A Composite Phase Function for Cometary Dust Comae

Comets are primitive planetesimals we can study to infer knowledge about the initial stages of formation of our Solar System. Dust is the largest mass component among cometary materials. Emitted dust particles are anysotropic scatterers of the incident solar light and their nature can be investigated with remote sensing studies. Among such studies, the measurement of the coma phase function curve has an important role for several aspects. Once inverted with theoretical and laboratory studies, the intimate nature (i.e. size, size distribution, composition, and shape) of the emitted dust particles can be investigated. The phase function knowledge is needed when adjusting cometary dust production rates for effects depending on the observational geometry when correlation of data obtained throughout large time intervals is performed. Finally, the coma phase function is useful for space instruments planning since it provides inputs for optimal exposure times, mainly in case observations are spanning a large range of phase angles during close approaches. This will be particularly valuable in the framework of the future ESA Comet Interceptor mission which is going to pass close to a Dynamically New Comet, carrying instruments such the EnVisS (Entire Visible Sky) camera which will image the coma with a large phase angle coverage in a short amount of time.

In order to provide an useful tool to address the aforementioned scientific topics, we used literature data to build a new composite phase function for cometary dust comae. We fitted Henyey-Greenstein (HG) functions to the literature data of 11 comets covering different dynamical classes and we connected them in a continuous way as all data values were coming from a single comet with average scattering properties. We then fitted our result with a compound HG curve and compared it with previous comprehensive models. Our approach contains recent literature data which were not included in previous models. These recent data are providing a good temporal coverage of the cometary coma scattering behavior at small and large phase angles. The most notable difference between our and previous models is found in the description of the forward scattering surge, where our model depicts a scattered intensity one order of magnitude larger than previous ones. This finding is extremely important since it shows that the choice of the phase function model may have severe consequences when scientific interpretation and planning of the observational strategy regarding forward scattering data are taken into account.

Acknowledgements: Contribution of I. Bertini and L. Inno was supported by the ASI-INAF agreement n. 2023-14-HH.0.