Atomic Oxygen in the Comae of Comets

TL;DR

This study detects atomic oxygen emission lines in eight comets, analyzing their intensities and widths to infer the parent molecules and excitation processes, highlighting the role of water and potential collisional effects.

Contribution

First high-resolution spectral analysis of atomic oxygen lines in multiple comets, providing insights into their excitation mechanisms and parent molecules.

Findings

Red line intensities match theoretical predictions.

Green line is about 10% of red lines' combined intensity.

Green line is broader than red lines, suggesting different excitation or parent molecules.

Abstract

We report on the detection of atomic oxygen lines in the spectra of 8 comets. These forbidden lines are a result of the photodissociation of the parent oxygen-bearing species directly into an excited state. We used high resolution spectra obtained at the McDonald Observatory 2.7m telescope to resolve the cometary oxygen lines from the telluric oxygen lines and from other cometary emissions. We find that the relative intensities of the two red lines (6300.304 and 6363.776A) are consistent with theory. The green line (5577.339A) has an intensity which is about 10% of the sum of the intensities of the two red lines. We show that collisional quenching may be important in the inner coma. If we assume the relative excitation rates of potential parents which have appeared in the literature, then H2O would be the parent of the cometary green oxygen line. However, those rates have been questioned. We measured the width of the three oxygen lines and find that the green line is wider than either of the two red lines. The finding of a wider line could imply a different parent for the green and red lines. However, the constancy of the green to red line flux ratio suggests the parent is the same for these lines but that the exciting photons have different energies.