University of Groningen

Evolved with circumstellar shells Oudmaijer, René Dick

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Publication date: 1995

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Download date: 25-09-2021 1 Introduction and outline of this thesis

1 Summary This thesis consists of two related topics. The ®rst part concerns the study of post- objects that are low to intermediate mass stars that have evolved off the AGB and are now in the transition to the phase. The second part of the thesis involves IRC +10420 , an object that was originally proposed to be a post- AGB , but is nowadays thought to be a post-Red Supergiant Branch object. This is an evolutionary phase of massive objects similar to the post-AGB stage. IRC +10420 is the only such known object, and therefore important for the study of the late stages of evolution of massive stars. In this introduction I will brie¯y discuss the background of the study of post-AGB stars and IRC +10420 , present a historical overview, discuss the current status of research in the ®eld of post-AGB evolution and give the outline of this thesis.

2 What is a post-AGB star? Post Asymptotic Giant Branch (post-AGB) stars are objects in the short-lived phase between the AGB and the Planetary Nebula (PN) stage. A star spends most of its lifetime on the during which it is burning hydrogen in its core. When the hydrogen in the core is exhausted, it is ignited in a shell around the core. At this point, the star evolves towards the Red Giant Branch, where its can reach values of a few hundred to a few thousand times solar. The helium formed in the hydrogen burning shell is dumped onto the core, increasing its pressure and temperature until helium is ignited. The star then evolves towards the Horizontal Branch, where the star is hotter and somewhat less luminous than on the Red Giant Branch. After some time, the heliumin the core gets exhausted as well, and helium shell burning provides the energy to prevent the star from collapsing. The star now evolves towards the ªEarly Asymptotic Giant Branchº. When there is not enough helium in the shell for helium fusion, hydrogen burning in a shell around the previous helium burning shell takes over, until enough helium has been formed, and the pressure has become large enough in the bottom of the shell to ignite the helium with a ª¯ashº. Then the story repeats itself: when the helium in the shell 12 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS

Figure 1 Theoretical evolutionary tracks for intermediate mass drawn in the HR diagram. The post-AGB tracks are taken from SchÈonberner 1983, the evolution from the Main Sequence to the AGB is taken from Schaller et al. 1992 (Figure provided by Bakker, 1995).

is exhausted, the hydrogen burning will take over again. This marks the beginning of a series of thermal pulses, phases of alternate H and He-shell burning. This phase is called ªThermal Pulsing AGBº, or most often simply called the AGB. In this phase the stars reach of around 1000 - 55000 times solar, depending on the initial mass, and

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undergo severe mass loss at rates in excess of 10 M /yr. At the moment when the envelope mass reaches a critically small value, the large mass loss rates decrease to much lower values marking the start of the post-AGB phase, or proto-planetary nebula phase. The central star contracts at constant luminosity and hence starts to increase in . When the star has become hot enough to ionize its circumstellar shell, it may become observable as a Planetary Nebula (PN). The

transition time from the AGB to a PN is estimated to be a few thousand . Almost all 

stars that are formed with a mass of less than 8M evolve through the AGB - post- AGB - PN sequence to end as White Dwarfs. Recent reviews on AGB stars, post-AGB stars and PNe are found respectively in Zijlstra (1995) on AGB stars, Waters et al. (1993) and Kwok (1993) on post-AGB stars, and Pottasch (1992) on PNe. WHAT IS A POST-AGB STAR?13

Table 1 Characteristic timescales (in years) during the evolution of a 1 and 5 M star

Initial mass ( M )1 5

10 8  Main Sequence 1  10 1 10

9 6  Red giant branch 3  10 3 10

8 7  Horizontal branch 1  10 2 10

7 6  Early AGB 1  10 1 10

5 5  TP - AGB 5  10 3 10

Final mass ( M ) 0.568 0.891 Data taken from Vassiliadis and Wood (1993)

Although the general picture of this evolution is relatively well understood, many topics are still unknown. For example, the post-AGB phase is not well understood, mainly because the mass loss rates, which dominate the evolution of low- to intermediate mass stars in the post-AGB phase, have not yet been well determined either theoretically or observationally. Although the mass loss rates can be determined for individual AGB stars relatively accurately, the overall behaviour of mass loss on the AGB is still a matter of debate (cf. the mass loss rates adopted by BlÈocker (1995a) and Vassiliadis and Wood (1993). Even today, it is still not known whether cool post-AGB stars lose mass at all (cf. Chapter 3 and 4). However, the timescale of the transition from the AGB to the PN phase is very sensitive to the actual mass loss rates. For instance, Trams et al. (1989) showed that when the adopted post-AGB mass loss rates for a low mass post-AGB star are raised by

7

only a factor of 5-10 to 10 M /yr, the transition time from the AGB to the PN phase is shortened from 100,000 years to only 5000 years. This would make a low mass star readily observable as a PN, while the (longer) predicted timescales would prevent such objects to become an observable PN, since the circumstellar shell would have moved far away from the star and dispersed into the interstellar medium long ago. The results of evolutionary calculations can only be improved by input from observations of real post-AGB stars. These studies can provide information on the mass loss rates, and when complete samples of post-AGB stars become available, the evolutionary rates may be derived independently from the observations. The study of evolved stars has its feedback on other branches in astronomy. AGB stars, post-AGB stars and Planetary Nebulae constitutea large fraction of the total energy output of , and make an important contribution to the mass cycle in galaxies through the mass loss. It is in the late AGB/post-AGB phase that the products of hydrogen and helium burning (notably carbon, nitrogen, oxygen, the s-process elements) are ejected into the interstellar medium, together with newly synthesised dust, which will eventually be used for the formation of new stars. 14 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS

The late stages of stellar evolution of low- to intermediate stars are important for the study of evolution in general. In the studies of galaxy evolution,stellar evolutionary calculations available in the literatureare used (see e.g. van den Hoek 1995). Especially the timescales of the post-AGB evolution have implications for population synthesis models; if most of the transition time is spent as a cool object, more energy will be radiated at optical wavelengths, while a longer time spent as a hot object will generate more UV output. Post-AGB stars may explain the observed UV excess emission observed in elliptical galaxies (cf. Binette et al. 1994). These studies will bene®t greatly from a better understanding of the later stages of the post-AGB evolution.

3 Historical overview

In the 18th century Messier (1784) compiled a catalog of nebulous objects. Higher res- olution observations showed that many of these nebulae could be resolved into stars and were mostly galaxies and Globular Clusters. Three objects however remained nebulous, because of their hazy, yet circular appearance, roughly like the planets, Herschel baptized these objects Planetary Nebulae (Herschel, 1791). The ®rst recorded spectrum of a PN was obtained by Huggins (1864). PN appeared to have spectral emission lines, whilenormal stars only showed absorption lines in their spec- tra. It was soon understood that the emission lines originated in a tenuous gas surrounding the central star. The rich emission line spectra gave rise to signi®cant progress in the ®eld of astro- physics of tenuous atmospheres. Bowen (1927) was able to identify the unknown emission lines in the spectra at 5007 and 4959 AÊ (originally attributed to a new element named `Nebulium', because it was ®rst found in PN) as forbidden lines of doubly ionized oxygen. PN appeared to be a rare species among the stars. Around the turn of the century only a few hundred objects were known, and even today the number of PNe amounts only to

 1500 (Acker et al. 1992). Two hypotheses for the nature of PNe were put forward at the beginning of this century; the ®rst one was that PN occurred only for one in every million stars, the second hypothesis was that the PN phase occurred for almost every star, but did not last very long. This question was solved several years later, when improved spectrographs made it possible to spectrally resolve the emission lines. At ®rst the double peaked emission lines in the spectra of PN (®rst described by Campbell and Moore, 1918) were thought to be the result of self absorption, but when Bowen (1927) showed that some of these lines were optically thin (forbidden), the line separation appeared the result from an out¯ow. Using this information, Zanstra (1931) proved that the nebulae expanded with

1 velocities of  10 - 20 km s from the central star. From the velocity and diameter of the nebula a crude estimate could be made of the `lifetime' of the circumstellar shell, which turned out to be of order tens of thousands of years. This showed that the PN phase was only a short-lived phase during the evolution of a star which apparently occurred for many stars. HISTORICAL OVERVIEW 15

3.1 Where do Planetary Nebulae come from?

Around the middle of this century a simple reasoning led to the suggestion that red giants were most likely the progenitors of PN. The strongest argument for the link between red giants and PN was discussed by Shklovskii (1956). He argued that the escape velocity of a large star is lower than the escape velocity of a smaller star with the same mass. Using the temperature and luminosity it was possible to determine the expected escape velocity of the central star of a PN, which amounted to several thousands km s1, much larger than the expansion velocity of the nebulae. Turning the argument around and calculating the stellar radius that is needed to obtain escape velocities of order 10-20 km s1 the central star should have been much larger, corresponding to the radii of the Red giants. In the sixties it was a generally accepted idea that red giants were the progenitors of PN (cf. Abell and Goldreich, 1966). Not much later, it turned out that not the red giants were the progenitors of PN, but that Asymptotic Giant Branch, or `second red giant branch', objects would evolve into PN. In the late ®fties an extra branch connected to the Horizontal Branch, asymptotically approaching the red giant branch was tentatively drawn in the HR diagrams of Globular Clusters (Sandage, 1953). Ten years later when the observations could be conducted with more precision, the presence of the Asymptotic Giant Branch was well established (Sandage and Walker, 1966). At the same time calculations showed that AGB stars are in a phase of alternate hydrogen and helium shell burning (Schwarzschild and HÈarm,1965).The®rst evolutionary tracks of stars in the HR diagram were presented by PaczyÂnski in 1970. When the entire envelope of the star was lost during the AGB, the star appeared to evolve with a constant luminosity to the blue in the HR diagram, con®rming the prediction by Shklovskii in 1956.

3.2 So there should be transition objects...

It is clear that objects in the transition from the AGB to PN should exist. To ®nd them is not trivial. Whereas PN are relatively easy to ®nd because of their ionised material and rich emission line spectra, and AGB stars are recognizable because of their location in the HR diagram of Globular Clusters, post-AGB objects are disguised as normal stars with K-G to B supergiant spectral types. The spectral types correspond to the temperature span between the AGB and the PN phase, while the supergiant luminosity class can be inferred from their predicted masses and luminosities. A simple observational test can distinguish post-AGB objects from normal G-B su- pergiants; the dusty wind that is formed during the AGB will emit mainly at infrared wavelengths. A post-AGB star will therefore exhibit a strong excess emission in the in- frared compared to normal supergiants. The emergence of (near-)infrared all-sky surveys like the Two Micron Survey (IRC, Neugebauer and Leighton, 1969) and the AFGL survey (Walker and Price, 1975) allowed astronomers for the very ®rst time to peek at the AGB remnant. 16 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS

Subsequent CO millimeter spectroscopy showed that a number of objects with infrared excess emission have an expanding circumstellar shell. The same objects were indepen- dently observed by Westbrook et al. (1975) who reported on the carbon-rich object AFGL 618, Lo and Bechis (1976) who presented CO measurements of AFGL 2688 (the `Egg' nebula) and AFGL 618, while Zuckerman et al. (1976) reported their work on AFGL 2688. The spectral type of the objects, F type for AFGL 2688 and B for AFGL 618, imply higher than normal temperatures for an AGB star. This ®nding, combined with the pres- ence of the familiar CO emission line and infrared excess emission led to the suggestion that these objects may in fact be `post-carbon stars', carbon-rich stars that have evolved off the AGB. The ®rst reference to the term proto-planetary nebula is found in the review paper by Zuckerman (1978). Not all objects in the list presented by him would nowadays be considered as post-AGB, but rather `young PNe', or `late AGB' stars (admittedly, the nomenclature is not without subjectivity).

4 Properties of post-AGB stars 4.1 Samples of post-AGB stars The work on the late stages of stellar evolution has undergone a huge boost since the IRAS mission in 1983 (IRAS Explanatory Supplement, 1985). The IRAS satellite observed

more than 250,000 objects at 12, 25, 60 and 100 m covering the wavelengths at which circumstellar dust shells emit most of their energy. Indeed, the number of proposed post- AGB stars has increased considerably, and amounts to more than a hundred (see e.g. the listings in Kwok 1993, Waters et al. 1993, and the catalogs provided in the conference proceedings on ªLuminous high latitude starsº, ed. Sasselov S.S., Astronomical society of the Paci®c conference series Vol. 45, ASP, USA) In short, one can divide the existing samples of post-AGB stars into the following groups: ± Infrared based samples: The IRAS colour-colour diagram has proven a useful tool to discriminate between various types of objects. Work by van der Veen and Habing (1988) and Pottasch (1984), showed that there is a colour sequence between obscured AGB stars and PN. With this information at hand, one can then de®ne locations in this diagram where the probability of ®nding transition objects is largest. Hrivnak et al. (1989), Volk and Kwok (1989), van der Veen et al. (1989) and Slijkhuis (1992) used infrared colour criteria, and follow-up observations to study post-AGB candidates. Interestingly,these searches yield large fractions of G type post-AGB stars. There appears to be a trend in that IRAS (i.e. infrared) based selections seem to preferably trace cool post-AGB stars. ± Optically based samples: There is a group of serendipitously discovered B type post-AGB stars by the group of Conlon et al. (1991) and McCausland et al. (1992, and references in these papers). Their program is aimed at studying young B-type halo stars, but in the course of PROPERTIES OF POST-AGB STARS 17

their study they found that some objects are evolved stars rather than young objects. Abundance studies using high resolution spectroscopy revealed that they are metal poor,haveasolar abundanceof helium,but an underabundance of He burning products, notably a strong depletion of carbon. The objects seem to ®t in reasonably with abundances expected for evolved stars. The carbon de®ciencies are however still not well understood. Other examples of optically based samples are the `UV bright stars' in Globular Clusters (Zinn et al. 1972), objects that are about 2 magnitudes brighter than the Horizontal Branch. The most recent work on these objects has been conducted by Gonzalez and Wallerstein (1992, 1994), and Gonzalez (1994). ± Other samples: The post-AGB objects that are studied in this thesis belong to the class of the so- called high latitude supergiants. These are stars with supergiant spectral types, but are located at unexpectedly high Galactic latitudes. They were originally discovered by Bidelman (1951) who drew attention to the existence of peculiar F-type supergiants at high galactic latitudes. When IRAS data showed them to have a considerable infrared excess caused by circumstellar dust (see e.g. Parthasarathy and Pottasch, 1986; Lamers et al. 1986; Waelkens et al. 1987; Trams et al. 1991), the post-AGB nature became widely accepted. The infrared excess is caused by thermally re-radiating dust that was formed in the AGB wind. The evolved status makes the objects less luminous and older compared to massive supergiants, placing them closer to the plane and allowing them more time to get there, solving the high latitude problem. Selections for such objects and the results are presented in Chapters 2, 5 and 6 of this thesis. Objects that are similar to the high latitude supergiants are the RV Tau stars (Jura, 1986).

It is inevitable that the above lists are contaminated by other objects than real post- AGB objects. Most catalogs have been compiled using simple criteria, and follow-up observations are needed to discriminate between the various possible identi®cations. Much work is done to actually try to establish the post-AGB nature of the objects. A nice example may be the Red Rectangle (= HD 44179), which, in its discovery paper was thought to be a young pre-main sequence object (Cohen et al. 1975), while later the post-AGB nature became accepted (e.g. Waelkens et al. 1992). Recently it has been found that the central star of the Red Rectangle is a member of a binary system (van Winckel et al. 1995a), which implies that single star scenarios may not be applicable for this object.

4.2 The spectral energy distribution (SED)

A typical post-AGB star shows two peaks in its SED, a peak in the optical due to the stellar radiation, and a peak in the infrared due to thermally re-radiating circumstellar dust. An example of such an SED is shown in Fig.2. By using dust radiative transfer codes to ®t the 18 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS

Figure 2 Energy distribution of the F5I post-AGB candidate star SAO 240664. The dashed line represents a Kurucz model (1979) ®t through the optical and near-infrared photometry (solid points), the solid line is the best ®t obtained with a dust model through the far-infrared points (see Chapter 5).

SED, one can determine the mass loss rates and kinematic ages of the dust shells which offer a ®rst determination of the evolutionary status of the objects. The types of models that are used to ®t the SED range from relatively simple models with few free parameters assuming the circumstellar dust to be optically thin (Waters et al. 1988, 1989; Bogaert 1994; van der Veen et al. 1994, Chapters 5 and 6), to more complicated models including optical depth effects, grainsizes and grain properties (e.g. Szczerba and Marten, 1993). Models are now being developed that take deviations from spherical geometry into account (e.g. Lopez et al. 1995) and models that include radiation effects on small grains (e.g. Siebenmorgen et al. 1994). Such models have many free parameters, and more observational constraints such as spectrophotometry and imaging have to be taken into account to describe the shells accurately.

4.3 Photospheric abundances

At several stages duing the evolution of a star, the products of hydrogen and helium burning are transported from the core to the stellar surface, changing the surface abundances of the star considerably. This process, called dredge-up, is the consequence of the fact that the convection cells in the stellar envelope reach the hydrogen and helium burning shells. The ®rst dredge-up occurs during the red giant branch, when hydrogen is burned in a shell

PROPERTIES OF POST-AGB STARS 19 +

12 13 + 13 14 15 15 12

  via the CNO cycle, C(p, ) N( ) C(p, ) N(p, ) O( ) N(p, ) C. Since the 14 15 cross section of the N(p, ) O reaction is the lowest, the abundance of N is increased at the expense of C, while the O abundance remains the same. The sum of their abundances remains constant. An additional effect is that the 13C/12C ratio will be enhanced with

respect to its original value (89 for solar composition). >

The second dredge-up only occurs for somewhat higher mass objects ( 4M ), and takes place after the horizontal branch evolution. Hydrogen burning products are again transported to the photosphere. The third, and most important, dredge-up occurs actually more than once. During a series of thermal pulses, the alternation of helium and hydrogen shell burning, products

of the helium burning cycle (mainly the triple process), are brought onto the surface.

Carbon will be more enhanced than N and O. Oxygen is formed trough capture by carbon atoms, nitrogen is formed during the hydrogen burning phase. After a certain number of thermal pulses, the photosphere contains more carbon than oxygen. The star has then transformed into a carbon star. A process called Hot Bottom Burning can however destroy the newly formed C, and may prevent an object to become a carbon star. This is thought to occur for relatively massive stars. Other elements that are dredged-up to the photosphere are the s-process elements, such as Sr, Tc, and Y (®rst proposed by Sanders, 1967). These elements are produced by the neutron capture of elements from the iron group. There are major uncertainties in the above theories. The process of convection is not well understood, the mixing length parameter, which is a measure of the ef®ciency of the dredge-up process, is not well known, and the parameters that govern the Hot Bottom Burning process are not known precisely. For a recent theoretical view of AGB evolution, see Groenewegen et al. (1995, and references therein). Although the absolute changes in surface abundances are dif®cult to predict theoret- ically, studies of the photospheric abundance patterns of post-AGB stars give important information on the evolutionary status of these objects. Several abundance studies of post-AGB stars have been performed. Luck et al. (1990), Bond (1991, and references therein), van Winckel (1995), and Trams (1991) showed that many ªoptically bright post- AGB starsº have iron abundances lower than solar. The C, N, and O abundances are solar, and are thus enhanced with respect to iron. Surprisingly many of the objects do not show strong evidence for s-process elements. An abundance study of a newly discovered post-AGB star is presented in Chapter 6. The iron de®ciencies in these stars re¯ect the primordial abundances, con®rming the populationII nature of the objects. The solar values of C, N, and O indicate that processing has occurred during their evolution. The lack of s-processed elements may mean that these stars have not undergone many thermal pulses, or that the third dredge-up was not very ef®cient (van Winckel, 1995). In fact, based on the surface abundances, only three objects are beyond doubt post-AGB stars. These are HD 187885, HD 158616 (van Winckel, 1995), and HD 56126 (Klochkova, 1995). They are all carbon-rich and show a wealth of lines from the s-process elements. 20 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS

An exciting discovery over the last years in this ®eld was the realization that a group of post-AGB candidate stars have extremely low iron abundances and peculiar abundance patterns. Their Fe abundances are 10,000 - 100,000 lower than the solar value. This group consists of HD 52961 (Waelkens et al. 1991), HR 4049 (Lambert et al.1988), BD +39 4926 (Kodaira et al. 1970), and the central star of the well known Red Rectangle nebula (HD 44179, Waelkens et al. 1992). According to conventional wisdom, a low Fe abundance is identi®ed with the primordial abundances from the parent cloud in which the star was formed. The lack of AGB objects and PNe with the same abundance patterns made this interpretation dif®cult. A breakthrough came with the ®nding that these objects share photospheric abundance patterns similar to the composition of the gaseous component of the interstellar medium. It is known that the abundances of elements in the interstellar medium are the result of an ef®cient gas-dust separation. Fe condenses much more easily on a dust grain than for example Zinc, the gaseous component of the interstellar medium is severely depleted in Fe. The conclusive con®rmation that a gas-dust separation indeed must have taken place was the detection of a Zinc line in HD 52961. The Zinc abundance was found to be three orders of magnitude larger than that of Fe (van Winckel et al. 1992; Trams et al. 1993). In order to explain these peculiar abundances, it was suggested that the out¯owing wind has been stripped of its metals that condensed onto grains (Lambert et al. 1988; Bond, 1991; Mathis and Lamers, 1992; Waters et al. 1992). The radiation pressure on the grains permit the grains to escape from the stellar gravitation ®eld, while the `cleaned' gas then has been reaccreted onto the stellar photosphere.

4.4 The chemistry of the circumstellar shell

The envelopes of AGB stars, post-AGB stars and PN can be classi®ed into two groups, the oxygen rich environments where the C/O ratio (by number) is smaller than one, and carbon-rich environments where C/O is larger than one. In both types of objects, the strongest and most often observed emission lines are the rotational transitions of CO, which trace cool gas. An example of such a spectrum is provided in Fig. 3, where the CO (1-0) line toward the G type object HD 179821 is presented. Spectra such as these were for a long time the only tools to determine the out¯ow velocity of the circumstellar shells around post-AGB stars. Examples are found in the work of Bachiller et al. (1988, 1993), Likkel et al. (1991), van der Veen et al. (1993), Bujarrabal et al. (1992), and Omont et al. (1993b). A useful reference is the catalog by Loup et al. (1993), who compiled all published CO and HCN measurements of circumstellar shells around evolved stars. Oxygen rich objects are also detected in the 18cm OH maser (e.g. te Lintel-Hekkert

et al. 1992; Lewis, 1989) and exhibit the silicate features at 10 and 18 m. The (sub)mm wavelength regime provides many transitional wavelengths of oxygen-rich molecules such as SO, SiO etc (Omont et al. (1993a). Carbon-rich objects display many `unidenti®ed' emission lines in the near-infrared spectra, which are attributed to transitions of the Poly-Aromatic Hydrocarbons (PAHs),

of which the 3.3 m feature is the most famous. In the IRAS-LRS spectra of AGB stars PROPERTIES OF POST-AGB STARS 21

Figure 3 Emission line of the CO (1-0) rotational transition measured toward the post-AGB candidate star HD 179821 (®gure taken from van der Veen et al. 1993).

the 11 m feature, attributed to SiC, is present. It has not been observed however in the spectra of post-AGB stars. In the (sub)mm regime the most frequently observed transitions next to CO are HCN thermal and maser emission. Circumstellar molecules have now been observed in the optical wavelength regime. Hrivnak (1995) using low resolution spectra and Bakker (1995) using high resolution optical spectra report the detection of C2 and HCN absorption lines in the spectra of carbon-rich post-AGB stars. An example of the importance of circumstellar envelopes in the ®eld of astrochemistry was the discovery by Kwok et al. (1989) who reported the presence of a broad emission

band at 21 m in the IRAS Low Resolution Spectrograph (LRS, 1986) data of 4 G- type post-AGB stars. Until now this feature, attributed to transitions from large PAH molecules has been observed in the spectra of post-AGB stars, but not in the spectra of their progenitors, the AGB stars, nor of their successors, the PN. The emission feature is most likely a transient phenomenon dependent on the stellar radiation ®eld, which is a useful constraint when searching for the carriers and excitation mechanisms. With higher spectral resolution than achieved by LRS, it becomes possible to resolve the emission band into individual lines, which will provide the necessary information to

constrain the range of possible carriers responsible for the 21 m emission. Extending the observed wavelength range further into the infrared gives rise to even more unidenti®ed

emission bands. In Fig. 4, a spectrum obtained with the Kuiper Airborne Observatory for  one of the 21 m emission stars shows a spectrum extending to 45 m. An emission 22 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS

Figure 4 Infrared spectrum of IRAS 22272+5435, the upper panel shows a composition of 5 - 8  m spectrum (Buss et al. 1990), the IRAS-LRS spectrum and a KAO spectrum from 16 - 48 m. The solid line is a combination of two blackbodies ®tted to the continuum, the dashed line is the result of a radiative transfer model. The lower panel presents the difference between the observed

spectrum and the continuum, highlighting the emission features. The emission bands from 6-9 m,  and 10-15 m, are probably due to clusters of PAHs, the 21 m emission feature proposed to be arising from PAHs (Justtanont et al. 1995. The most common identi®cation to date for the strong

30 m emission feature is grain material composed of MgS (Figure kindly provided by Pierre Cox,

it is published in Omont et al. 1995).  band is present at 30 m, exceeding the strength of the 21 m bymorethananorder

of magnitude. As in the case of the 21 m feature, the carrier is still not identi®ed, it is thought to be based on MgS, but then 50% of the available Mg and S should be locked in

the grains (Omont et al. 1995). An investigation of the 21 m feature in post-AGB stars and HII regions is discussed in Chapter 8.

Exciting results can be expected from the Infrared Space Observatory (ISO), which will be launched at the end of 1995. ISO is both capable of a higher spectral resolution

than previously possible, and will observe spectroscopically from 2 - 160 m, opening up large parts of this wavelength window. POST-RED SUPERGIANT EVOLUTION AND IRC +10420 23

4.5 Morphology of the circumstellar material

Many Planetary Nebulae show a variety of shapes, ranging from spherically symmetric to butter¯y geometries (Balick, 1987; see also Corradi and Schwarz, 1995 and references therein), while AGB stars show relatively spherical envelopes (Olofsson et al. 1992, Waters et al. 1994). The change of the roughly spherical AGB geometries to the shapes of the PNe must have occurred during the post-AGB phase. An original idea was presented by Kwok et al. in 1978. They considered the possibility that the circumstellar material of PN is subject to dynamical processes from the interaction of a high velocity radiation driven wind colliding with the dense AGB wind. Following up on this suggestion, authors as Mellema (1993), and Frank et al. (1993) used hydro- dynamical models to simulate the interacting winds and showed that a slight asymmetry in the AGB wind can result in strong asymmetries of the shells after the collision. This slight asymmetry in the AGB wind is however not well understood. Several mechanisms have been proposed to account for these a-spherical winds, as for example the interaction of a binary companion that forces the mass loss of the AGB star to be preferably planar (Livio, 1993) or the presence of a strong magnetic ®eld induced by the rotation of the star (Pascoli et al. 1992). As post-AGB stars are in a phase between the AGB and the PN, they are the natural laboratories to investigate the shaping process. This is a ®eld that is now starting to be explored for larger samples of objects. Recent data have been obtained for a subset of the objects (e.g. Waters, in preparation, Skinner et al. 1995 with mid-IR imaging; Trammell et al. 1994, Kastner and Weintraub 1994, with spectropolarimetry resp. imaging polarimetry; Beuzit et al. 1994 and Roddier et al. 1995, using adaptive optics) with important results. Most of the observed objects have not yet developed a fast radiation driven wind, but the observations show asymmetric shells. This result puts into question the proposed mechanisms that need a high velocity radiation driven wind. Such a wind only occurs when

the star has reached high temperatures (> 20,000 K). Instead, the change in geometry must have taken place already at the end of the AGB. Clearly, the question of the shaping of PN is not yet settled.

5 Post-red supergiant evolution and IRC +10420

The second part of this thesis involves the peculiar F8 hypergiant IRC +10420 . Originally it appeared in lists of post-AGB candidate stars, but nowadays IRC +10420 is believed to be in the `post-Red SuperGiant' (post-RSG) phase of evolution (Jones et al. 1993). This is

an evolutionary phase occurring for massive objects (> 10 times solar) and is very similar to the post-AGB phase. In this phase the stars evolve from a low effective temperature on the Red Supergiant branch toward higher temperatures in the Wolf-Rayet phase, possibly passing the Luminous Blue Variable phase (see Humphreys 1991). Although several A- F hypergiants are known, IRC +10420 is the only one with a strong infrared excess, implying a huge mass loss in the previous Red Supergiant phase. 24 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS

The evolution of objects that evolve from the Red Supergiant branch back to the blue has gained interest after the explosion of SN 1987A (Wampler et al. 1990). At that , evolutionary models did not predict that a star with the mass of the progenitor of SN 1987A would explode as a blue star. Nowadays it is realised that mass loss plays a crucial role in the evolutionof massive post-main-sequence stars, while, as in the AGB/post-AGB case, no well-de®ned value for the mass loss rate during the RSG phase is incorporated in theoretical models. IRC +10420 was discovered for its huge infrared excess emission in 1973 (Humphreys et al. 1973) and appears to be variable on timescales less than decades; Lewis et al. (1986) found the OH strength linearly variable with time, the onset of a rapid ionization of the

stellar wind was found by the discovery of H and near-infrared lines that went into emission less than a decade ago (Irvine, 1986; Oudmaijer et al. 1994), and ®nally the optical part of the spectral energy distribution considerably changed over the last 20 years (Jones et al. 1993).

6 Thesis outline

6.1 Post-AGB stars

In order to obtain a well-de®ned sample of candidate post-AGB objects, I selected all SAO stars in the IRAS Point Source Catalog (PSC) that have infrared excess emission due to circumstellar dust. Contrary to previous such selections, the entire colour-colour diagram was taken into account instead of selected parts of this diagram. This selection, presented in Chapter 2, yielded a catalog of more than 450 objects, covering basically all known post-AGB objects, and a large sample of other sources like Be stars, proto- planetary systems (stars surrounded by a dusty disc, the precursor of a planetary system), and pre-main sequence stars. This chapter was published as Oudmaijer et al. (1992). Chapter 3 concerns a survey of post-AGB candidate stars in the near-infrared spectral

region. One of the main results was the discovery of variable emission at 2.3 m, where the CO molecule has its transition levels of the ®rst-overtone of the rotation-vibration states. It is speculated that the emission may be due to post-AGB mass loss. HD 52961

was found to be only the second post-AGB star with an emission feature at 3.5 m(after HR 4049, Geballe et al., 1989). A correlation is proposed between the presence of the 3.5

m feature that is attributed to de-hydrogenated PAH molecules and the extreme metal de®ciencies (Oudmaijer et al. 1995a).

In Chapter 4, the short-term variability in the H line of the post-AGB star HD 56126 is investigated. If real, it might indicate the presence of post-AGB mass loss with important implications for the transition timescales. A careful examination of high quality

data shows however that no variability is present on timescales of minutes. The H is variable on a time scale of months, which may be due to pulsation (Oudmaijer and Bakker, 1994). THESIS OUTLINE 25

Chapter 5 concerns a search for hotter post-AGB objects than found so far in infrared searches. Three candidates were found. Model calculations show that the circumstellar shells appear to have been ejected longer ago than found for most objects studied so far in the literature. The apparent discrepancy between evolutionary calculations, that predict that more B-type post-AGB stars should be observed than are actually found, is explained in terms of selection effects. Most optical catalogs are biased against hotter objects, because of the bolometric correction effect. Due to this effect a hot object that has the same luminosity as a cool object is up to 3 magnitudes fainter in the V band, the band at which most optical catalogs are compiled (Oudmaijer 1995, Oudmaijer et al. 1993). In Chapter 6 an abundance study has been carried out for one of the objects discovered in Chapter 5 (SAO 225457 = HD 133656). The abundances, combined with spectral features and the location above the Galactic plane are similar to the known post-AGB stars, suggest that SAO 225457 is indeed a post-AGB object (Van Winckel, Oudmaijer and Trams, 1995b). In Chapter 7 we investigate the spectral evolution of post-AGB stars using the photo- ionization model CLOUDY and the latest evolutionarypredictions from BlÈocker (1995a+b). The emphasis is laid on the evolution of the infrared colours during the post-AGB phase. We ®nd that the effect of the evolving star, that increases in temperature, can not be neglected (Van Hoof, Oudmaijer and Waters, 1995).

Chapter 8 concerns a comparative study of the 21 m emission band that has been observed in the IRAS LRS spectra of post-AGB stars and HII regions. In this chapter

we investigate the data again, and ®nd that the 21 m feature in HII regions may well be explained as an artefact of LRS, because the HII regions are resolved by the IRAS LRS-spectrometer, while this instrument was designed for point sources. (Oudmaijer and de Winter, 1995).

6.2 IRC +10420 I obtained a large set of data, mainly consisting of high resolution optical spectra from the Utrecht Echelle Spectrograph mounted on the 4.2m WHT on La Palma, Spain, high resolution near-infrared UKIRT spectra and (sub)mm CO spectra obtained with the JCMT. Chapter 9 presents a study of the spectral energy distribution and mass loss history of IRC +10420 . The main conclusions from this work are 1) that the photometric changes over the last 20 years indicate that the temperature of IRC +10420 increased more than 1000 K and 2) that the spectral energy distribution can not be ®tted with a single shell model, but that a hot component has to be invoked. This hot component is likely to be a circumstellar disk seen pole-on and 3) evidence is presented that IRC +10420 is most likely not a post-AGB star. (Oudmaijer et al. 1995b). Finally, in Chapter 10, the entire optical spectrum of IRC +10420 , is studied. The change in surface temperature discussed in the previous chapter is con®rmed using the new optical spectra. The spectral type is A-type which is a signi®cant change from the F8 determination in the seventies. We ®nd that the bi-polar ¯ow scenario that was proposed by 26 CHAPTER 1 j INTRODUCTION AND OUTLINE OF THIS THESIS us some years ago (Oudmaijer et al. 1994), is not compatible with the new data. Instead, we suggest, based on the consistently red-shifted absorption line pro®les that IRC +10420 probably undergoes infall of material. Overall conclusions, and suggestions for future research are given in Chapter 11.

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