Small-Scale Behavior of the Physical Conditions and the Abundance Discrepancy in the Orion Nebula

Abstract

We present the results of long-slit spectroscopy, in several positions, of the Orion Nebula. Our goal is to study the spatial distributions of a large number of nebular quantities, including line fluxes, physical conditions, and ionic abundances, at a spatial resolution of about 1''. In particular, we have compared the O⁺⁺ abundance determined from collisionally excited and recombination lines in 671 individual one-dimensional spectra covering different morphological zones of the nebula. We find that protoplanetary disks (proplyds) show prominent spikes of T_e≤ft(≤ft[{N} ^\textsc{, which is probably produced by collisional deexcitation due to the high electron densities found in these objects. Herbig-Haro objects show also relatively high values of T_e≤ft(≤ft[{N ^\textsc{, but these are probably produced by local heating due to shocks. We also find that the spatial distribution of the pure recombination O II and [O III] lines is fairly similar. The abundance discrepancy factor (ADF) of O⁺⁺ remains rather constant along the slit positions, except in some particular small areas of the nebula, such as at the locations of the most conspicuous Herbig-Haro objects. There is also an apparent slight increase of the ADF in the inner 40'' around θ¹ Ori C. We find a negative radial gradient of T_e≤ft(≤ft[{O ^\textsc{ and T_e≤ft(≤ft[{N ^{\textsc{ in the nebula, based on the projected distance from θ¹ Ori C. In addition, the ADF of O⁺⁺ seems to increase very slightly with the electron temperature. Finally, we estimate the value of the mean-square electron temperature fluctuation, the so-called t²