NH₃ and NH₂ in the coma of comet Brorsen-Metcalf

Narrow-band CCD images of comet Brorsen-Metcalf have been obtained using interference filters (fwhm = 20 Å) centered at λ = 6250 and 6338 Å to isolate continuum and NH₂ (8-0) òA₁-X̃²B₁ emission, respectively. The 6338 Å images corrected for background sky and dust-scattered solar continuum isolate the NH₂ coma in the comet. The distribution of NH₂ is symmetric and shows no evidence for jet structure at the 3 σ significance level above background emission. An azimuthal average of the NH₂ image produces an NH₂ surface brightness profile for comet Brorsen-Metcalf which gives a factor of ∼ 10 improvement in the signal-to-noise ratio over previous one-dimensional long-slit NH₂ observations, and provides a significant constraint on the NH₂ photodissociation time scale in comets. A Monte Carlo simulation of the comet coma assuming that NH₃ is the primary source of NH₂ is described and compared with the observations. The effects on the surface brightness distribution of NH₂ due to (1) collisions in the inner coma and (2) non-steady state production rates were investigated with the Monte Carlo model. For an observed production rate, Q(H₂ O) ∼ 7 × 10²⁸ molecules s^-1, collisional effects on the NH₃ and NH₂ outflow had at most a ∼ 10% effect on the NH₂ surface brightness profile. The models also indicate that large variations in the gas production rate on time scales ∼ τ_NH2 could cause significant differences from the steady state profiles. Because comet Brorsen-Metcalf showed no significant dust or gas production rate variability, we argue that steady state conditions best match the comet at the time of observations. We find for steady state conditions and a recently revised NH₂ photodissociation time scale, τ_NH2 ∼ 3.3 × 10⁴ s at 1 AU, a satisfactory match between the observed and computed NH₂ surface brightness profiles. For a steady state model with τ_NH2 ∼ 5.0 × 10⁴ s at 1 AU the observations are fitted to an estimated accuracy of ∼ 10%. We conclude that NH₃ is probably the dominant source of NH₂ in the coma of comet Brorsen-Metcalf. The conclusive identification of the NH₂ percursor in comets will require (1) the direct detection of NH₃ , (2) observations of NH₂ emission over several successive nights to monitor possible production rate variations, or (3) accurate experimental photodissociation cross sections for NH₂.