The aim of this page is to promote coverage of a very puzzling star, the triple or possibly quadruple eclipsing system, KIC 2856960. This is due to show multiple eclipse events as a compact binary passes in front of a third star around July 10, 2019. Given KIC 2856960's position (RA=19 29 31.521, Dec=+38 04 35.94), this is almost perfectly timed for ground-based follow-up by northern hemisphere observers, with the object typically higher than 40 degrees elevation all night, and only a moderatly illuminated Moon (two thirds). These events happen every 204 days and are are often not easily observable. The last time was in August 2015. The July 2019 events are the best placed since the end of Kepler coverage in May 2013, with the multi-dip eclipse last observed by Kepler in December 2012.
KIC 2856960 has SDSS magnitudes of g=16.4, r=15.6, i=15.3.
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Left: The Kepler light curve of KIC 285690 shows a fuzzy appearance owing to the primary and secondary eclipses of a 6 hour binary along with sharp drops of flux marking the tertiary eclipses of the binary crossing the face of a third star. The insets show blown-up views of two of these events, the second of which was observed in Kepler's 1-minute short cadence mode. You can also see the binary eclipses away from the tertiary eclipse dips. (Figure taken from the model paper.) |
In 2014 I and collaborators published a paper on a remarkable star found in the Kepler field, known as KIC 2856960. This star features the eclipses of a close binary with a period just over 6 hours, along with "tertiary events" that recurr every 204 days when the binary passes in front of a third star (see above). KIC 2856960 thus appears to be a triple star consisting of a tight, probably circular-orbit, binary which is in turn in a much wider (eccentric) orbit with another star. Such arrangements are rare, but of high interest for their potential harvest of precision stellar parameters. Initially confident that we would be able to model these events, we found to our surprise that we simply couldn't. We wrote up this failed modelling attempt in the paper, titling it the "impossible" star, reflecting the current impossibility of modelling (clearly nature does not find it impossible!).
Sadly, the original Kepler survey was brought to an end by a failure of one of Kepler's reaction wheels in May 2013. This was doubly sad for KIC 2856960 since the failure occurred a matter of days before another tertiary event was expected. I organised observers to try to see tertiary eclipses in the fairly well-placed event of August 2015. Four years later and we have an even better opportunity in July 2019, and four years further down the line we can really expect some significant changes in the structure of the tertiary eclipses for reasons I explain in a separate page on modelling KIC 2856960. This is strong motivation to try to cover the July 2019 events.
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If you think you can contribute, please . I will be particularly interested in your location, telescope aperture and possible filters. Please include "KIC 2856960" somewhere in the message to help me when searching later; clicking the previous link should do this automatically.
If possible, we need round-the-clock (which means round the globe) coverage of KIC 2856960 over the 3 day interval July 9 to July 11, 2019 (UTC). The main events are predicted to last a bit more than a day and to be centred on July 10, but this prediction is uncertain! The August 2015 event suggests that if anything the dips will arrive early, so I think July 9 & 10 are the most important, and observations during the latter part of July 8 could be useful too.
CCD photometry is needed as the variations are only ~6% deep at maximum. The events are synchronised to the orbit of the 6 hour binary which means that they occur at a relatively sedate pace, so long exposures (multiple minutes) are possible so long as tracking is accurate. The ideal observations would be to acquire the star as early as possible, and then simply take a time-series of observations for as long and as uniformly as possible until the star is no longer visible. Exposure times from 20 to 600 seconds according to the equipment in use should be fine, but obviously the exposures should be short enough to avoid image trailing and/or saturation, and long enough to avoid too much deadtime between exposures. If possible a colour filter should be used to reduce the effect of colour difference between stars. R is best I think; I or V good alternatives, but filterless ("clear") data are better than none: just the detection of an event is useful. I suggest not cycling through filters. I think intensive observations in a single band are best as they will allow us to build up S-to-N best. The eclipses of the M dwarf binary will be most prominent in I so if you try to observe on July 8 or 11/12, I might be preferred. It would be nice to get a solid detection of the M dwarf binary in I from the ground.