aa r X i v : . [ a s t r o - ph . S R ] J a n Planetary Nebulae: Multi-Wavelength Probes of Stellar and Galac-tic EvolutionProceedings IAU Symposium No. 323, 2016X. Liu, L. Stanghellini, and A. Karakas, eds. c (cid:13) Post-AGB nebular studies
Eric Lagadec Universit´e Cˆote d’Azur, Observatoire de la Cˆote d’Azur, CNRS, Lagrange, Franceemail: [email protected]
Abstract.
This review presents the latest advances in the nebular studies of post-AGB objects.Post-AGB stars are great tools to test nucleosynthesis and evolution models for stars of low andintermediate masses, and the evolution of dust in harsh environment. I will present the newlydiscovered class of post-RGB stars, formed via binary interaction on the RGB. Binary systemscan also lead to the formation of two class of aspherical post-AGB, the Proto-Planetary Nebulaeand the naked post-AGBs (dusty RV Taus , a.k.a. Van Winckel’s stars).
Keywords. post-AGB stars
1. Introduction
In this review, I present the most recent results based on the study of post-AGB stars.For a thorough review of the post-AGB objects, I refer the reader to the very nice workby Hans van Winckel (Van Winckel (2003)).Post-AGB stars, as it can be guessed from their names, are evolved stars of low tointermediate initial masses, in the evolutionary phase after the Asymptotic Giant Branch.The beginning of the post-AGB phase is usually defined as when the envelope of theAGB star gets detached. The post-AGB phase ends when this envelope gets ionised bythe central star, which has shrunk and became hotter. Observationally, post-AGB starsare often identified by a double-peaked spectral energy distribution, with a peak in theoptical due to the contribution from stellar emission, and a peak at longer wavelengths,in the infrared, due to cool dust in the detached envelope. We will see in this review thatthis observational identification is changing.The advances in instrumentation, with new instruments/telescopes such as ALMA,SPHERE/VLT and the VLTI enable us to probe deep in the core of post-AGB objectsand study them intensively. We can now map the different components of these objects(discs/tori, jets, molecular envelope, shocks) with great details down to a few milliarc-seconds using either narrowband filters or integral field spectroscopy. One of the nicestillustrations for this is the impressive work by Gledhill & Forde(2015), where they map aspherically symmetric ionised envelope (in Br γ ) expanding in a bipolar neutral gas enve-lope (in H ). I will review here how these advances are helping us to study these objects,which are great test benches for nucleosynthesis and the timing of stellar evolution, theevolution of gas and dust in a harsh radiation field and the understanding of the shapingof non-spherical planetary nebulae.
2. Finding post-AGB stars
As the central star quickly shrinks and get hotter during the pos-AGB phase, thisphase of stellar evolution is fast, making post-AGB stars rather rare (less than 500 post-AGB stars are known). As the temperature of the central star quickly increase duringthis phase, post-AGB stars harbour spectral types from B to M. As they are embedded1 Eric Lagadecin the dusty envelope formed during the AGB phase, they are highly obscured. All thismakes it very difficult to find post-AGB stars.The success of the IRAS mission was a great leap forward for the field, as post-AGBstars were identified as luminous object with infrared excess. IRAS colour-colour diagrams(particularly the ([12]-[25] vs [25]-[60]) were thus widely used for follow-up studies toidentify stars located between the AGB and the PN phases. Post-AGB objects werealso found by correlating optical catalogues and infrared ones looking for bright opticalobjects with an infrared excess (Van de Steene et al. 2000 and references therein).As the post-AGB objects have circumstellar environment very similar to other dustyobjects, confusion can occur, mostly with Young Stellar Objects or massive evolved stars(post-Red SuperGiants). A very useful resource to study post-AGB objects is the online,evolving Torun catalogue of post-AGB objects (Szczerba et al. 2012).
3. Post-RGB stars
The success of IRAS in the 80s lead to the discovery of many Galactic post-AGBs.The
Spitzer Space Telescope mission now enables the study of extragalactic post-AGBstars. Studying post-AGB stars in different galaxies can bring a lot of new insights,despite the distances to the objects. With well known distance to the host galaxies,it is straightforward to determine the luminosities of the studied objects. With nearbygalaxies such as the Magellanic Clouds that an overall lower metallicity than the MilkyWay, one can also study the effect of metallicity on the late stages of the evolution of lowand intermediate mass stars.A very nice study of post-AGB objects in the Large and the Small Magellanic clouds ispresented by Kamath et al.(2014) and Kamath et al.(2015). Their work combines pho-tometry from Spitzer (to look for IR excess due to dust) and ground-based optical spec-troscopy (to suppress contaminants). This led to the identification of 63 post-AGB can-didates in the SMC and 154 in the LMC.As the distances to these objects are well constrained, which is not the case for mostof the Galactic post-AGBs, they estimated their luminosities. Surprisingly, most of theobjects appear to be emerging from the Red Giant Branch, rather than from the Asymp-totic Giant Branch. It is very likely that these object have evolved blue-wards beforereaching the AGB phase. Kamath et al.(2016) suggest that these objects are the resultsof binary interaction, which led to the ejection of the star’s envelope before it could reachthe AGB phase. They thus identified a new class of objects, the post-RGB stars. Theseobjects certainly have a Spectral Energy Distribution (SED) consistent with the presenceof a disc due to the presence of a binary.The SEDs of these binary objects are different to the ones classically assumed forpost-AGB objects with a double peak (a blue peak due to emission from the star and aninfrared peak due to dust emission from the shell). If a disc is formed via the interactionwith the companion, warm dust will form and be trapped in the disc, leading to a near-infrared excess, filling the gap between the two peak of the SED. Such a shape of theSED is observed for the dusty RV Tau objects (a.k.a. the Van Winckel’s stars), whichare binary systems on the post-AGB with a stable dusty disc and no visible nebulae(van Winckel et al.(2009)).
4. What can we learn from post-AGB stars studies?
As we just mentioned, post-AGB objects are not easy to identify but can be key objectsto understand stellar evolution and a large variety of physical processes. Determining the ost-AGB nebular studies
Test for nucleosynthesis and stellar evolution
As stated before, post-AGBs are ideal to study the products of nucleosynthesis on theAGB (as they suffer from less molecular absorption). Studying post-AGB stars in theSMC and LMC can help understand the effect of metallicity on nucleosynthesis. Forexample, a discrepancy appears for Galactic objects between models and observationsfor the abundance of lead (one of the product of s-process nucleosynthesis on the AGB).De Smedt et al.(2014) have shown that this discrepancy is even larger at low metallicity.Neutron capture with neutron densities between s- and r- neutron capture processes mayprovide an explanation to this discrepancy (Lugaro 2015). A recent study of post AGBstars in the LMC also confirms a correlation between the third dredge-up efficiency andthe neutron exposure (van Aarle et al.(2013)).It has also been shown recently that some meteorites have isotopic ratios consistentwith a post-AGB origin (Jadhav 2013) with a composition similar to the prediction forthe hydrogen injection phase during a Very Late Thermal Pulse (a thermal pulse duringthe post-AGB phase).Finally, and this came out to be one of the most important results from this sym-posium, the study of post-AGB stars have revealed that the timing of stellar evolutionwas certainly wrong. A study of PNe in the Galactic Bulge (Gesicki 2014) has shownthat Bloecker’s evolution models are too slow to explain the presence of PNe aroundsuch low mass stars as found in the Bulge (the models takes too much time to reach atemperature high enough for the gas to be ionised). The predicted final stellar massesare also higher than what is measured for White Dwarfs or measurements from astero-seismology. Bloecker’s post-AGB evolution models need to be accelerated by a factor of3. This helps produce new stellar evolutionnary models, that should replace Bloecker’smodels (Miller Bertolami(2016)).4.2.
Dust evolution after the AGB
At the end of the AGB, the envelope gets detached, so that the dust goes further awayfrom the star and gets cooler. At the same time, the star becomes hotter and the dustis exposed to a harsher radiation field. The presence of a companion can also lead tothe formation of a disc or a torus and dust processing. All these factors lead to a verycomplex chemistry on the post-AGB, with very complex components observed such asPAHs, fullerenes, and the unidentified 21 microns feature. The most recent knowledgeabout dust evolution during the post-AGB phase has been obtained thanks to the studyof objects in the Magellanic Clouds. A very thorough work by Sloan et al.(2014) usinginfrared spectra from the
Spitzer Space Telescope has identified 5 classes of objects clearly Eric Lagadecclustered in color-color diagrams. The difference of chemistry is thus likely linked to adifference in density that could be linked to the presence of discs/torii. A morphologicalstudy of these objects would be necessary to understand their dust properties.These studies however enabled a better understanding of the dust/gas compositionafter the AGB. For example, the so-called 21 micron feature, an unidentified broad featureseen only in carbon-rich post-AGB stars seems to be very common at low metallicity(Volk et al. 2011) and to be always associated with objects harbouring PAHs in theirspectra. Thus, Cerrigone et al.(2011) proposed that it was associated with hydrocarbons.Sloan et al.(2014) showed that it was always associated with two unidentified featuresat 15.8 and 17.1 microns. They showed that these two features were clearly linked toalkynes, confirming that the 21 micron feature is associated with hydrocarbons. Finally,stars harbouring the 21 micron feature have a special class of newly defined PAHs (Sloanet al., 2014), and seem to have a line of sight to the central star is free of dust absorption.
Spitzer spectroscopy of post-AGB stars in the Magellanic Clouds also revealed a new classof PAHs (Matsuura et al. 2014), with a peak at 7.7 microns. They also showed that theobserved class of PAHs for a given object was linked to its evolutionary state rather thanits metallicity, i.e. that the different kinds of PAHs observed is due to the dust densityand central star’s radiation field rather than to its initial composition.Finally, an unexpected discovery from the
Spitzer surveys of post-AGB stars in theMagellanic Clouds was the presence of very strong and broad emission features around11.3 microns. A very common feature due to SiC is usually observed at this wavelengthbut AGB studies have shown that the strength of this feature tend to decrease withmetallicity (Sloan et al. 2006; Lagadec et al.(2007)), as less Si is available in low Z envi-ronments to produce silicon carbide. One would thus expect post-AGBs and PNe in theMagellanic Clouds to have a rather weak SiC emission. But these lower metallicity ob-jects harbour more prominent 11.3 microns features than in the Galaxy (where less thana handful of PNe with SiC are known). This 11.3 microns feature could be due to PAHsemission, but its shape is fully consistent with SiC (Sloan et al.(2014)). SiC can indeedbe less abundant but a bright SiC feature can be observed if the condensation sequenceis different at low Z, leading to grain coating, with SiC forming on top of amorphouscarbon grains (Sloan et al.(2014); Lagadec et al. 2007; Leisenring et al. 2008).
5. Binary post-AGB stars and shaping
Determining the morphology of post-AGB objects
While most of the stars on the RGB and the AGB are thought to be more ore lessspherical, Planetary Nebulae (PNe) can harbour a wide variety of shapes and be elliptical,bipolar or multipolar. It is also widely accepted that the shaping of these objects is likelyto occur during the post-AGB phase and that binarity and magnetic field (sustained bya companion) are important shaping agents.The observed morphology of the objects appears as projected on the sky, making itdifficult to know their intrinsic shape. In a recent work, Koning et al.(2013) constructeda model for post-AGB nebulae based on a pair of hollow cavities in a spherical dustenvelope. They were able to reproduce most of the observed shapes by only changing theorientation and dust densities. One thus has to keep that in mind when trying to studythe morphology of an object.Another difficulty arises when one wants to determine the morphology of an object isthe wavelength-dependence induced by radiative transfer effects. This is very well illus-trated by a study of the highly aspherical post-AGB star HD 161796 by Min et al.(2013). ost-AGB nebular studies
Binaries as shaping agents
Hydrodynamical models explain many of the observed aspherical structures from astructure-magnification mechanism, where a fast wind from the smaller, hotter, cen-tral star ploughs into the earlier slow, dense, AGB wind (Kwok et al. 1978) and amplifiesany density asymmetry already present (Balick et al. 1987): the Generalised InteractingStellar Wind model or GISW. Fast, collimated and precessing jets could also explain theformation of multipolar nebulae , not explained by the GISW model (Sahai & Trauger1998). The presence of jets was confirmed by a study of PPNe by Bujarrabal et al.(2001).They found that in about 80% of the PPNe, the momentum of the outflowingmaterial is too high to be powered by radiation pressure only. An extra source of angularmomentum is thus needed to explain the presence of these jets. To explain the formationof jets, two kinds of models have been proposed, implying either a magnetic field or abianry companion, leading to a long debate in the PN community. Noam Soker and JasonNordhaus settled the debate by showing that magnetic fields could play an importantrole, but a single star could not supply enough angular momentum to shape the nebulae:a companion is needed (Soker 2006; Nordhaus & Blackman 2006).More and more binary systems are being discovered in PNe (see the contributionby David Jones to these proceedings (Jones 2016). The detection of binaries in post-AGB systems is made complex by the pulsation of the central stars and their dustyenvelopes. Binaries have however been discovered in two emblematic bipolar post-AGBs,OH 231.8+4.2 (G´omez & Rodr´ıguez 2001) and the Red Rectangle (Waelkens et al. 1996).A long lasting quest for binaries in post-AGBs has been performed by Bruce Hrivnakand his undergraduate students at Valparaiso University, with radial velocity monitoringsince 1994. They might have detected a binary system with P >
22 years (Hrivnak et al.2011). Binaries are also being discovered by Hans van Winckel and his collaborators,mostly in dusty RV Tau stars, that certainly form a special class of post-AGB stars, aswe will discuss now.
6. Two classes of post-AGB objects
It is indeed important to make the distinction between two kind of observed post-AGBobjects. Proto-Planetary Nebulae are post-AGB objects that (as their name indicates)will from PNe, but not all post-AGB objects will form PNe.6.1.
Proto-Planetary Nebulae
PPNe have nebulae visible in reflection in the optical and emission in the infrared. Themost bipolar PPNe are often associated with massive central torii (masses of the orderof about a solar mass or more: Lagadec et al. 2006). They have a low expansion velocity(typically a few km/s: Peretto et al. 2007. They have a small angular momentum and Eric Lagadecexpand radially (i.e. they have almost no rotation). If the mass loss that created themstops, they will vanish rapidly.In the recent years, very interesting studies of such objects have been obtained, inparticular thanks to new instruments like ALMA, that enable the study of gas kine-matics at very high angular resolution, and thus a better understanding of the forma-tion of torii/jets. Sahai et al.(2013) resolved a hourglass-shaped torus in the core of theBoomerang nebula using ALMA CO observations. They found this torus to be a hollowcavity with very thick walls of gas and dust. This torus is surrounded by a roughly round,but patchy, high velocity outflow. This outflow appears to be ultra-cool, cooler than thecosmic microwave background, so that they dubbed this object the coolest object in theuniverse (PPNe are very cool objects to study!). Using the SMA, Lee et al.(2013) spa-tially resolved the CO gas around the PPN CRL 618. The circumstellar environmentsconsist of a dense equatorial torus and different fast molecular outflows roughly perpen-dicular to it. Two episodes of bullet-like ejection are needed to explain these outflows,that clearly are not long lived jets. Similar structures were found in the Water FountainNebula (Sahai et al. 2016). The magneto-rotational explosion model of Matt et al.(2006)seems to be a good explanation for these bullet like outflows.That leads us to the study of magnetic fields at the surface of these objects. For a thor-ough review, I will refer the reader to the contribution to this proceedings by LaurenceSabin. More and more magnetic fields measurements have been lately obtained for post-AGB stars. One of the most impressive one is the detection of a magnetically collimatedjet in the water fountain post-AGB IRAS 15445 − ∼ Naked post-AGB stars
Many post-AGB stars are also of the dusty RV Tau type, also called naked post-AGBstars, discs post-AGB stars or the Van Winckel’s objects. They harbour a near-infraredexcess in their SED, due to the presence of dust near the sublimation temperature,in a compact (R ∼
10 AU) disc. This was confirmed by their infrared interferometricmeasurements (Deroo et al. 2007). These discs are long lived, as indicated by crys-talline dust, have a very small aperture angle and a Keplerian kinematics (see e.g.Bujarrabal et al.(2013)[Bujarrabal et al. 2013]; Bujarrabal et al. 2015. Their lifetimes ost-AGB nebular studies
Naked vs clothed post-AGBs
As described in the previous sections, two very distinct classes of post-AGB stars areobserved, the dusty RV Taus (Naked post-AGBs) and the PPNe. Naked post-AGBs allhave dusty Keplerian discs (with almost no gas), crystalline dust, binaries but no long-lived nebulae. They are thus very unlikely to form PNe, as when the central stars willbecome hot enough for the ionisation to occur, no gas will be present around them. PPNeemerging from binary systems have slowly, radially, expanding torii (no Keplerian rota-tion), and contain amorphous dust and gas. They are short-lived. One of the explanationfor the naked post-AGB stars is that the configuration of the binary system led to gasaccretion from the central star. It then does not lose mass on the post-AGB and re-mains cold longer. There are no ionising photons while gas is present in the circumstellarenvironment and thus no PN (O. De Marco, private communication).
7. Conclusion and perpectives
Post-AGB stars are great tools to test nucleosynthesis and evolution models for starsof low and intermediate masses. The post-AGB phase starting by definition when theenvelope of an AGB star gets detached and the star becomes hotter. They are also greattest-benches for the study of the evolution of dust in harsh environments of differentdensities and harbour a very larger variety of complex molecules and dust features suchas PAHs, crystaline and amorphous dust. They are also not easy to identify as the post-AGB phase is short and the stars are embedded in dust. If they are in close binarysystems, the envelopes can be ejected before the AGB phase, leading to the formationof objects of a newly discovered class: the post-RGB stars. If binary systems manageto evolve till the end of the AGB without ejecting the envelope, they can lead to theformation of bipolar post-AGB stars. Two kinds of aspherical post-AGB object can beformed. PPNe harbour a dense torus and will become bipolar planetary nebulae. Nakedpost-AGBs harbour Keplerian discs and no visible nebulae and will not form PNe. Finally,the recent advances in observing techniques with high angular resolution now enable usto map circumbinary discs around post-AGB stars and study in real-time the evolutionof the discs and the binaries. These high angular resolution techniques also show that,contrarily to what has been thought for decades, the shaping of aspherical nebulae doesnot necessarily start during the post-AGB phase, as more and more AGB stars appear tobe aspherical. This was demonstrated very strikingly by the observations of the nearbyAGB star L Pup using the extreme adaptive optics system SPHERE on the VLT, whichrevealed the clear presence of a disc, outflows perpendicular to it and a companion to Eric Lagadecthe central star (Kervella et al.(2015)). We might thus have to study stars on the AGBphase rather than on the post-AGB to understand the beautiful shapes of PNe.
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