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

Abstract

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