A variability found in the Seven Sisters of the Pleiades

The Pleiades seen by the Kepler Space Telescope (Image NASA / Aarhus University / T. White)
The Pleiades seen by the Kepler Space Telescope (Image NASA / Aarhus University / T. White)

An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes the most detailed study ever made on the variability of the Pleiades. A team of astronomers led by Dr. Tim White of the Stellar Astrophysics Center at the Danish University of Aarhus used a new algorithm to improve the observations made by NASA’s Kepler Space Telescope in its K2 mission.

The Kepler Space Telescope is designed to examine thousands of dim stars at a time with the result that some bright stars turn out to be too bright to be observed with precision. What happens is that in the image of such a star the central pixels are saturated causing a sharp loss of precision in the measure of the star’s overall brightness. This is a problem similar to that of ordinary digital cameras, which can’t see dim and bright details in the same image.

Tim White explained that the solution to bright stars was in the end quite simple, but in these cases everything is simple after somebody thought of it. Measuring changes in brightness instead of absolute values ​​can solve the problem but several factors may still hide the signal of star variability. To eliminate this problem, the team of researchers developed a new technique known as halo photometry, a simple and fast algorithm released as free / open source software, available on GitHub’ website.

This method was applied to the Seven Sisters, the seven brightest star of the Pleiades (or M45) cluster: Sterope, Merope, Electra, Maia, Taygeta, Celaeno, and Alcyone. The cluster is very young in astronomical terms with an estimated age of one hundred million years and the Seven Sisters are massive class B stars.

From the new analysis, six of them turned out to be part of the class of slowly-pulsating B variable stars. In those stars, the brightness changes with day-long periods. The frequencies of those pulsations are a key to exploring some processes so far not well understood in those stars’ cores.

Maia is different from its “sisters” because its variability has a regular 10-day period. Previous research showed that it belongs to a class of stars with an abnormal concentrations of certain chemical elements such as manganese on its surface. To see if there was a connection between these characteristics, a series of spectroscopic observations was performed with the Hertzsprung SONG telescope.

The astronomers noticed that the changes in brightness had the same pattern of variations in intensity of the manganese absorption lines in Maia’s atmosphere. Their conclusion is that variations are caused by a large area on the surface of the star that contains that element that appears and disappears every ten days, which is Maia’s rotation period.

The new method can also be useful in the search for exoplanets, even in the next space telescopes’ missions. This is a development useful for example in the observations of nearby stars such as Alpha Centauri and which for the moment brought an interesting result in the study of the Seven Sisters of the Pleiades.

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