Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21543117b&link_type=abstract
American Astronomical Society, AAS Meeting #215, #431.17; Bulletin of the American Astronomical Society, Vol. 42, p.365
Astronomy and Astrophysics
Astrophysics
Scientific paper
We suggest that small carbonaceous molecules (SCMs) may be the sources of the unidentified infrared bands (UIR) and the underlying continuum. We show that the IR spectroscopy of ethylene oxide (EO, c-C2H4O) and cyclopropenylidene (CP, c-C3H2) closely correlates with the major UIR bands at 3.3, 6.2, 7.7, 8.6, and 11.2 um, the often seen strong bands at 12.7 and 16.4 um, as well as many minor features. The differences in band locations and shapes between laboratory EO absorption spectra and astrophysical UIR emission spectra are attributed to vibrational anharmonicity, Fermi resonance splitting of nearly degenerate vibration levels, and rotational envelop narrowing due to the low temperatures in space. The excitation mechanism is absorption of UV radiation, primarily Ly-alpha, by SCMs. Photon trapping for this very optically thick transition enhances the absorption by several orders of magnitude. Our abundance analysis for NGC 7027 reveals that the SCM abundance, relative to H2, is 3E-9 which compares well to radio measurements of the CP abundance range of 1E-9 - 1E-7. Radio lines of CP have been seen in numerous astronomical objects, most displaying the UIR bands. EO is also seen, but in fewer objects, none displaying the UIR bands. We theorize that in UIR objects, EO is formed on, and primarily resides on, carbonaceous grains, precluding radio detection of rotational lines. We present a detailed spectral analysis of the 11.2 um UIR band and its associated continuum. An asymmetric top rotational model is used to fit the feature profiles observed from a variety of astrophysical objects. We show that the narrower 11.2 um feature can be fit based on EO or CP rotational constants at a temperature around 10 K and the broader continuum corresponds to 200 K. We suggest laboratory experiments, astronomical observations, and theoretical investigations to further evaluate the SCM mechanism.
Bernstein Lawrence
Lynch David
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