Stellar Occultation by Comet 67P/Churyumov–Gerasimenko Observed with Rosetta?s Alice Far-ultraviolet Spectrograph

摘要

Following our previous detection of ubiquitous H O2and O 2 absorption against the far-ultraviolet 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 ?) spectrograph, both of which show H O 2 absorption with column density >10 17.5 ?cm ?2 and significant O 2 absorption ( O 2 / H O2?≈?5%–10%). Because the projected distance from the star to the nucleuschanges between exposures, our ability to study the H O2 column density profile near the nucleus (impactparameters <1 km) is unmatched by our previous observations. We find that the H O 2 and O 2 column densities decrease with increasing impact parameter, in accordance with expectations, but the O 2 column decreases ～3 times more quickly than H O 2 . When combined with previously published results from stellar appulses, we conclude that the O 2 and H O 2 column densities are highly correlated, and O 2 / H O 2 decreases with the increasing H O 2 column.