Physical and dynamical characterization of hyperbolic comet C/2017 U7 (PANSTARRS)

Evangelista-Santana, M.; Carvano, J. M.; De Prá, M.; de la Fuente Marcos, R.; Schambeau, C.; Licandro, J.; de la Fuente Marcos, C.; Souza-Feliciano, A. C.; Pinilla-Alonso, N.
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We present here a dynamical and observational study of the comet C/2017 U7 (PANSTARRS). This comet was discovered in 2017 and found to have a hyperbolic orbit. Our dynamical analysis shows that the object has probably originated in the Oort cloud, however an interstellar origin cannot be discarded. The observations were obtained in 2018 and 2019 using the Goodman High Throughput Spectrograph (GHTS) at the SOAR telescope. We obtained visible spectra covering the wavelength range of 0 . 5 - 0 . 9 μm and also images in the SDSS filters system. Both the low-resolution reflectance spectrum and the reflectance spectra derived from the SDSS filters show an atypical band at ∼ 0 . 595 μm . We conducted a comparative study of the colors and reflectance spectra of different small body populations (e.g., comets, Centaurs, and trans-Neptunian objects or TNOs) from the literature and concluded that the spectra and the colors of this comet are atypical, showing only some overlap with those of some known members of the TNOs and Centaurs, within the large uncertainties of the measurements of those populations. It is found that the feature and overall spectral shape can be reproduced by laboratory spectra of kerite, a template for aliphatic-rich hydrocarbons that has been previously identified in NIR cometary spectra absorptions. It is tentatively proposed that the unusual spectral shape is the result of a particle size distribution of dust grains in the coma or on the surface that has arisen due to a low grain ejection velocity from the surface and large nucleus size.
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