Detection of C2, CN, and NaID absorption in the AGB remnant of HD56126

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We present the detection of molecular absorption lines in the optical spectrum of the post-AGB star HD56126. The C2 Phillips A¹Πᵤ-X¹Σ⁺g (1,0), (2,0), and (3,0); Swan d³Πg-a³Πᵤ (0,0) and (1,0); and CN Red system A²Π-X²Σ⁺ (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 kms 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 log NC2=15.3+-0.3 cm-2 and log NCN=15.5+-0.3 cm-2, 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.