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Session 48 - Circumstellar Envelopes.
Display session, Tuesday, January 14
The ultraviolet spectrum of the peculiar star HD 44179 shows many bands of the A--X (Fourth Positive) electronic system of carbon monoxide, first noted in IUE spectra obtained by Sitko (1983). Recently, Hubble Space Telescope (HST FOS and GHRS spectra of HD 44179 have become publicly available, and confirm the presence of the CO A--X bands. In addition, bands of the a--X (Cameron) system are present in emission. The CO absorption originates in cool circumstellar gas around HD 44179; this circumstellar shell is probably a remnant of the previous AGB evolutionary phase of this supergiant star. HD 44179 is notable for several spectroscopic anomalies: extremely low photospheric abundances of the refractory elements (e.g. [Fe/H] = -3.5), but almost solar CNO; (2) an intense source of the unidentifed infrared emission (UIR) bands, particularly the 7.7\micron\ band; and (3) the strongest known source of the extended red emission (ERE) between 5500--7500 ÅThe origin of both the UIR and ERE emissions remains unclear, although large carbon molecules (PAHs) have been widely proposed as the source. Analysis of the low-excitation absorption lines present in the UV spectrum confirms that the circumstellar gas is cool. This result is consistent with previous determinations of a CO rotational excitation temperature near 3000 K.\ These temperatures would appear to be too low to collisionally excite the CO Cameron bands. Therefore, we examine various mechanisms that would permit radiative pumping of the Cameron bands, and consider possible links to the optical ERE spectrum.
Program listing for Tuesday