Large‐Aperture [O i ] 6300 A Photometry of Comet Hale‐Bopp: Implications for the Photochemistry of OH

Abstract

Large-aperture photometric observations of comet Hale-Bopp (C/1995 O1) in the forbidden red line of neutral oxygen ([O I] 6300 Å) with the 150 mm dual-etalon Fabry-Pérot spectrometer that comprises the Wisconsin Hα Mapper and a 50 mm dual-etalon Fabry-Pérot spectrometer at the McMath-Pierce main telescope from 1997 late February to mid April yield a total metastable Q( 1 D) production rate of (2.3-5.9) × 10 30 s -1 . Applying the standard H 2 O and OH photodissociation branching ratios found in Huebner, Keady, & Lyon and van Dishoeck & Dalgarno, we derive a water production rate, Q(H 2 O), of (2.6-6.1) × 10 31 s -1 , which disagrees with Q(H 2 O) ≈ 1 × 10 31 s -1 determined by independent H 2 O, OH, and H measurements. Furthermore, our own [O I] 6300 Å observations of the inner coma ( < 30,000 km) using the 3.5 m Wisconsin-Indiana-Yale-NOAO telescope Hydra and Densepak multi-object spectrographs yield Q(H 2 O) ≈ 1 × 10 31 s -1 . Using our [O I] 6300 Å data, which cover spatial scales ranging from 2,000 to 1 × 10 6 km, and a complementary set of wide-field ground-based OH images, we can constrain the sources of the apparent excess O( 1 D) emission to the outer coma, where photodissociation of OH is assumed to be the dominant Q( 1 D) production mechanism. From production rates of other oxygen-bearing volatiles (e.g., CO and CO 2 ), we can account for at most 30% of the observed excess O( 1 D) emission. Since even less O( 1 D) should be coming from other sources (e.g., electron excitation of neutral O and distributed nonnuclear sources of H 2 O), we hypothesize that the bulk of the excess O( 1 D) is likely coming from photodissociating OH. Using the experimental OH photodissociation cross section of Nee & Lee at Lyα as a guide in modifying the theoretical OH cross sections of van Dishoeck & Dalgarno, we can account for ≈60% of the observed O( 1 D) excess without requiring major modifications to the other OH branching ratios or the total OH photodissociation lifetime.