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Astron. Astrophys. 335, 985-990 (1998)

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6. Conclusions

PG 1323-086 and PG 1704+222 are giant stars with normal helium abundances. Both stars share a characteristic abundance pattern with BD +33[FORMULA]2642 , the central star of a halo planetary nebula, and with two high galactic latitude B-giants discussed by Conlon et al. (1991), most notably the strong carbon deficiency. Depletions of N, O, Mg, Al, and Si by 1 dex or more in PG 1323-086 and PG 1704+222 point towards a low metallicity and, hence, membership of an old stellar population, while they are not compatible with the values found for young B stars. The positions of both stars in the ([FORMULA], log g) diagram are consistent with evolutionary tracks starting from the AGB for masses of about 0.55  [FORMULA] . We therefore conclude that both stars are in the post-AGB stage of their evolution.

The similarity of the abundance patterns to that of BD +33[FORMULA]2642 suggests that the successors of the B-type PAGB stars can be found amongst the halo CSPNe as already suggested by McCausland et al. (1992). Potential progenitors could be found among the optically bright post-AGB stars discussed by van Winckel (1997). However, van Winckel finds only one star, HD 107369, for which the abundance pattern comes close to that of B-type PAGB stars. The CNO abundances of this star are consistent with being the product of CN cycling. In the B-type PAGB stars, however, the C/N ratio can be explained by CN cycling, but not the N/O ratio.

Conlon et al. (1991), McCausland el. (1992) and Napiwotzki et al. (1994) discussed various scenarios to explain the abundance pattern of B-type PAGB stars in terms of nuclear processing and subsequent dredge-up, but no conclusion could be reached. The high [O/Fe] ratio (+1.2[FORMULA]0.5) observed in BD +33[FORMULA]2642 prompted Napiwotzki et al. (1994) to suggest that its abundance pattern could be affected by gas-dust separation invoked by Bond (1991) to explain the extreme [O/Fe] [FORMULA] 4 ratio found in two peculiar PAGB stars of spectral type F (Lambert et al., 1988and Waelkens et al., 1992). Therefore, the determination of the iron abundance in the B-type PAGB stars is a prerequisite for an interpretation of their abundance patterns.

According to the kinematic behaviour of PG 1704+222 and the large distance from the galactic plane for PG 1323-086 both stars are probably Population II objects.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998