OBSERVATIONS OF [S-IV] AND THE O S ABUNDANCE RATIO IN 2 HALO POPULATION PLANETARY-NEBULAE/

We have observed two planetary nebulae in the Galactic halo population , K648 and BB-1, to look for the [S IV] 10.5 mum line. In both cases w e detected no [S IV] at a 3 sigma upper flux limit of 3 x 10(-14) ergs cm-2 s-1. Our measurements indicate that there is no significant amou nt of sulfur hidden in the form of S+3 in these objects, confirming th at the O/S abundance ratio is well above the solar and interstellar ra tio in these two nebulae. This contrasts with results for dwarf halo f ield stars and low-abundance H II regions, which show essentially a co nstant, solar, O/S ratio at all metallicities. The high O/S ratios cou ld be explained if the sulfur abundance reflects the initial compositi on of the progenitor stars, while nearly all of the oxygen in the nebu lae is the product of helium burning in the very late evolution of the progenitors.