Small scale behavior of the physical conditions and the abundance discrepancy in the Orion nebula

TL;DR

This study investigates the small-scale physical conditions and abundance discrepancies in the Orion nebula using high-resolution spectroscopy, revealing localized temperature variations and the behavior of the abundance discrepancy factor in relation to nebular features.

Contribution

It provides detailed spatial analysis of nebular physical conditions and abundance discrepancies at a 1" resolution, highlighting the influence of proplyds and Herbig-Haro objects.

Findings

Proplyds show high electron densities causing T([N II]) spikes.

Herbig-Haro objects exhibit elevated T([N II]) likely due to shocks.

The O II and [O III] recombination line distributions are similar.

Abstract

We present results of long-slit spectroscopy in several positions of the Orion nebula. Our goal is to study the spatial distribution of a large number of nebular quantities, including line fluxes, physical conditions and ionic abundances at a spatial resolution of about 1". We find that protoplanetary disks (proplyds) show prominent spikes of T([N II]) probably produced by collisional deexcitation due to the high electron densities found in these objects. Herbig-Haro objects show also relatively high T([N II]) but probably produced by local heating due to shocks. We also find that the spatial distribution of pure recombination O II and [O III] lines is fairly similar, in contrast to that observed in planetary nebulae. The abundance discrepancy factor (ADF) of O^{++} remains rather constant along the slit positions, except in some particular small areas of the nebula where this quantity reaches somewhat higher values, in particular at the location of the most conspicuous Herbig-Haro objects: HH 202, HH 203, and HH 204. There is also an apparent slight increase of the ADF in the inner 40" around theta^1 Ori C. We find a negative radial gradient of T([O III]) and T([N II]) in the nebula based on the projected distance from theta^1 Ori C. We explore the behavior of the ADF of O^{++} with respect to other nebular quantities, finding that it seems to increase very slightly with the electron temperature. Finally, we estimate the value of the mean-square electron temperature fluctuation, the so-called t^2 parameter. Our results indicate that the hypothetical thermal inhomogeneities --if they exist-- should be smaller than our spatial resolution element.