EFFECTS OF CONDUCTION FRONTS ON DERIVED COMPOSITIONS IN H Ⅱ REGIONS

摘要

In both H Ⅱ regions and planetary nebulae, the (O~(+2)/H~+) and (C~(+2)/H~+) abundances derived from collisionally excited lines analyzed with the temperature from [O Ⅲ] λλ4363/5007 are ≈ 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 confines the visible plasma to ≤ 10% of the nebular volume. The O~(+2) 14363 produced in the transition layer increases the λλ4363/5007 ratio and artificially decreases the derived value of the nebular (O/H) abundance. We find that the λ4363 from the transition layer is not large. In the Orion Nebula, a factor of 2 decrease in measured (O/H) would arise from having the typical line of sight cross ≈ 40-60 separate free-standing ionized sheets. We regard this requirement as excessive. In spite of the unexpected strength of the O~+ and C~+ recombination lines, there is also evidence that the effects of temperature fluctuations are minimal. B stars formed recently from the nebular gas have the same abundances (about 60% solar) as derived from H Ⅱ regions without temperature fluctuations. It is also difficult to account for more than about 60% of solar abundances of O and N in the interstellar medium. We briefly discuss other possible causes of temperature fluctuations and comment on a commonly used method of their analysis.