighted. In our search for the origin of the magnetic fields, recent observations show the signatures of possible magnetic activity and rotation, indicating that the magnetic fields might be intrinsic to the AGB stars.
During the last years, many observational studies have revealed that binaries play an active role in the shaping of non spherical planetary nebulae. We review the different works that lead to the direct or indirect evidence for the presence of binary companions during the Asymptotic Giant Branch, proto-Planetary Nebula and Planetary Nebula phases. We also discuss how these binaries can influence the stellar evolution and possible future directions in the field.
It is now well established that FGK post-AGB stars that are surrounded by both hot and cold dust (as derived from the spectral energy distribution), are almost always part of a binary system with $100 < P_{orb} < 5000$~days. The properties and long-term stability of the dust emission requires it to arise from a gas- and dust-rich, puffed-up and (semi-)stable circumbinary disk. This interpretation has been confirmed with spatially resolved observations at a range of wavelengths for various individual objects. Here I present the first results of the first mid-IR interferometric survey of this class of objects. Our sample comprises 18 sources, most of which are confirmed binaries and which cover a range in IR excess. Our analysis clearly shows the compactness of the dust structures in these systems. We perform a statistical comparison with radiative transfer disk models, showing that most objects are indeed continuous disks from the sublimation radius outwards.
Post-AGB stars evolve on a very fast track and hence not many are known. Their spectral properties make them, in principle, ideal objects to test our theories on the late phases of stellar evolution. This has, however, proven much more difficult than anticipated, mainly because the morphological, dynamical and chemical diversity in Galactic post-AGB stars is very large indeed. Here I focus on recent results and touch upon the bright near future of post-AGB research.
Binary post-asymptotic giant branch (post-AGB) stars are thought to be the products of a strong but poorly-understood interaction during the AGB phase. The aim of this contribution is to update the orbital elements of a sample of galactic post-AGB binaries observed in a long-term radial-velocity monitoring campaign. Radial velocities are computed from high signal-to-noise spectra by use of a cross-correlation method. The radial-velocity curves are fitted by using both a least-squares algorithm and a Nelder-Mead simplex algorithm. We use a Monte Carlo method to compute uncertainties on the orbital elements. The resulting mass functions are used to derive a companion mass distribution by optimising the predicted to the observed cumulative mass-function distributions, after correcting for observational bias. As a result, we derive and update orbital elements for 33 galactic post-AGB binaries, among which 3 are new orbits. The orbital periods of the systems range from 100 to about 3000 days. Over 70 percent (23 out of 33) of our binaries have significant non-zero eccentricities ranging over all periods. Their orbits are non-circular despite the fact that the Roche-lobe radii are smaller than the maximum size of a typical AGB star and tidal circularisation should have been strong when the objects were on the AGB. We derive a distribution of companion masses that is peaked around 1.09 $M_odot$ with a standard deviation of 0.62 $M_odot$. The large spread in companion masses highlights the diversity of post-AGB binary systems. Furthermore, we find that only post-AGB stars with high effective temperatures (> 5500 K) in wide orbits are depleted in refractory elements, suggesting that re-accretion of material from a circumbinary disc is an ongoing process. It appears, however, that chemical depletion is inefficient for the closest orbits irrespective of the actual surface temperature.
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