Prompt Emission by Oh in Comet Hyakutake

Abstract

Laboratory studies predict that the photo-dissociation of H 2 O by solar Ly a photons in the comae of comets should lead to a small percentage of OH in high rotational states of the A 2 S + electronic state. These states should promptly emit a near-ultraviolet (near-UV) photon in a transition to the X 2 P state. From there, the radicals decay to the lowest rotational states by direct rotational transitions and via ro-vibrational cascade in the 1-0 vibrational band, all within the X 2 P state. Normally in Earth-based observations the lines are extremely weak compared to the fluorescence of OH in sunlight. Since the prompt emission rate is directly proportional to the column density of water, whereas the fluorescent emission of OH is proportional to the column density of OH, the lines due to prompt emission are relatively strongest very close to the nucleus, a region not often accessible from Earth. We report here the first spectrally resolved detection of near-UV prompt emission by cometary OH, as seen in the bright comet Hyakutake (C/1996 B2), on the day of its closest approach to Earth. All the expected doublets are seen, in both the P and Q branches, up to rotational quantum number N′ = 22 with relative strengths in good agreement with the laboratory results. The fluxes of the lines are in agreement with what one expects from the various measurements of the water production rate of the comet, including that deduced from the OH fluorescent lines in these spectra.

The Rotation and Other Properties of Comet 49P/Arend–Rigaux, 1984–2012

“…We also compute abundance ratios, water production rates, the effective active area on the surface of the nucleus required to produce the water based on a standard vaporization model, and the active fraction based on the surface area of the nucleus. For the dust, the fluxes and the now standard proxy for dust production, Afρ, (A'Hearn et al 1984b) are determined from the continuum measurements (see Tables 4,5 and 6). Because of the wide range of solar phase angles, particularly in 1984/85, phase adjustments were made to yield A(0°)fρ.…”

ACTIVITY OF 50 LONG-PERIOD COMETS BEYOND 5.2 au

“…Since all comets were observed in activity and all had an apparently extended coma, we could determine the Af ρ quantity and its slope (A'Hearn at al. 1984).…”

Comets: looking ahead

“…It is not yet clear to what extent these new insights will matter for remote sensing observations of comets [32]. Certainly prompt emission has been observed for decades in the [O I] red and green lines at large distances from the nucleus by numerous investigators up to the present [33] and prompt emission in OH has similarly been observed, albeit much less extensively, up to 1000 km from the nucleus both in the near IR [34,35] and in the near UV [36,37]. It is therefore clear that the broader issues raised by the inner-coma observations from Rosetta need to be explored for their implications for remote sensing because 1000 km resolution is common in spectroscopy of comets.…”