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Astron. Astrophys. 332, 928-938 (1998)

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Photospheric composition of the carbon-rich 21 µm post-AGB stars IRAS 22223+4327 and IRAS 04296+3429 *

Leen Decin ** 1, Hans Van Winckel *** 1, Christoffel Waelkens 1 and Eric J. Bakker 2

1 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, B-3000 Leuven, Belgium
2 Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712, USA

Received 15 September 1997 / Accepted 15 December 1997

Abstract

We present a detailed chemical analysis on the basis of high-resolution, high signal-to-noise optical spectra of two post-AGB objects IRAS 22223+4327 and IRAS 04296+3429. Both display the unidentified 21 µm feature in their IR-spectra. The analysis is performed using HR 1865 (F0Ib) and HR 1017 (F5Iab) as reference stars.

The spectroscopic indicators provide accurate atmospheric parameters of [FORMULA] =6500 K, [FORMULA] =1.0 and [FORMULA] =5.5 [FORMULA] for IRAS 22223+4327 and [FORMULA] =7000 K, [FORMULA] =1.0 and [FORMULA] =4.0 [FORMULA] for IRAS 04296+3429. Our high-resolution data are inconsistent with the significantly lower temperatures deduced from spectral-type determinations in the literature based on low resolution spectra and highlight the need of high-resolution spectroscopy for the determination of accurate fundamental parameters of chemically peculiar supergiants.

Both photospheres are found to be metal-deficient with [Fe/H]=-0.4 and -0.7 respectively. C and N are found to be overabundant. Useful O-lines were only detected in the brighter IRAS 22223+4327 and the O abundance is found to follow the Fe deficiency: the C/O photospheric abundance is about 1.3, but due to the lack of oxygen lines it is difficult to determine accurately. This corroborates the fact that the carriers of the 21 µm feature are formed in a carbon rich circumstellar chemistry. Moreover, these IRAS-stars have large overabundances of s-process-elements. The mean abundance of all the measured s-process-elements is [s/Fe]=+1.0 for IRAS 22223+4327 and +1.4 for IRAS 04296+3429. The distribution of the s-process elements can best be described as due to a distribution of neutron exposures with a low mean neutron exposure of [FORMULA]. The 21 µm stars form an interesting sub-group in the total post-AGB sample of stars, not only for their IR characteristics, but also in a broader context of stellar (chemical) evolution theory. They show, in contrast to other post-AGB stars, that the 3rd dredge-up has been efficient during their AGB evolution. The mean neutron exposure is lower than expected for their metallicity.

The spectroscopic parameters found for the massive spectral analogues ([FORMULA] =7500 K, [FORMULA] =2.0 and [FORMULA] =3.0 [FORMULA] for HR 1865 and [FORMULA] =6500 K, [FORMULA] =2.0 and [FORMULA] =3.5 [FORMULA] for HR 1017) agree well with the values found in the literature. Both stars display an enrichment in Na, which is commonly observed in massive supergiants but theoretically not well understood.

Key words: stars: individual: HR 1017, HR 1865, IRAS 04296+3429, IRAS 22223+4327 – stars: post-AGB – stars: abundances

* Based on observations with the Utrecht Echelle Spectrograph, fed by the 4.2m William Herschel Telescope at La Palma
** Scientific researcher of the Fund for Scientific Research, Flanders
*** Postdoctoral fellow of the Fund for Scientific Research, Flanders

Send offprint requests to: Leen Decin (e-mail: Leen.Decin@ster.kuleuven.ac.be)

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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