Measurements of [C I] emission from comet Hale-Bopp

We present quantitative measurements of cometary [C I] 9850 Å emission obtained during observations of comet Hale-Bopp (C/1995 O1) in 1997 March and April. The observations were carried out using a high-resolution (λ/Δλ ≈ 40,000) Fabry-Pérot/CCD spectrometer at the McMath-Pierce Solar telescope on Kitt Peak. This forbidden line, the carbon analog of [O I] 6300 Å, is emitted in the radiative decay of C(¹D) atoms. In the absence of other sources and sinks, [C I] 9850 Å emission can be used as a direct tracer of CO photo-dissociation in comets. However, in Hale-Bopp's large, dense coma, other processes, such as collisional excitation of ground-state C(³P), dissociative recombination of CO⁺, and collisional dissociation of CO and CO₂ may produce significant amounts of C(¹D). The long C(¹D) radiative lifetime (∼4000 s) makes collisional deexcitation (quenching) the primary loss mechanism in the inner coma. Thus, a detailed, self-consistent global model of collisional and photochemical interactions is necessary to fully account for [C I] 9850 Å emission in comet Hale-Bopp.