Sunday, April 23, 2017

Using the Pleiades as guinea-pigs for new photometric method

Target pixel mask of Alcyone from Kepler image. The four squared-in pixels are used for the registration of the brightness of the star. 
The white pixels are useless due to saturation.

Tim White as lead author and several other researchers related to SAC have a paper out in Monthly Notices of the Royal Astronomical Observatory, titled 'Beyond the Kepler/K2 bright limit: variability in the seven brightest members of the Pleiades'. The astronomers will never be content! They strive to observe the faintest stars possible, and this means that some of the brighter stars are actually too bright to observe with modern equipment. A workaround to this has now been developed by an international group of astronomers led by Tim White of Stellar Astrophysics Centre, Aarhus University and the method has been tested successfully on the seven brightest stars in the open cluster named the Pleiades or the Seven Sisters.

Aiming a beam of light from a bright star at a point on a CCD detector will cause several of the central pixels of the star's image to be saturated, and the construction of the CCD will cause long ghost images of saturated pixels in various directions out from the center of the image. Saturation means a loss of precision in the measurement of the total brightness of the star. The solution is simple: the star is bright enough that you can skip all the saturated pixels, selecting a set of unsaturated positions on the CCD hit by enough light that you can still make a reliable measurement of the brightness variations that are of interest if you want to do asteroseismology, observing the regular short time variations or if you want to see if an exoplanet passes in front of the star causing the intensity to drop shortly.

This new method has been named halo photometry. It is simple and fast and it has been used by the authors for observing the seven brightest named stars in the open cluster using data from the extended K2 mission by the NASA Kepler satellite.

The Pleiades with the light curves of the seven brightest stars inserted. Maia is obviously the odd sister out.

The seven stars are closely of the same age and six of the show regular B-star pulsations. This is interesting for determining the values of some of the poorly understood processes in the core of these stars. The seventh star, Maia is different. It is not variable in any way comparable to the other bright stars in the cluster, but it does vary with a regular period of 10 days. Previous studies have shown that Maia belongs to a class of stars with a deficiency of both He and Hg, and the authors conclude that the variability is due to a large spot on the surface of the star of different chemical composition. This means that Maia itself does definitely not belong to the controversial group af stars named Maia variables, and the authors implore for the sanity of future astronomers that this designation should not be used anymore!

No signs of exoplanets were detected during the study.