Circumstellar molecular line absorption and emission in the optical spectra of post-AGB stars

Abstract

We present a list of post-AGB stars showing molecular line absorption and emission in the optical spectrum. Two objects show CH⁺, one in emission and one in absorption, and 10 stars show C₂ and CN in absorption. The Doppler velocities of the C₂ lines and the rotational temperatures indicate that the line forming region is the AGB remnant. An analysis of the post-AGB stars of which CO millimeter data is available suggests that the C₂ expansion velocity is of the same order as the CO expansion velocity. HD 56126 has been studied in detail and we find a mass-loss rate of M=2.8 × 10⁻⁴ M_⊙yr⁻¹,$f_{C_2 }$ = 2.4 × 10⁻⁸ and f _CN = 1.3 × 10⁻⁸. The mass-loss derived from C₂ is significantly larger than M=1.2 × 10⁻⁵ M_⊙yr⁻¹ derived from CO. We find that all objects with the 21µm feature in emission show C₂ and CN absorption, but not all objects with C₂ and CN detections show a 21µm feature.