Abstract :
[en] E. Jehin, J. Manfroid, and D. Hutsemekers, Universite de Liege; O. Hainaut, C. Opitom, Y. Moulane, and Bin Yang, European Southern Observatory; P. Rousselot, Universite Bourgogne Franche-Comte; A. Fitzsimmons, Queen's University, Belfast; K. Meech, J. Kleyna, and J. Keane, University of Hawaii; C. Snodgrass, University of Edinburgh; and M. Micheli, NEO Coordination Centre, European Space Agency, report on the near-ultraviolet observations of the interstellar comet 2I/Borisov with the 8.2-m Very Large Telescope (+ UVES spectrograph) at the European Southern Observatory on 2019 Dec. 24 and 26, two weeks after perihelion (when the comet was at r = 2.04 AU). Analysis of two 6600-s exposures obtained through a 1".8-wide slit reveals several lines of the OH(0-0) band at 310 nm, a photodissociation product of H_2O. The spectra were extracted over a slit length of 10", centered on the nuclear condensation, and flux-calibrated with a spectrophotometric standard star. The dust continuum was fitted and then removed with a solar analogue star. The two spectra were then averaged, and the seventeen brightest lines were summed. The total emission flux measured is 4.1 x 10**-15 erg/s/cm**2, and a water-production rate of Q(H_2O) = 2.0 x 10**26 molecules/s, assuming 91 percent of H_2O dissociates to OH (Crovisier 1989, A.Ap. 213, 459), was determined with a standard Haser model using the scale lengths from Cochran and Schleicher (1993, Icarus 105, 235). The same velocity was assumed for the daughter and parent species: 595 m/s, derived from the formula V(H_2O) = 850/sqrt(Rh) m/s (Cochran and Schleicher, op.cit.). To compare these new measurements to those of Xing et al. (2020, website URL https://arxiv.org/abs/2001.04865), the authors also computed Q(H_2O) using the same parameters and found 3.4 x 10**26 molecules/s (with 85 percent of H_2O producing OH).
