Ionized magnesium in the planetary nebula NGC 7027

Observations of NGC 7027 are presented for six ionic lines: Mg(+3) (4.48 microns), Mg(+4) (5.61 microns), H(0) (4.05 and 7.46 microns), Ne(+5) (7.64 microns), and Ar(+5) (4.53 microns). The magnesium lines are consistent with the measurements of Russell, Soifer, and Willner (1977), and the hydrogen lines are consistent with the line strengths predicted from the radio flux. Upper limits were obtained for the neon and argon lines. The abundance of magnesium in the central part of the nebula is highly uncertain because the fine-structure collision strengths are poorly known. The strong gradient of magnesium abundance from the inner to the outer portions of the nebula derived by Pequignot and Stasinska (1980) could be an artifact of this uncertainty. A brief analysis of the effective stellar temperature derived from the magnesium line ratios is given.