Searching for Outbursts in Ground-Based Photometry of 67P/Churyumov-Gerasimenko

Comets are the primitive building blocks of the Solar System. In order to understand the extent of the pristine nature of comets, we must understand the mechanisms that affect their surfaces and comae – their activity.

Activity can be tracked in a variety of ways, such as observing dust production in the coma and tail. We can study the activity in a quantitative way through photometry of the reflected sunlight, as flux is proportional to the reflecting area of the dust lifted off the comet by its activity. Activity varies from comet to comet so we must try to distinguish whether these differences in activity are because of ageing or reflect primordial differences. Ageing refers to effects that have chemically or physically altered the nucleus since its formation and may cause a change in the activity. The most obvious of which is the production of comae and tails, caused by escaping gas and dust lifted off the surface of the comet. Other signs of activity include outbursts, where material is suddenly ejected into the coma, causing an immediate and rapid brightening in telescopic observation.

All processes that physically and chemically alter a cometary nucleus can be regarded as ageing. The ageing processes are too slow and irregular to become detectable during a single observation. When comets are observed over a number of Solar System returns some changes can be detected (Meech & Svoren, 2004). As such, that evolution of comets is best studied on a statistical basis.

Distinguishing the effects of ageing from primordial differences is important since the wide range of formation scenarios should imply significantly different observational qualities. Once the differences are established, it will allow us to use the comets as time capsules of the chemical and physical conditions in the early Solar System.

67P/Churyumov-Gerasimenko is a Jupiter family comet that was the target of the Rosetta mission, the first mission to successfully orbit and land a probe on a comet, and this mission was accompanied by a large ground-based observing campaign (Snodgrass et al., 2017). Some of the data from this campaign have been previously calibrated approximately but we present a more accurate calibration using background stars.

We are developing a pipeline to calibrate and measure photometry of comet 67P during its 2016 perihelion passage, making use of all visible wavelength imaging collected at a wide range of facilities. The pipeline calibrates the brightness to a common (PanSTARRS1) photometric system using background stars in the field via the calviacat program.

The pipeline has been successfully applied to two data sets of 67P so far: images taken with the FORS2 instrument on the 8m Very Large Telescope (VLT) and with IO:I on the 2m Liverpool Telescope (LT). Both light curves are presented and overlaid in Fig 1. Preliminary results follow predictions based on observations in previous orbits (Snodgrass et al., 2013) and show no obvious deviations, indicating that 67P is a ‘well behaved’ comet and that it doesn’t show significant changes due to ageing from orbit-to-orbit. Further data sets and more detailed analysis will be presented in this talk.

The calibrated data will act as a companion to the Rosetta mission. We will search for small-scale variation in activity that can be related to outbursts measured in situ by the spacecraft (Vincent et al., 2016) to place constraints on these outbursts and their detectability from Earth-based observations. This will help us constrain how well global activity measures relate to local changes on the comet nucleus. The lessons learned from Rosetta can then be applied to the wider comet population, many of which will never be visited directly and only ever observed from the ground.

Figure 1: Preliminary light curves of 67P photometry taken from VLT and LT telescopes. Light curves are in good agreement with each other and predictions.

References
Meech, K. J.  and Svoren, J. 2004, In Comets II, pages 317-335. Univ. of Arizona, Tuscon
Snodgrass, C. et al. 2013, A&A, 557:A33
Snodgrass, C. et al. 2017, Phil. Trans. R. Soc. A, 375(2097):2016024
Vincent, J.-B. et al. 2016, MNRAS, 462(Suppl_1):S184-S194