FAR-INFRARED ABUNDANCE MEASUREMENTS IN THE OUTER GALAXY

Five H II regions at large distances from the center of the Galaxy (R = 13-17 kpc) have been observed in the far-IR emission lines of [O III ] (52 and 88 mu m), [N III] (57 mu m), and [S III] (19 mu m) using the Kuiper Airborne Observatory. These observations have been combined wi th Very Large Array radio continuum observations of these sources to d etermine the abundances of O++, N++, and S++ relative to hydrogen. A s imple ionization correction scheme has been used to determine the tota l abundances of nitrogen and sulfur relative to hydrogen as well as th e relative abundance N/O. For the two sources in common with previous optical studies (S127 and S128), we find good agreement between the fa r-infrared and optical determinations of N/H, S/H, and N/O. Our result s from the outer Galaxy have been combined with previous far-infrared results to determine the abundance gradient of these elements in the M ilky Way over a range of Galactocentric radii from R = 0 to R = 17 kpc . Our results are consistent with a gradient of log N/H = -0.111 +/- 0 .012 dex kpc(-1) and a gradient of log SH = -0.079 +/- 0.009 dex kpc(- 1). Our method is not able to determine: independently the abundances of both S and 0, although other evidence suggests that the O/S ratio i s approximately constant. While these results differ from recent optic al studies, which suggest that these abundance gradients flatten in th e outer Galaxy, we do not vet have sufficient data to rule out such a change in the gradient. The log N/O data are better fitted by a two-st ep function with a value of -0.50 +/- 0.02 for R < 6.2 kpc and -0.83 /- 0.04 for R > 6.2 kpc. Both of these values are consistent with seco ndary production of nitrogen, However, the outer Galaxy oxygen abundan ces are in the low abundance regime where nitrogen is expected to be p roduced by primary processes.