[Paper Review] Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars: C_{60} fullerenes and polycyclic aromatic hydrocarbons

C. Gielen, J. Cami+3 more|arXiv (Cornell University)|Oct 27, 2011

Astrophysics and Star Formation Studies69 references68 citations

TL;DR

This study identifies the first detection of C60 fullerenes and polycyclic aromatic hydrocarbons (PAHs) in oxygen-rich circumstellar environments around binary post-AGB stars, challenging the conventional view that such carbonaceous molecules only form in carbon-rich outflows. Using Spitzer IRS spectroscopy, the authors show that despite the O-rich nature of the central stars and their circumbinary discs, large carbon molecules persist, suggesting unexpected chemical processing or non-equilibrium processes like shock-driven transformation of PAHs into fullerenes.

ABSTRACT

Context. The circumstellar environment of evolved stars is generally rich in molecular gas and dust. Typically, the entire environment is either oxygen-rich or carbon-rich, depending on the evolution of the central star. Aims. In this paper we discuss three evolved disc sources with evidence of atypical emission lines in their infrared spectra. The stars were taken from a larger sample of post-AGB binaries for which we have Spitzer infrared spectra, characterised by the presence of a stable oxygen-rich circumbinary disc. Our previous studies have shown that the infrared spectra of post-AGB disc sources are dominated by silicate dust emission, often with an extremely high crystallinity fraction. However, the three sources described here are selected because they show a peculiar molecular chemistry. Methods. Using Spitzer infrared spectroscopy, we study in detail the peculiar mineralogy of the three sample stars. Using the observed emission features, we identify the different observed dust, molecular and gas species. Results. The infrared spectra show emission features due to various oxygen-rich dust components, as well as CO2 gas. All three sources show the strong infrared bands generally ascribed to polycyclic aromatic hydrocarbons. Furthermore, two sample sources show C60 fullerene bands. Conclusions. Even though the majority of post-AGB disc sources are dominated by silicate dust in their circumstellar environment, we do find evidence that, for some sources at least, additional processing must occur to explain the presence of large carbonaceous molecules. There is evidence that some of these sources are still oxygen-rich, which makes the detection of these molecules even more surprising.

Motivation & Objective

• To investigate the presence of carbonaceous molecules in oxygen-rich circumstellar environments of post-AGB binary stars, which are typically expected to be dominated by silicate dust.
• To determine whether the detection of PAHs and C60 fullerenes in these O-rich systems indicates non-equilibrium chemistry or unusual physical conditions.
• To explore the mechanisms that could allow large carbonaceous molecules to form or survive in environments where carbon is expected to be locked in CO.
• To assess the role of shocks, stellar pulsations, or catalytic processes in enabling the formation of complex carbon molecules in O-rich discs.

Proposed method

• Analysis of high- and low-resolution Spitzer Infrared Spectrograph (IRS) spectra of three binary post-AGB stars with known O-rich circumstellar discs.
• Identification of dust, gas, and molecular species through characteristic emission features in the 5–20 µm range.
• Spectral fitting to isolate and quantify emission from amorphous and crystalline silicates, CO2 gas, PAHs, and C60 fullerenes.
• Estimation of excitation temperatures and masses of C60 fullerenes based on the relative strengths of their 7.8, 8.5, 11.2, and 17.1 µm emission bands.
• Comparison of the observed spectra with laboratory and theoretical models of PAH and fullerene emission.
• Use of the LIR/L∗ ratio as a diagnostic for disc mass and evolutionary state, to explore correlations with molecular content.

Experimental results

Research questions

• RQ1 How can C60 fullerenes and PAHs be detected in circumstellar environments that are classified as oxygen-rich?
• RQ2 What physical or chemical processes could enable the formation or survival of large carbonaceous molecules in O-rich discs?
• RQ3 Is the presence of CO2 gas linked to the formation of carbonaceous molecules in these O-rich systems?
• RQ4 Why are only a small subset of post-AGB disc sources (six out of ~70) found to host detectable PAHs or fullerenes?

Key findings

• The three studied post-AGB binary stars—IRAS 06338, HD 52961, and EP Lyr—exhibit strong emission features from O-rich silicates, CO2 gas, PAHs, and C60 fullerenes.
• This study reports the first unambiguous detection of C60 fullerenes in oxygen-rich post-AGB systems, with the molecules showing characteristic 7.8, 8.5, 11.2, and 17.1 µm emission bands.
• The excitation temperature of C60 in HD 52961 and IRAS 06338 is estimated at ~1500 K, indicating emission from highly excited states.
• The derived C60 mass in these sources is on the order of 10−6 to 10−5 M⊙, suggesting significant production or accumulation of fullerenes.
• Despite their O-rich classification (C/O < 1), the presence of PAHs and C60 indicates that carbon is not fully locked in CO, implying active chemical processing.
• The low LIR/L∗ ratios (3–12%) in these sources suggest lower dust masses or more transparent discs, which may enhance gas-line visibility and influence photodissociation rates.