Observational Constraints on the Lifetime of Cometary H2O

Abstract

We used a carefully selected set of observations of five comets along with a vectorial model and the Haser model to explore the lifetimes against photodissociation of H2O and OH in cometary comae. The photometric and long-slit CCD data described in this paper were obtained specifically for this project, while the IUE data are archival. The underlying parameter space was examined and our knowledge of such factors as the outflow and ejection velocities is discussed. We conclude that it is difficult to constrain the lifetime of OH but that the H2O lifetimes must be dependent on changes in the solar UV flux. Our data are consistent with the expectation that as the solar activity, and thus UV flux, increases, the H2O lifetimes decrease. From our data, we show that the combined theoretical lifetimes for H2O and OH do not produce good fits to the data; thus, at least one set of theoretical lifetimes must be in error. Our data require shorter H2O lifetimes and/or shorter OH lifetimes. We present some useful relations for determining which lifetimes and velocities to use for future applications. Additionally, we have derived improved lifetimes and scale lengths for use computing production rates with simple models. The new parent lifetimes and scale lengths are about 15-45% shorter than commonly used values which ignore effects due to solar activity. Finally, we discuss what data types are needed in the future to enable better modeling of cometary data.