Community Research and Development Information Service - CORDIS


BEAMING Report Summary

Project ID: 291352
Funded under: FP7-IDEAS-ERC
Country: Israel

Final Report Summary - BEAMING (Detecting massive-planet/brown-dwarf/low-mass-stellar companions with the beaming effect)

The main goal of the Kepler space mission was to discover transiting planets that pass in front of their parent stars, causing a minute decrease of the stellar brightness for a few hours. The transits should occur periodically, as the planetary motion around its parent star acts like a precise clock. However, sometimes the transits appear with a time shift—TTV (Transit Time Variation), caused by dynamical interaction with other planets in their systems. Observed TTVs can help discovering unseen planets and estimating their mass ratios. We applied a novel systematic analysis to all transits observed by Kepler, and derived exact timing for all available transits with enough signal in their light curves. Over 225,000 timings were presented in two catalogs for the community to use. Including systems found by previous works, we have found 274 planets that showed highly significant TTVs.

Some of the observed TTVs are induced by stellar spots that rotate on the visible hemisphere of their parent stars. We present an approach that can distinguish between a prograde and a retrograde planetary motion with respect to the stellar rotation, using the derived TTVs of a planet and the local slope of the stellar flux at the time of transit. We applied this approach to the Kepler KOIs, identifying nine systems where the photometric spot modulation is large enough and the transit timing accurate enough to allow detection of a TTV-brightness-derivatives correlation. Of those systems, five show highly significant prograde motion, while no system displays retrograde motion, consistent with the suggestion that planets orbiting cool stars have prograde motion. Our analysis further suggest that stellar spots, or at least the larger ones, tend to be located at low stellar latitude, but not along the stellar equator, similar to the Sun.

We have developed the BEER algorithm to find unseen companions of stars observed by the Kepler space mission. Using the BEER algorithm, we have discovered many binaries and brown dwarfs. We have identified and confirmed a new extra-solar planet—Kepler-76, the first of its kind. We have also discovered four new white dwarfs orbiting stars observed by Kepler.

The BEER analysis led to the discovery of evidence for equatorial super-rotation of hot Jupiters—strong winds in the planetary atmosphere that carry energy from the area heated by the stellar radiation to the cold side of the planets. Our analysis detected the super-rotation effect for the first time in optical data. Apparently, super-rotation planetary winds are a common phenomenon in hot Jupiters.

We have invented a new way to derive stellar rotation from the Kepler light curves by using the autocorrelation technique. The novel automated autocorrelation-based method detected rotation periods for 34,030 of the 133,030 main-sequence Kepler targets, making this the largest sample of stellar rotation periods to date.

Throughout their main-sequence lifetime, stars lose angular momentum and spin down via a magnetized wind that is linked to the stellar outer convection zone. Therefore, the present-day rotation periods reflects the integrated angular momentum loss history of the stars. Apparently, the present-day rotation period does not depend on the initial conditions, and therefore a tight relationship exists between the period, age and mass, which is known as gyrochronology, for which a range of empirical relations have been derived. The large sample of derived stellar rotations is therefore a novel way to measure the ages of a large sample of main-sequences stars, and confront stellar theory with observations.

The derived stellar rotation of stars with known transits reveals a notable lack of close-in planets around rapid rotators. It appears that only slowly spinning stars with rotation periods longer than 5-10 days host planets on orbits shorter than 2 or 3 days. This is a strong hint for the inter-connection between the evolution of the stellar spin and the planetary orbit.

Another surprising result of the stellar rotation work was the application for the study of the obliquity of planets—the angle between the stellar axis of rotation and the planetary orbital plane of motion. The analysis of the amplitudes of the stellar rotation modulations suggests that the cool stars have their planets aligned with their stellar rotation, while the planets around hot stars have large obliquities. We show that the low obliquity of the planets around cool stars extends up to at least 50 days. The results have far reaching consequences for our understanding of planetary formation and evolution.

In another application of the autocorrelation technique we analyzed the light curves of 14 normal hot white dwarfs (WDs). In five, and possibly up to seven of the WDs, we detect periodic variations, with semi-amplitudes of 60-2000 parts per million, lower than ever seen in WDs. We consider various explanations. Each mechanism could be behind some of the variable WDs, but could not be responsible for all five to seven variable cases. Alternatively, the periodicity may arise from UV metal-line opacity, associated with accretion of rocky material, a phenomenon seen in about 50 per cent of hot WDs.

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