EFFECTS OF CONDUCTION FRONTS ON DERIVED COMPOSITIONS IN H-II REGIONS

In both H II regions and planetary nebulae, the (0(+2)/H+) and (C+2/H) abundances derived from collisionally excited lines analyzed with th e temperature from [O III] lambda lambda 4363/5007 are approximate to 60% of those derived from the recombination lines produced by the same ions. We investigate the line emission from the conductive transition layer between the visible plasma and very hot gas that presumably con fines the visible plasma to less than or equal to 10% of the nebular v olume. The 0(+2) lambda 4363 produced in the transition layer increase s the lambda lambda 4363/5007 ratio and artificially decreases the der ived value of the nebular (O/H) abundance. We find that the lambda 436 3 from the transition layer is not large. In the Orion Nebula, a facto r of 2 decrease in measured (O/H) would arise from having the typical line of sight cross approximate to 40-60 separate free-standing ionize d sheets. We regard this requirement as excessive. In spite of the une xpected strength of the O+ and C+ recombination lines, there is also e vidence that the effects of temperature fluctuations are minimal. B st ars formed recently from the nebular gas have the same abundances (abo ut 60% solar) as derived from H II regions without temperature fluctua tions. It is also difficult to account for more than about 60% of sola r abundances of O and N in the interstellar medium. We briefly discuss other possible causes of temperature fluctuations and comment on a co mmonly used method of their analysis.