NH3 and NH2 in the coma of comet Brorsen-Metcalf

Stephen C. Tegler, Luke F. Burke, Susan Wyckoff, Maria Womack, Uwe Fink, Michael Disanti

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


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) Ã2A1-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 ∼ τNH2 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, τNH2 ∼ 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 τNH2 ∼ 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.

Original languageEnglish (US)
Pages (from-to)292-297
Number of pages6
JournalAstrophysical Journal
Issue number1
StatePublished - 1992

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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