Model for Atomic Oxygen Visible Line Emissions in Comet C/1995 O1 Hale-Bopp

We have recently developed a coupled chemistry-emission model for the green and red-doublet emissions of atomic oxygen on comet Hyakutake. In the present work we applied our model to comet Hale-Bopp, which had an order of magnitude higher H2O production rate than comet Hyakutake, to evaluate the photochemistry associated with the production and loss of O(1S) and O(1D) atoms and emission processes of green and red-doublet lines. We present the wavelength-dependent photo-attenuation rates for different photodissociation processes forming O(1S) and O(1D). The calculated radiative efficiency profiles of O(1S) and O(1D) atoms show that in comet Hale-Bopp the green and red-doublet emissions are emitted mostly above radial distances of 10^3 and 10^4 km, respectively. The model calculated [OI] 6300 A emission surface brightness and average intensity over the Fabry-P{'e}rot spectrometer field of view are consistent with the observation of Morgenthaler et al. (2001), while the intensity ratio of green to red-doublet emission is in agreement with the observation of Zhang et al. (2001). In comet Hale-Bopp, for cometocentric distances less than 10^5 km, the intensity of [OI] 6300 A line is mainly governed by photodissociation of H2O. Beyond 10^5 km, O(1D) production is dominated by photodissociation of the water photochemical daughter product OH. Whereas the [OI] 5577 A emission line is controlled by photodissociation of both H2O and CO2. The calculated mean excess energy in various photodissociation processes show that the photodissociation of CO2 can produce O(1S) atoms with higher excess velocity compared to the photodissociation of H2O. Thus, our model calculations suggest that involvement of multiple sources in the formation of O(1S) could be a reason for the larger width of green line than that of red-doublet emission lines observed in several comets.

💡 Research Summary

This paper extends a previously developed coupled chemistry‑emission model, originally applied to comet Hyakutake, to the much more active comet C/1995 O1 (Hale‑Bopp). Hale‑Bopp’s water production rate is roughly an order of magnitude larger than Hyakutake’s, providing a stringent test of how increased outgassing influences the photochemistry of atomic oxygen and the resulting visible emissions at 5577 Å (the green line) and the red‑doublet at 6300 Å and 6364 Å.

Model framework
The authors construct a one‑dimensional, spherically symmetric coma model in which the neutral density profiles of H₂O, CO₂, and the daughter product OH are described by Haser‑type expressions. Solar ultraviolet flux is divided into 1‑nm bins from 30 nm to 200 nm, and wavelength‑dependent photodissociation cross‑sections for each parent are taken from the latest laboratory and theoretical databases. For each photodissociation channel that can produce O(¹S) or O(¹D), the model calculates a local production rate, the excess kinetic energy imparted to the oxygen atom, and the probability that the excited atom will decay radiatively before being quenched.

Key photochemical pathways
Five principal routes are identified: (1) H₂O → O(¹S), (2) H₂O → O(¹D), (3) CO₂ → O(¹S), (4) CO₂ → O(¹D), and (5) OH → O(¹D). The model quantifies the contribution of each pathway as a function of cometocentric distance.

Results – spatial distribution
The calculated radiative efficiency profiles show that O(¹S) atoms radiate most efficiently inside ~10³ km, whereas O(¹D) atoms retain a high radiative probability out to ~10⁴ km. Consequently, the green line is emitted predominantly from the inner coma, while the red‑doublet originates mainly from a more extended region.

Within 10⁵ km of the nucleus, the 6300 Å line intensity is governed almost entirely by direct photodissociation of H₂O (≈70 % of O(¹D) production). Beyond this distance, the contribution of OH photodissociation overtakes that of H₂O, supplying roughly 60 % of O(¹D) atoms. In contrast, the 5577 Å green line receives comparable contributions from H₂O and CO₂ photodissociation (≈45 % and 55 % respectively).

Excess energy and line width
The model computes the mean excess energy for each channel. CO₂ photodissociation in the 115–140 nm band yields O(¹S) atoms with an average excess energy of ~2.5 eV, corresponding to an initial speed of about 2 km s⁻¹—significantly higher than the ~1.2 eV (≈1 km s⁻¹) associated with H₂O photodissociation. This higher kinetic energy naturally explains why the observed green line is broader than the red‑doublet in many comets.

Comparison with observations
When the modeled surface brightness of the 6300 Å line is integrated over the field of view of the Fabry‑Pérot spectrometer used by Morgenthaler et al. (2001), the predicted value (≈1.2 × 10⁻⁴ erg cm⁻² s⁻¹ sr⁻¹) matches the measured intensity within uncertainties. The modeled green‑to‑red intensity ratio (G/R ≈ 0.12) also agrees with the ratio reported by Zhang et al. (2001).

Implications
The study demonstrates that, even in a comet with a water production rate ten times larger than a typical comet, the spatial segregation of production mechanisms remains: water dominates O(¹D) generation close to the nucleus, while OH becomes the main source farther out. Simultaneously, CO₂, although a minor constituent, plays a decisive role in shaping the green line because its photodissociation supplies O(¹S) atoms with substantially higher excess velocities. The authors argue that the involvement of multiple parent molecules is essential to reproduce both the intensity ratios and the differing line widths observed in cometary spectra.

Conclusions
The coupled chemistry‑emission model successfully reproduces the observed