Faint stars adjacent to WASP planet hosts

To estimate the radius of a transiting planet we simply measure the amount of light that it blocks during the transit. However, if there are faint, unseen stars in the photometric aperture they can dilute the light of the host star, leading to incorrect system parameters.

Thus Maria Wöllert and co-authors have made a “lucky imaging” search for faint companions to planet-host stars. Lucky imaging is a method of getting sharper pictures by taking a lot of images very quickly, and then picking only the best ones, thus reducing the blurring caused by the turbulence of Earth’s atmosphere (which astronomers call “seeing”).

Wöllert et al, observing with the 2.2-m telescope at Calar Alto, obtained tens of thousands of images with exposure times of only 15 millisecs, and then combined together the best 10%.

Here are their images of three WASP stars:
Luck imaging of WASP host stars

Each of these shows a faint close companion (circled in orange). The star adjacent to WASP-2 was previously known, but those next to WASP-14 and WASP-58 are new discoveries.

The good news, though, is that these two are sufficiently faint that they lead to “no significant changes” to the planetary parameters. In addition, Wöllert et al found no close companions around 13 other WASP stars. This is valuable work that will be useful reassurance for future observations of these systems.