Detection of C 2 , CN , and NaID absorption in the AGB remnant of HD 56126 ⋆ ⋆ ⋆

We present the detection of molecular absorption lines in the optical spectrum of the post-AGB star HD 56126. The C2 Phillips A Πu − X Σg (1,0), (2,0), and (3,0); Swan dΠg − a Πu (0,0) and (1,0); and CN Red system AΠ − XΣ (1,0), (2,0), (3,0), and (4,0) bands have been identified. From the identification of the molecular bands we find an expansion velocity of 8.5 ± 0.6 km s independent of excitation condition or molecular specie. On the basis of the expansion velocity, rotational temperatures, and molecular column densities we argue that the line-forming region is the AGB remnant. This is in agreement with the expansion velocity derived from the CO lines. We find column densities of logNC2 = 15.3±0.3 cm −2 and logNCN = 15.5±0.3 cm , and rotational temperatures of Trot = 242 ± 20 K and Trot = 24± 5 K respectively for C2 and CN. By studying molecular line absorption in optical spectra of post-AGB stars we have found a new tracer of the AGB remnant. From comparison with the results of CO and IR observations it is possible to obtain information on non-spherical behavior of the AGB remnant. Using different molecules with different excitation conditions it should be possible to study the AGB remnant as a function of the distance to the star, and thus as a function of the evolutionary status of the star on the AGB. Send offprint requests to: Eric J. Bakker, present address: Astronomy Department, University of Texas, RLM 16.218, Austin, TX 78712, U.S.A., [email protected] ⋆ Based on observations with the Utrecht Echelle Spectrograph on the William Herschel Telescope (La Palma, Spain) ⋆⋆ Tables of App. A are only available in electronic form at the CDS via anonymous ftp 130.79.128.5