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Astron. Astrophys. 323, 469-487 (1997)

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Circumstellar [FORMULA], CN, and C [FORMULA] in the optical spectra of post-AGB stars * **

Eric J. Bakker 1, 2, 3, Ewine F. van Dishoeck 4, L.B.F.M. Waters 5, 6 and Ton Schoenmaker 7

1 University of Texas, Department of Astronomy, TX 78712, USA
2 Astronomical Institute, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
3 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Sterrewacht Leiden, University of Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands
5 Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
6 SRON Laboratory for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands
7 Kapteyn Sterrenwacht Roden, Mensingheweg 20, 9301 KA Roden, The Netherlands

Received 16 April 1996 / Accepted 2 October 1996


We present optical high-resolution spectra of a sample of sixteen post-AGB stars and IRC +10216. Of the post-AGB stars, ten show C2 Phillips ([FORMULA]) and Swan ([FORMULA]) and CN Red System ([FORMULA]) absorption, one CH [FORMULA] ([FORMULA]) emission, one CH [FORMULA] absorption, and four without any molecules. We find typically [FORMULA], and 18-50 K, [FORMULA], and 15.03-16.47 cm-2 for C2, CH [FORMULA], and CN respectively, and [FORMULA] (CN)/N (C [FORMULA]. We did not detect isotopic lines, which places a lower limit on the isotope ratio of [FORMULA]. The presence of C2 and CN absorption is correlated with cold dust ([FORMULA]  K) and the presence of CH [FORMULA] with hot dust ([FORMULA]  K). All objects with the unidentified 21 µm emission feature exhibit C2 and CN absorption, but not all objects with C2 and CN detections exhibit a 21 µm feature. The derived expansion velocity, ranging from 5 to 44 km s-1, is the same as that derived from CO millimeter line emission. This unambiguously proves that these lines are of circumstellar origin and are formed in the AGB ejecta (circumstellar shell expelled during the preceding AGB phase). Furthermore there seems to be a relation between the C2 molecular column density and the expansion velocity, which is attributed to the fact that a higher carbon abundance of the dust leads to a more efficient acceleration of the AGB wind. Using simple assumptions for the location of the molecular lines and molecular abundances, mass-loss rates have been derived from the molecular absorption lines and are comparable to those obtained from CO emission lines and the infrared excess.

Key words: molecular processes – circumstellar matter - stars: AGB and post-AGB – line: identification

* Based on observations with the Utrecht Echelle Spectrograph on the William Herschel Telescope (La Palma, Spain), and the McDonald observatory 2.7m telescope (Texas).
** Tables A.1/2/3/4/5 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/Abstract.html, or from the authors.

Send offprint requests to: Eric J. Bakker at the University of Texas, ebakker@viking.as.utexas.edu

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998