Astronomy and Astrophysics – Astrophysics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0604c&link_type=abstract
American Astronomical Society, AAS Meeting #212, #6.04; Bulletin of the American Astronomical Society, Vol. 40, p.196
Astronomy and Astrophysics
Astrophysics
Scientific paper
Polycyclic aromatic hydrocarbon (PAH) molecules have characteristic emission bands that have been observed in many astrophysical environments. However, there is one notable exception: carbon-rich AGB (C) stars. Whereas formation of PAHs in the circumstellar shells of C stars is expected, only a handful of C stars exhibit the PAH spectral features and all have hot (A or B star) companions. This makes C stars with hot companions an ideal location to investigate the physical conditions associated with PAH formation.
PAHs absorb single photons and then re-emit the energy in spectral features at characteristic wavelengths where the precise wavelengths and strength ratios of the features depend on the size, composition and charge state of the individual PAHs. Additionally, the wavelength of photons needed to excite PAHs depends on the individual PAHs size and charge state. While small PAHs undoubtedly need higher energy (UV) photons, it has been suggested that large or ionized PAHS (>100; C atoms) can be excited by visible or even near-IR photons. The lack of PAH emission from single carbon stars suggests that either PAHs do not form around C stars or that only small neutral grains form, which cannot be excited by a C star's cool radiation field.
There are two competing formation mechanisms for PAHs around C stars: (1) "bottom-up” where acetylene molecules (C2H2) react to form aromatic carbon rings, building up to PAHs; or (2) "top-down", where small carbon grains react with H atoms and desorb PAHs. These two scenarios should lead to differences in the spatial distribution of the PAH emission features around these stars.
We present preliminary results from Gemini/MICHELLE spatially resolved spectroscopic observations of C stars with hot companions.
Barlow Michael J.
Caputo Daniel P.
Clayton Geoff C.
Speck Angela Karen
Volk Kathryn
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