Far-ultraviolet Spectroscopy of Recent Comets with the Cosmic Origins Spectrograph on the Hubble Space Telescope

Since its launch in 1990, the Hubble Space Telescope (HST) has served as a platform with unique capabilities for remote observations of comets in the far-ultraviolet region of the spectrum. Successive generations of imagers and spectrographs have seen large advances in sensitivity and spectral resolution enabling observations of the diverse properties of a representative number of comets during the past 25 years. To date, four comets have been observed in the far-ultraviolet by the Cosmic Origins Spectrograph (COS), the last spectrograph to be installed in HST, in 2009: 103P/Hartley 2, C/2009 P1 (Garradd), C/2012 51 (ISON), and C/2014 Q2 (Lovejoy). COS has unprecedented sensitivity, but limited spatial information in its 2 ''.5 diameter circular aperture, and our objective was to determine the CO production rates from measurements of the CO Fourth Positive system in the spectral range of 1400-1700 angstrom. In the two brightest comets, 19 bands of this system were clearly identified. The water production rates were derived from nearly concurrent observations of the OH (0,0) band at 3085 angstrom by the Space Telescope Imaging Spectrograph. The derived CO/H2O production rate ratio ranged from similar to 0.3% for Hartley 2 to similar to 22% for Garradd. In addition, strong partially resolved emission features due to multiplets of S I, centered at 1429 angstrom and 1479 angstrom, and of C I at 1561 angstrom and 1657 angstrom, were observed in all four comets. Weak emission from several lines of the H-2 Lyman band system, excited by solar Ly alpha and Ly beta pumped fluorescence, were detected in comet Lovejoy.