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.
We present Spitzer IRS spectra of four carbon stars located in the Galactic Halo and the thick disc. The spectra display typical features of carbon stars with SiC dust emission and C$_2$H$_2$ molecular absorption. Dust radiative transfer models and i
nfrared colors enable us to determine the dust production rates for these stars whilst prior CO measurements yield expansion velocities and total mass-loss rates. The gas properties (low expansion velocities (around 7 km/s) and strong C$_2$H$_2$ molecular absorption bands) are consistent with the stars being metal-poor. However the dust content of these stars (strong SiC emission bands) is very similar to what is observed in metal-rich carbon stars. The strong SiC emission may indicate that the carbon stars derive from a metal-rich population, or that these AGB stars produce silicon. The origin of the halo carbon stars is not known. They may be extrinsinc halo stars belonging to the halo population, they may have been accreted from a satellite galaxy such as the Sagittarius Dwarf Spheroidal Galaxy, or they may be escapees from the galactic disk. If the stars are intrinsically metal-rich, an origin in the disc would be most likely. If an $alpha$-element enhancement can be confirmed, it would argue for an origin in the halo (which is known to be $alpha$-enhanced) or a Galactic satellite.
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 Asymptotic Giant Branch (AGB) towards the Planetary Nebula stage. There is growing evidences 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 VISIR/VLT, T-Recs/Gemini South and Michelle/Gemini North. We found that all the 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 in two categories. The nebulae with a dense central core, that are either bipolar and multipolar. The nebulae with no central core have an elliptical morphology.The dense central torus observed likely host binary systems which triggered fast outflows that shaped the nebulae.
We report the detection, from observations using the James Clerk Maxwell Telescope, of CO J $=$ 3$to$ 2 transition lines in six carbon stars, selected as members of the Galactic Halo and having similar infrared colors. Just one Halo star had been det
ected in CO before this work. Infrared observations show that these stars are red (J-K $>$3), due to the presence of large dusty circumstellar envelopes. Radiative transfer models indicates that these stars are losing mass with rather large dust mass-loss rates in the range 1--3.3 $times$$10^{-8}$M$_{odot}$yr$^{-1}$, similar to what can be observed in the Galactic disc. We show that two of these stars are effectively in the Halo, one is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr dSph), and the other three stars certainly belong to the thick disc. The wind expansion velocities of the observed stars are low compared to carbon stars in the thin disc and are lower for the stars in the Halo and the Sgr dSph stream than in the thick disc. We discuss the possibility that the low expansion velocities result from the low metallicity of the Halo carbon stars. This implies that metal-poor carbon stars lose mass at a rate similar to metal-rich carbon stars, but with lower expansion velocities, as predicted by recent theoretical models. This result implies that the current estimates of mass-loss rates from carbon stars in Local Group galaxies will have to be reconsidered.
We present spectroscopic observations from the {it Spitzer Space Telescope} of six carbon-rich AGB stars in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C$_2$H$_2$ and
SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the LMC, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the DUSTY radiative transfer model and determine their dust mass-loss rates to be in the range 1.0--3.3$times 10^{-8} $M$_{odot}$yr$^{-1}$. The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars show the strongest SiC feature in our present Local Group sample.
The driving mechanism of the AGB superwind has become controversial in recent years. The efficacy of dust-driven mass loss has been queried. Spitzer observation of AGB stars in Local Group Galaxies show the surprising result that at low metallicity,
AGB mass loss occurs at low luminosity, possibly lower than in the Galaxy, but only for carbon-rich stars. Oxygen-rich stars in the Galaxy and in lower metallicity galaxies have similar mass-loss rates only at high luminosities. To explain this dichotomy, we propose that the superwind has a dual trigger. The superwind starts either when sufficient excess carbon builds up for efficient formation of carbonaceous dust (which we propose occurs when $X_{rm CO} = rm (C - O)/ O_odot = 0.1$), or when the luminosity reaches a value sufficient for a silicate-dust-driven wind (proposed at $L = 10^4 Z^{-4/3} rm L_odot)$. We show that this dual trigger fits the current observational constraints: the luminosity at which the superwind begins, and the predominance of carbon superwind star at low metallicity. We use stellar evolution models to check the consistency of our explanations and present detailed predictions of the luminosities at which the superwind is triggered for different metallicities and initial stellar masses.
To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies with the 10-$mu$
m camera VISIR at the VLT. We derive mass-loss rates for 29 AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated from the $K-[9]$ and $K-[11]$ colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between $K-[12]$ colour and dust mass-loss rates is observed for AGB stars from galaxies with different metallicities, i.e. the Galaxy, the LMC and the SMC. The derived dust mass-loss rates are in the range 5$times10^{-10}$ to 3$times10^{-8}$ M$_{odot}$yr$^{-1}$ for the observed AGB stars in Sgr dSph and around 5$times10^{-9}$ M$_{odot}$yr$^{-1}$ for those in Fornax; while values obtained with the two different methods are of the same order of magnitude. The mass-loss rates for these stars are higher than the nuclear burning rates, so they will terminate their AGB phase by the depletion of their stellar mantles before their core can grow significantly. Some observed stars have lower mass-loss rates than the minimum value predicted by theoretical models.
The high angular resolution and dynamic range achieved by the NACO adaptive optics system on the VLT is an excellent tool to study the morphology of Planetary Nebulae (PNe). We observed four stars in different evolutionary stages from the AGB to the
PNe phase. The images of the inner parts of the PN Hen 2-113 reveal the presence of a dusty torus tilted with respect to all the other structures of the nebula and the present of hot dust close to the hot central star. The NACO observations of Roberts 22 reveal an amazingly complex nebular morphology with a S-shape that can be interpreted in terms of the warped disc scenario of Icke (2003). Combined NACO and MIDI (the VLTI mid-infrared interferometer) observations of the nebula OH 231.8+4.2 have enabled us to resolve a very compact (diameter of 30-40 mas, corresponding to 40-50 a.u.) dusty structure in the core of the nebula. Finally, recent observations of the AGB star V Hydrae show that this star present a departure from spherical symmetry in its inner shell and is probably on its way to become an asymmetrical planetary nebula. These observations show that NACO is a great instrument for the discovery and study of small structures in circumstellar envelopes and PNe and a good complement to interferometric devices.
