Fine-Scale Temperature Fluctuations in the Orion Nebula and the t 2 Problem

Abstract

We present a high spatial resolution map of the columnar electron temperature (T c ) of a region to the southwest of the Trapezium in the Orion Nebula. This map was derived from Hubble Space Telescope images that isolated the primary lines of H i for determination of the local extinction and of the [O iii] lines for determination of T c . Although there is no statistically significant variation of T c with distance from the domi- nant ionizing star, h 1 Ori C, we find small-scale variations in the plane of the sky down to a few arcseconds, which are compatible with the variations inferred from comparing the value of T e derived from forbidden and recombination lines, commonly known as the t 2 problem. We present other evidence for fine-scale variations in conditions in the nebula, these being variations in the surface brightness of the nebula, fluctuations in radial velocities, and ionization changes. From our T c map and other considerations we estimate that t 2 = 0.028 Æ 0.006 for the Orion Nebula. Shadowed regions behind clumps close to the ioniza- tion front can make a significant contribution to the observed temperature fluctuations, but they cannot account for the t 2 values inferred from several methods of temperature determination. It is shown that an anomalous broadening of nebular emission lines appears to have the same sense of correlation as the temperature anomalies, although a causal link is not obvious.