The Ortho-Para Ratio of Water Vapor in Comet p/ Halley

Details The Ortho-Para Ratio of Water Vapor in Comet p/ Halley The Ortho-Para Ratio of Water Vapor in Comet p/ Halley

Nov 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987a%26a...187..419m&link_type=abstract

Astronomy and Astrophysics, Vol.187, NO. 1+2/NOV, P. 419, 1987

Astronomy and Astrophysics

Astrophysics

73

Scientific paper

The abundance ratio of ortho-H2O to para-H2O is shown to be an invariant in the cometary coma, its dependence on temperature in thermal equilibrium is given, and the nuclear-spin-temperature is defined. The relation between the physical temperature of the cometary ice and the nuclear-spin-temperature is discussed, and the prospects for using the observed ortho-para ratio to infer properties of the cometary nucleus are explored. The ortho-para ratio in Halley's comet, derived from high resolution infrared spectra of the v3-band near 2.7 μm wavelength, is given. On UT 1985 December 24.1, it was 273 ± 0.17 and on UT 1986 March 22.7/26.7, it was 3.23 ± 0.37. The error estimates contain a sizeable subjective component for atmospheric transmittance, and may be reduced pending further modelling. The nuclear-spin-temperature was 35 K (+9 K, -5 K) pre-perihelion, and >40 K post-perihelion, at the 67 % confidence limit. Both numbers are consistent with modelled values of the equilibrium temperature of Halley's nucleus at aphelion (˜ 47 K). However, at the 95 % confidence limit they are also fully consistent with temperatures exceeding 50 K, corresponding to an ortho-para ratio of ˜ 3.0.

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