Source of water and emission processes in the coma of comet 46P/Wirtanen

Water is one of the most abundant species in comets in the solar system [4]. The progression from water ice stored in the comet's nucleus to the tenuous gas in the outer coma goes through various processes. Photodissociation in the coma produces fragments such as OH and H, and subsequent fluorescent excitation results in OH A-X emission in the near ultraviolet (NUV) and Lyman-alpha in the far ultraviolet (FUV) wavelength regions of its spectrum. However, in the inner coma (<1000 km), other excitation mechanisms such as prompt excitation [1] and electron impact [3] can contribute. The close approach to Earth of comet 46P/Wirtanen in December 2018 was a good opportunity to study the inner coma. During this apparition, the comet came as close as 0.077 AU on 16th December 2018, just four days after its perihelion passage. This work will summarize and compare observations by the Neil Gehrel-Swift observatory and the Hubble space telescope in order to understand the source of water and the emission from its fragments in the inner and outer coma. We used the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope to observe comet 46P between January 10 and 20, 2019. Different gratings spanning the NUV and FUV wavelengths were used to observe the comet. Finally, the spectra were obtained by placing spectrograph slit at three different locations across the coma. The spectrum of the comet was obtained using the long slit of the spectrograph which covers a total of ~3000 km across the coma, which allowed us to obtain the spatial emission profiles of different molecular bands of OH and CS. We acquired near-contemporaneous observations with the UltraViolet and Optical Telescope (UVOT) on board the Neil Gehrels-Swift Observatory during three epochs between November 2018 and January 2019. The UVW1 filter of UVOT includes the OH A-X emission[2]. Therefore, the images obtained using the UVW1 filter can be used to estimate the water production rate. The Swift/UVOT image spans a total radius of more than 40000 km of the coma, thus covering a much larger range than the HST/STIS spectra, connecting the inner coma observations with the outer coma.

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