A Further Investigation of Apparent Periodicities and the Rotational State of Comet 103P/Hartley 2 from Combined Coma Morphology and Light Curve Data Sets

We present an analysis of Kitt Peak National Observatory and Lowell Observatory observations of comet 103P/Hartley 2 obtained from 2010 August through December. The results are then compared with contemporaneous observations made by the EPOXI spacecraft. Each ground-based data set has previously been investigated individually; the combined data set has complementary coverage that reduces the time between observing runs and allows us to determine additional apparent periods at intermediate times. We compare CN coma morphology between ground-based data sets, making nine new measurements of apparent periods. The first five are consistent with the roughly linearly increasing apparent period during the apparition found by previous authors. The final four suggest that the change in apparent period slowed or stopped by late November. We also measure an inner coma light curve in both CN and R-band ground-based images, finding a single-peaked light curve which repeats in phase with the coma morphology. The apparent period from the light curve had significantly larger uncertainties than from the coma morphology, but varied over the apparition in a similar manner. Our ground-based light curve aligns with the published EPOXI light curve, indicating that the light curve represents changing activity rather than viewing geometry of structures in the coma. The EPOXI light curve can best be phased by a triple-peaked period near 54-55 hr that increases from October to November. This phasing reveals that the spacing between maxima is not constant, and that the overall light curve shape evolves from one triple-peaked cycle to the next. These behaviors suggest that much of the scatter in apparent periods derived from ground-based data sets acquired at similar epochs are likely due to limited sampling of the data.