Journal
ASTRONOMY & ASTROPHYSICS
Volume 557, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201322020
Keywords
comets: individual: 67P/Churyumov-Gerasimenko
Categories
Funding
- European Union [268421]
- ESO
- NASA [NNX13A151G, NNA09DA77A]
- Canadian Space Agency
- Robert Martin Ayers Sciences Fund
- Alfred P. Sloan Foundation
- National Science Foundation
- US Department of Energy Office of Science
- University of Arizona
- Brazilian Participation Group
- Brookhaven National Laboratory
- University of Cambridge
- Carnegie Mellon University
- University of Florida
- French Participation Group
- German Participation Group
- Harvard University
- Instituto de Astrofisica de Canarias
- Michigan State/Notre Dame/JINA Participation Group
- Johns Hopkins University
- Lawrence Berkeley National Laboratory
- Max Planck Institute for Astrophysics
- Max Planck Institute for Extraterrestrial Physics
- New Mexico State University
- New York University
- Ohio State University
- Pennsylvania State University
- University of Portsmouth
- Princeton University
- Spanish Participation Group
- University of Tokyo
- University of Utah
- Vanderbilt University
- University of Virginia
- University of Washington
- Yale University
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Context. Comet 67P/Churyumov-Gerasimenko was selected in 2003 as the new target of the Rosetta mission. It has since been the subject of a detailed campaign of observations to characterise its nucleus and activity. Aims. We present previously unpublished data taken around the start of activity of the comet in 2007/8, before its last perihelion passage. We constrain the time of the start of activity, and combine this with other data taken throughout the comet's orbit to make predictions for its likely behaviour during 2014/5 while Rosetta is operating. Methods. A considerable difficulty in observing 67P during the past years has been its position against crowded fields towards the Galactic centre for much of the time. The 2007/8 data presented here were particularly difficult, and the comet will once again be badly placed for Earth-based observations in 2014/5. We make use of the difference image analysis technique, which is commonly used in variable star and exoplanet research, to remove background sources and extract images of the comet. In addition, we reprocess a large quantity of archival images of 67P covering its full orbit, to produce a heliocentric lightcurve. By using consistent reduction, measurement and calibration techniques we generate a remarkably clean lightcurve, which can be used to measure a brightness-distance relationship and to predict the future brightness of the comet. Results. We determine that the comet was active around November 2007, at a pre-perihelion distance from the Sun of 4.3 AU. The comet will reach this distance, and probably become active again, in March 2014. We find that the dust brightness can be well described by Af rho proportional to r(-3.2) pre-perihelion and proportional to r(-3.4) post-perihelion, and that the comet has a higher dust-to-gas ratio than average, with log(Af rho/Q(H2O)) = -24.94 +/- 0.22 cm s molecule(-1) at r < 2 AU. A model fit to the photometric data suggests that only a small fraction (1.4%) of the surface is active.
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