Stellar Occultation by Comet 67P/Churyumov-Gerasimenko Observed with Rosetta's Alice Far-Ultraviolet Spectrograph

Abstract

Following our previous detection of ubiquitous H2O and O2 absorption against the far-UV continuum of stars located near the nucleus of Comet 67P/Churyumov-Gerasimenko, we present a serendipitously observed stellar occultation that occurred on 2015 September 13, approximately one month after the comet's perihelion passage. The occultation appears in two consecutive 10-minute spectral images obtained by Alice, Rosetta's ultraviolet (700-2100 A) spectrograph, both of which show H2O absorption with column density $>10^{17.5}{\mathrm{cm}}^{-2}$ and significant O2 absorption ($O2/H2O\approx 5$-10%). Because the projected distance from the star to the nucleus changes between exposures, our ability to study the H2O column density profile near the nucleus (impact parameters $<1$ km) is unmatched by our previous observations. We find that the H2O and O2 column densities decrease with increasing impact parameter, in accordance with expectations, but the O2 column decreases $\sim 3$ times more quickly than H2O. When combined with previously published results from stellar appulses, we conclude that the O2 and H2O column densities are highly correlated, and O2/H2O decreases with increasing H2O column.

Accepted for publication in the Astronomical Journal; 9 pages, 7 figures

References in corpus (2)

• Alice: The Rosetta Ultraviolet Imaging Spectrograph
• Evolution of CO${}_2$, CH${}_4$, and OCS abundances relative to H${}_2$O in the coma of comet 67P around perihelion from Rosetta/VIRTIS-H observations

Cited by in corpus (1)

• Upper Limits for Emissions in the Coma of Comet 67P/Churyumov-Gerasimenko Near Perihelion as Measured by Rosetta's Alice Far-Ultraviolet Spectrograph