5.3.4. PNe in the Galactic halo
Only a small number of PNe in the halo are known so far, less than 20, for an expected total of several thousands(see e.g. Tovmassian et al. 2001). This number is however rapidly growing, thanks to systematic sky surveys at high Galactic latitudes for the search of emission line galaxies, and in which PNe are discovered serendipitously. Halo PNe belong to an old metal poor stellar population, and therefore serve as probes of the halo chemical composition at the time of the formation of their progenitors. They also give the opportunity to study mixing processes in metal poor intermediate mass stars.
Using published spectral line data, Howard et al. (1997) rederived the chemical composition of 9 halo PNe in a consistent way. They found that all had subsolar O/H, the most oxygen poor being K648, with log O/H + 12 = 7.61 (i.e. about 1/20 of the Anders & Grevesse 1989 solar value). They also found that the spread in Ne/O, S/O and Ar/O is much larger than can be accounted for by uncertainties alone. This scatter in PNe abundances is similar to the scatter observed in halo stars (Krishnaswamy-Gilroy et al. 1988), and suggests that accretion of extragalactic material occured during formation of the halo. It must be noted however that, among PNe considered to be in the halo, some actually probably belong to an old disk population (Torres-Peimbert et al. 1990).
After the study of Howard et al. (1997), a few other PNe were discovered in the halo and their chemical composition analyzed (Jacoby et al. 1997, Napiwotzki et al. 1994, Tovmassian et al. 2001). The most spectacular one is SBS 1150+599A (renamed PN G 135.9+55.9), which has an oxygen abundance less than 1/100 solar (Tovmassian et al. 2001). This makes it by far the most oxygen poor PN known (and perhaps the most oxygen poor star known). One may ask whether the oxygen abundance in this object really reflects that of the initial star. Indeed, bright giants in metal poor globular clusters seem to present star to star oxygen abundance variations (see e.g. Ivans et al. 1999), and mixing processes have been invoked to explain these abundance patterns (see Charbonnel & Palacios 2001 for a review). One could invoke that a similar process affects the oxygen abundance in PN G 135.9+55.9. However, Ne is found to be also strongly underabundant in this object (Ne/O ~ 0.3, paper in preparation), indicating that this object is indeed extremely metal poor. In that case, the progenitor must have formed very early in the Galaxy but given rise to a PN only recently. Alternatively, it could have formed out of infalling metal poor material at a more recent epoch.