1 Introduction

The atmospheric chemical composition of post-asymptotic giant branch (post-AGB) stars and circumstellar environments is determined by nucleosynthesis and dredge-up events at the late AGB phases. At AGB, the star has a C-O core surrounded by helium and hydrogen burning shells above which lies a deep convective envelope. The thermally pulsating phase (TP-AGB), though much shorter than early AGB phase (E-AGB), is responsible, through considerable mass-loss, for the ejection of large fraction of carbon and s-process elements into the ISM. At this phase, thermal pulses are caused by instabilities in the He-burning shell first discovered by Schwarzschild & Härm (1965) and Weigert (1966). The excess energy generated by He shell flashes is transported by convection over the region that extends from the base of He-burning shell and the hydrogen-helium discontinuity. The He shell instabilities strongly influence the chemical composition of the convective envelope. The expansion and cooling of the intershell layers during a powerdown phase of the He shell flash causes the deepening of the convective envelope into regions containing the products of partial He-burning. The ¹²C, ¹⁹F and the s-process elements are mixed into the outer envelope causing abundance variations at the surface of these stars. This process, known as the third dredge-up (TDU), is able to explain the formation of carbon stars (Busso et al. 1999), Wallerstein et al. (1997), Mowlavi (1999). At AGB the star is constantly losing mass, but a final phase of enhanced mass-loss by the superwind is believed to terminate the AGB phase producing a planetary nebula. Therefore, studying the chemical composition of the atmospheres and envelopes of evolved stars with IR fluxes, one expects to identify post-AGB stars and to provide important observational constraints for the theoretical work on nucleosynthesis, internal structure and mass-loss in evolved intermediate and low-mass stars.

Post-AGB stars, as they evolve across the H-R diagram towards the white dwarf stage, form families of rather exotic objects like the R CrB stars, other subgroups of H-deficient and He-rich stars, planetary nebulae etc. In the H-R diagram, they populate the region generally occupied by massive young supergiants evolving redwards from the main sequence, and having similar temperatures and luminosities. To differentiate the massive and young stars from the highly evolved low-mass post-AGB stars, detailed atmospheric abundance analysis is crucial. Chemical analysis of high galactic latitude A-F supergiants have led to the discovery of many interesting post-AGB stars such as HR 7671 (Luck et al. 1990), HR 4912 (Lambert et al. 1983), HR 4114 (Giridhar et al. 1997) or of selected IRAS sources such as IRAS 22223+4327 and IRAS 04296+3429 (Decin et al. 1998). However, most of these high galactic latitude stars are field stars of unknown distances. It is therefore likely that a significant fraction of them could possibly turn out to be disk objects of nearly solar compositions. A search of post-AGB stars among high galactic latitude stars could be more rewarding if we put the additional constraint of IR detection. The wavelength dependence of IR fluxes and also the detection of submillimeter fluxes could give valuable information on the circumstellar matter surrounding the evolved star. The IRAS two colour diagrams such as those published by Olnon et al. (1984), van der Veen & Habing (1988) etc., are extremely useful in separating stars with different kinds of envelopes.

In this study, we have undertaken the abundance analysis of a selected sample of stars likely to be post-AGB stars. From the published lists of high galactic latitude stars ( $b > 20^{\rm o}$) (e.g. Bidelman 1990 and others) we chose the ones with known infrared fluxes. Among them, the ones falling into the regions VIa and VIb of Fig. 5b of van der Veen & Habing (1988) were preferred as they were more likely to be post-AGB stars. We have also included a few objects belonging to the regions IIIa and IIIb that are likely to be evolved stars with oxygen-rich envelopes.

The hot star HD 172324 was also included in spite of not being an IRAS source since it has high radial velocity (-110 km s^-1) and very complex structures in hydrogen line profiles. It appeared to be a possible hot post-AGB star similar to those investigated by Conlon et al. (1993a,b).

A search for post-AGB stars among supergiant-like stars of high galactic latitude is expected to be more efficient since the possibility of forming stars at truly large distances from the galactic plane is low.

This program is also aimed at providing calibrators for photometric empirical calibrations of atmospheric abundances (Arellano Ferro & Mantegazza 1996), temperatures, and gravities in particular, since gravities are better determined from the ionization equilibrium.

This paper is organized in the following way: Sect. 2 describes the observations and data reduction; Sect. 3 discusses the methodology of abundance calculation; Sect. 4 gives an account of the sources of uncertainty in the derived abundances; in Sect. 5 the results are given and discussed for each star; in Sect. 6 these results are discussed in terms of the evolutionary status of each star while in Sect. 7 we summarize our results.