Abstract
High-resolution spectra and spectral-synthesis methods have been used to search for diffuse absorption bands in two protoplanetary nebulae candidate stars HD 179821 and SAO 34504. We have found strong evidence for eight diffuse bands (5780, 5797, 5850, 6196, 6270, 6614, 6660, 6699) in HD 179821. The mean value of the heliocentric velocity (V⊙) of these bands is −11.5 km s−1, which may be compared to V⊙≃ -11 km s−1 obtained for the Na i D12 interstellar component. A feature on the red wing of the diffuse interstellar band at 5780 A is likely to have a circumstellar origin. Its Doppler velocity is close to that for one of the Na i D12 absorption components. No significant interstellar absorption bands are detected in SAO 34504. However, we have found a number of unidentified absorption features. Those at 6196, 6597 and 5780 A may be diffuse circumstellar bands blueshifted with respect to the stellar velocity derived from the photospheric absorption lines, V⊙ = -39.7 km s−1, in agreement with the expansion velocity of the circumstellar remnant derived from the radio observations, Vexp≃ 10 km s−1. The Na i D12 lines for SAO 34504 consist of two absorption components at V⊙≃ -13 and -49 km s−1.
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