Tag Archives: WASP-85

Outer orbits of binary stars hosting WASP planets

Many of the WASP transiting exoplanets have a companion star visible close to the planet-host star, and these are usually genuine binary companions rather than chance alignments. This raises questions as to whether the gravitational perturbation of the companion affects the planet formation, and whether the cumulative affect of perturbations alters the planetary orbits.

The very existence of close-in hot-Jupiter planets might owe to the Kozai effect, in which companion stars perturb planets into highly eccentric orbits that have very close approaches to their host star, leading to tidal capture into close, circular hot-Jupiter orbits.

A new paper led by Daniel Evans, from Keele University, uses lucky-imaging techniques to look for close companions of known exoplanet hosts. For the first time, they also report observations of the companions over several epochs, which then gives constraints on their orbits.

The above figure for WASP-77 (left) and WASP-85 (right) shows the observed locations of the companion stars (black symbols; the scale is in Astronomical Units from the planet-host star). The blue lines are possible orbits, computed to be consistent with the data. In both cases the companion stars are shown to be in moderately eccentric orbits with separations of hundreds of AU.

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Starspots on WASP-85 from K2 transits

If, during a transit of its star, an exoplanet crosses a star spot, it will be covering a region that is dimmer than the rest of the star. Since less light will be being occulted, we will see a small increase or “bump” in the transit profile. WASP-85 was recently observed by the K2 mission, getting sufficiently high-quality photometry that it could reveal such starspot `bumps”.

Here is the transit proflle, from a paper by Teo Močnik et al, which contains all the K2 data folded in:

WASP-85b transit profile observed with Kepler K2.

Teo Močnik then subtracted the overall transit profile, thus showing the departures from the average behaviour, and produced a plot of each transit:

WASP-85 starspots observed with K2

The vertical dashed lines show the regions in transit. The lightcurve bumps circled in red are starspots being occulted. (Blue arrows are times when K2 fired its thrusters, which can cause a feature in the lightcurve.)

The interesting question is whether a bump recurs in the next transit, but shifted later in phase, as it would if the same starspot is being occulted again. This would happen if the planet’s orbit is aligned with the stellar rotation. In that case, as the star rotates, the spot moves along the line of transit, to be occulted again next transit.

Aligned orbit star spot occultation

An illustration of a planet occulting a star spot when the planet’s orbit and the star’s rotation are aligned. Graphic by Cristina Sanchis Ojeda

To judge whether the starspot bumps repeat, Teo gave all the co-authors a set of lightcurves and asked them to judge which features in the lightcurve were genuine bumps. But, to avoid human bias, he first scrambled the order of the lightcurves, so that the co-authors didn’t know which lightcurve came next.

The result is that we think that starspots do repeat, shown by the red linking lines in the above figure. This shows that the planet’s orbit is aligned, and it also allows us to estimate the rotational period of the star.

WASP and Kepler K2

WASP-85 is a binary star, with the hot Jupiter WASP-85Ab orbiting the brighter star of the pair. It was in the Campaign 1 field of the revamped Kepler K2 mission, and thus we have the first extensive Kepler-quality lightcurve of a WASP planetary system.

K2 light curve of WASP-85

The WASP discovery paper by David Brown et al presents an initial look at the long-cadence K2 data. The upper plot shows the entire light curve, with obvious variability of the star (presumably because it is magnetically active) and narrow dips caused by the transits. The lower plot shows the data folded on the transit.

K2 light curve of WASP-85 transit

The higher-time-resolution “short cadence” data will be available soon, and should allow a high-quality analysis of this system. The WASP planets WASP-47b and WASP-75b are being observed in the current K2 Campaign 3, which should lead to more space-quality light curves of WASP systems.

In other news, WASP played a minor role in the discovery of the first K2 planet, a super-Earth-sized planet orbiting the bright K-dwarf star HIP 116454. There is extensive WASP data on this star, and while the transits (only 0.1% deep) are too shallow to see in WASP data, the WASP data contribute by showing a possible 16-day rotation period of the host star. The discovery paper by Andrew Vanderburg et al featured in a NASA press release.