Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...18110007b&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #100.07; Bulletin of the American Astronomical Society, Vol. 24, p.1279
Astronomy and Astrophysics
Astronomy
Scientific paper
The unidentified infrared (UIR) bands at 3.3, 6.2, 7.7, 8.7 and 11.3microns are usually identified with various vibrational and bending modes of polycyclic aromatic hydrocarbons (PAHs), and typically appear when dust grains and an UV radiation field are present together. The star HD 44179 is a strong source of these UIR features. Recent spectroscopic analyses of HD 44179 reveal the photosphere is strongly depleted in the refractory elements Fe, Ca, Al, Ti, Mg, Cr and Si, but has approximately solar abundances of C, N, O, and S. This abundance pattern is consistent with the depletion of refractory elements by silicate grain formation in an oxygen-rich (C/O < 1) gas. HD 44179 is probably an immediate post-AGB object, and I propose that recent evolution off the AGB has resulted in the heating and evaporation of grains formed in the AGB phase. The most abundant product of such processes should be the silicon monoxide molecule. Gaseous SiO has its fundamental vibration-rotation band near 8microns , and this feature should be observationally conspicuous. I have calculated synthetic emission spectra of the SiO Delta v = 1 fundamental band and find a double-peaked feature is produced. The R-branch has forms a band head at J ~ 125 producing a sharp emission peak near 7.7microns , while the P-branch forms a broad feature of lower amplitude with a peak in the region of 8.5 -- 8.8microns . I propose that these peaks correspond to the UIR bands at 7.7 and 8.7microns , and thus the carrier of these two features is actually the (gaseous) SiO molecule.
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