Chemical Synthesis in the Circumstellar Environment

Abstract

We discuss the spectral distinctions between B[e] stars and compact planetary nebulae. The differentiation between proto-planetary nebulae, transition objects between the asymptotic giant branch and planetary nebulae, and reflection nebulae in binary systems is also discussed. Infrared and millimeter-wave observations have identified many inorganic and organic molecules, as well as solid-state minerals, in the circumstellar environment. There is evidence that complex organics in the form of mixed aromatic/aliphatic nanoparticles (MAONs) are synthesized during the proto-planetary nebulae phase of evolution. Their ejection into the interstellar medium may have enriched the primordial Solar System, and the complex organics found in comets, asteroids, and planetary satellites could be stellar in origin.

1. Introduction

The observation of emission lines in the planetary nebula NGC 6543 by William Huggins in 1864 provided the first definitive evidence for the existence of gaseous nebulae in the Galaxy. Objective prism surveys in the mid-20th century led to the discovery of many emission-line objects, a large fraction of which are stellar in appearance [1,2]. Among these emission-line objects are Be stars, which are young stars with temperature between 10,000 and 30,000 K with circumstellar materials (Miroshnichenko, these proceedings) [3]. A sub-class of Be stars are B[e] stars, which show strong collisionally excited forbidden lines in the spectrum. Some of these B[e] stars could be compact planetary nebulae that appear stellar in optical images.

With the advent of high-resolution radio [4] and optical imaging [5], the nebulosity of compact planetary nebulae can be resolved, allowing them to be distinguished from B[e] stars, which have much smaller circumstellar envelopes. Another distinguishing feature is the degree of infrared excess. Planetary nebulae generally have larger infrared excess from cool dust [6], whereas dust in B[e] stars is warmer and the excess is smaller. In this paper, we review the chemical properties of compact planetary nebulae and their immediate progenitors.

2. Discovery of Proto-Planetary Nebulae

Planetary nebulae have been recognized as a post-asymptotic giant branch (AGB) phenomenon since the 1970s, with the central stars of planetary nebulae being powered by hydrogen-shell burning on top of a carbon–oxygen electron-degenerate core [7]. As the central star evolves to temperatures higher than 30,000 K, ultraviolet continuum output from the central star photoionizes the circumstellar nebula, giving rise to recombination lines of hydrogen and helium and collisionally excited lines of metals [8]. These emission lines are responsible for the optical brightness of planetary nebulae.

However, the intermediate evolutionary stage between the end of the AGB and planetary nebulae remained a missing link in our understanding of planetary nebulae evolution. The observations of infrared colors of AGB stars and of young planetary nebulae by the Infrared Astronomical Satellite (IRAS) made the prediction of the infrared colors of these transition objects possible (often called proto-planetary nebulae) [9,10]. The search for IRAS objects with these intermediate infrared colors led to the identification of these transition objects [11,12].

The central stars of proto-planetary nebulae generally have spectral types of F and G, and their spectral energy distribution shows a reddened photosphere and cold dust component characteristic of a detached dust shell. The relative strengths of these two components are often the result of inclination effects [13].

Proto-planetary nebulae are distinct from the class of B stars with infrared excess (see van Winckel, these proceedings) [14], which are often referred to as post-red-giant-branch or post-AGB objects. The infrared emission of these objects is due to a circumstellar disk, whereas the infrared emission from proto-planetary nebulae arises from the remnants of the circumstellar envelopes of the progenitor AGB stars. Because of their larger separation from the central star, the dust components of proto-planetary nebulae have lower temperatures (~100 K). As progenitors of planetary nebulae, the central stars of proto-planetary nebulae also have higher luminosities (>3000 L_ʘ.).

3. Confusion Between Planetary Nebulae and Other Objects

One of the persistent problems of planetary nebulae research has been misclassification. Objects in planetary nebulae catalogs have been found to be confused with emission-line galaxies, reflection nebulae, Be stars, O-type subdwarfs, Hii regions, Wolf–Rayet nebulae, supernovae remnants, novae, and symbiotic stars. The problems of misclassification and a proposed observational definition of planetary nebula are given by Frew and Parker [15]. However, a purely observational definition may not adequately determine the evolutionary state of the object, and a combined observational–theoretical definition may be necessary [16].

Even the nature of the well-known planetary nebula M2-9 is not clear. M2-9 has a classical bipolar morphology and is often cited as a proto-typical planetary nebula. However, the total observed infrared luminosity of M2-9 over all spectral bands is only 1500 L_ʘ. When the projection effect and possible leakage of optical/UV photons from the central star are taken into consideration, the estimated total luminosity of the object is 2500 L_ʘ. [17,18], which is less than the lower limit of a post-AGB star [19].

The Red Rectangle (HD44179) is a reflection nebula with a central star of spectral type B9–A0 in a binary system with a period of 319 ± 3 days [20]. It is deficient in iron ([Fe/H] = –3.3) and other refractory elements (Mg, Si, and Ca) but has normal solar C, N, O, and S abundances [21]. It is also a strong source of unidentified infrared emission (UIE) bands in the infrared [22] (Figure 1) and extended red emission (ERE) in the optical range [23]. Narrow emission features resembling those of the diffuse interstellar bands (DIB) can be seen on top of the continuum emission [24]. Although the nature of the DIBs has not been identified, they are likely to be electronic transitions of gas-phase molecules [25]. The ERE could be due to photoluminescence of a carbonaceous semiconductor, whereas the UIE bands are probably vibrational bands of complex organics (see Section 5). Interesting circumstellar molecular synthesis is ongoing in this object.

Although the Red Rectangle is often referred to as a proto-planetary nebula, its evolutionary nature is unclear. Its characteristic X shape is the result of light reflected off the walls of the cavity created by a directional outflow [26]. Model simulations show that if the Red Rectangle is rotated to be viewed at an angle of 30°, its ladder rung structures resemble the multiple rings seen in the bipolar lobes of the compact planetary nebula Hb12. Other reflection nebulae with large infrared excesses similar to the Red Rectangle include M1-92, Mz-3, and Roberts 22.

There are various evolutionary scenarios that can lead to an emission-line object with an infrared excess. The energy source can be a single post-AGB star evolving towards the white dwarf stage or a binary system undergoing mass transfer. The infrared excess can be the result of remnants of AGB mass loss or a circumstellar disk formed by mass transfer. The X-ray emission can be due to a shocked fast wind from a pre-white dwarf (as in planetary nebulae) [27,28] or collisions of stellar winds from two binary components [29,30]. In a planetary nebula, the emission lines originate from a high-density shell formed by the sweeping up of AGB wind by a fast wind from the central star [31], whereas in the case of B[e] stars, the emission lines originate from the accretion disk. Time-dependent and/or orientation-changing collimated fast winds can also complicate the morphological structure of the circumstellar nebula.

4. Synthesis of Complex Organics in Post-AGB Evolution

In addition to optical spectroscopic and infrared continuum observations, recent millimeter-wave and mid-infrared spectroscopic observations have led to new insights into the chemical structure of the circumstellar nebula. Gas-phase molecules have been detected through their rotational and vibrational transitions, and inorganic and organic solids have been identified through their vibrational modes [32]. These observations open the new arena of circumstellar chemistry under low-density conditions very different from the physical conditions of terrestrial chemistry.

Stellar molecular synthesis begins during the AGB phase of stellar evolution. With the nucleosynthesis of carbon in the core and dredged up to the surface of AGB stars, dozens of molecules have been observed to form in the circumstellar envelopes of AGB stars [33,34]. Solid minerals in the form of amorphous silicates and silicon carbide are detected in over 4000 and 700 AGB stars, respectively [35]. These detections suggest that molecules and solids can be synthesized under very low-density conditions and over very short (10²–10³ years) time scales.

A family of broad infrared emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 μm (commonly referred to as unidentified infrared emission or UIE bands) was first detected in the spectrum of the planetary nebula NGC 7027 [36]. The UIE bands are now widely seen in planetary nebulae, reflection nebulae, Hii regions, and external galaxies. The 3.3 and 11.3 μm were identified with C–H stretching and out-of-plane-bending modes of aromatic compounds, respectively [37]. After the discovery of proto-planetary nebulae, aliphatic features around 3.4 μm corresponding to C–H stretching modes of methyl (–CH₃) and methylene (–CH₂–) groups were identified [38,39]. Also present are broad plateau features around 8 and 12 μm, which can be due to superposition of in-plane and out-of-plane bending modes of various aliphatic side groups [40]. These results suggest that amorphous organics with mixed hybridization states can form under circumstellar conditions.

5. Mixed Aromatic/Aliphatic Organic Nanoparticles (MAON) as Carrier of UIE Bands

For the past 30 years, the most popular theory on the origin of the UIE bands is the polycyclic aromatic hydrocarbon (PAH) hypothesis [41]. In spite of its popularity, the PAH hypothesis has a number of problems [42]. As an alternative to the PAH hypothesis, we proposed that the UIE bands are due to mixed aromatic/aliphatic organic nanoparticles (MAONs) [43,44]. Unlike PAH molecules, which are planar in geometry and consist of pure C and H atoms, MAONs are three-dimensional in structure with a mix of aromatic islands of different sizes linked by aliphatic chains of different lengths and orientations. As the circumstellar/interstellar environment is not a clean laboratory, other elements (N, O, S, etc.) are also likely to be incorporated in the structure (Figure 2).

MAON-like particles are the natural products of combustion. When a mixture of hydrocarbon molecules is subjected to external energy injection (electric discharge, laser pyrolysis, radio, flame combustion, etc.), the products are often amorphous hydrocarbons with mixed aromatic/aliphatic structures [45,46,47,48]. Using gas-phase hydrocarbon molecules as ingredients, MAON-like particles can be produced in the circumstellar environment during the proto-planetary nebulae phase of evolution.

With advancements of quantum chemistry techniques, it is now possible to calculate the theoretical spectra of complex molecules. In order to understand the origin of the UIE bands, we have explored the spectral behavior of complex organics by first adding aliphatic side groups to PAH molecules and analyzing their vibrational modes [49]. This is followed by changing the sizes of the aromatic islands and orientations of the aliphatic groups [50]. Unlike simple organic molecules, complex organics show many coupled vibrational modes. Further work is needed to compare the vibrational spectra of MAONs to the astronomical UIE bands.

6. Stellar Enrichment of the Solar System

Complex organics are known to be commonly present in the Solar System. Solids of mixed aromatic/aliphatic structures have been found in carbonaceous meteorites, comets, asteroids, planetary satellites, and interplanetary dust particles [51]. Although it is commonly believed that such organics are produced during the early phase of Solar System formation, it is possible that the primordial Solar System had been enriched by ejecta from planetary nebulae [52]. While molecules are likely to be photodissociated by interstellar radiation fields [53], large, complex organics like MAON are more likely to survive their journey through the interstellar medium, as evidenced by the existence of pre-solar grains in meteorites [54,55]. The early Earth was subjected to large-scale external bombardments [56], and externally delivered organics may have played a role in the development of life on Earth [57].

7. Summary

While traditional astronomical observations have focused on the stellar or gaseous (primarily ionized) form of matter, recent observations in the millimeter-wave and infrared domains have broadened our awareness of the molecular and solid forms in the Universe. Organic compounds are now found to be not just confined to the terrestrial environment, as we used to believe 50 years ago. We now know that organic matter is routinely synthesized in the circumstellar environment of evolved stars. Signatures of the UIE bands can be found in galaxies over 10 billion years old [58]. The 220 nm feature, often attributed to complex organics, has been detected in a Galaxy dated 13 billion years ago [59], suggesting that organic synthesis had started as early as 800 million years after the Big Bang. Abiotic synthesis of complex organics is common in the Galaxy and occurred even during the early evolution of the Universe. A better understanding of the origin of these organics is central to our picture of the chemical evolution of the Universe and may be relevant to the problem of the origin of life on Earth.