Using a novel photometric technique to enhance observations from NASA’s Kepler Space Telescope, astronomers have performed the most detailed study yet of the variability in the seven brightest members of the Pleiades open cluster.
This image from Kepler shows members of the Pleiades star cluster taken during Campaign 4 of the K2 Mission. Kepler was not designed to look at stars this bright; they cause the camera to saturate, leading to long spikes and other artifacts in the image. Despite this serious image degradation, the new technique has allowed astronomers to carefully measure changes in brightness of these stars as Kepler observed them for almost three months. Image credit: NASA / Aarhus University / T. White.
“Named halo photometry, the new method is simple, fast and does not require extensive pixel allocation, and will allow us to use K2 and other photometric missions, such as the Transiting Exoplanet Survey Satellite (TESS), to observe very bright stars for asteroseismology and to search for transiting exoplanets,” said Aarhus University researcher Dr. Tim White and co-authors.
“We applied this method to the seven brightest stars in the Pleiades open cluster.”
The Pleiades were the first collection of stars to be recognized as a star cluster, and together with many fainter members they remain one of the most prominent and well-studied open clusters.
While the cluster contains over 1,000 members, with an age of 125 million years, it is visually dominated by seven hot, massive stars (Alcyone, Atlas, Electra, Maia, Merope, Taygeta, and Pleione).
Most of the seven stars are slowly-pulsating B stars, a class of variable star in which the star’s brightness changes with day-long periods.
The frequencies of these pulsations are key to exploring some of the poorly understood processes in the core of these stars.
The unique brightness fluctuations of each star reveal clues about their physical properties such as their size and rotation rate. Most of the bright stars in the Pleiades are a type of variable star called a slowly-pulsating B star, but Maia is different, and shows evidence of a large chemical spot that crosses its surface as the star rotates with a 10-day period. Image credit: Aarhus University / T. White.
The seventh star, Maia, is different: it varies with a regular period of 10 days.
Previous studies have shown that Maia, the fourth-brightest member of the Pleiades, belongs to a class of stars with abnormal surface concentrations of some chemical elements such as manganese.
To see if these things were related, a series of spectroscopic observations were taken with the Hertzsprung SONG Telescope.
“What we saw was that the brightness changes seen by Kepler go hand-in-hand with changes in the strength of manganese absorption in Maia’s atmosphere,” said team member Dr. Victoria Antoci, also from Aarhus University.
“We conclude that the variations are caused by a large chemical spot on the surface of the star, which comes in and out of view as the star rotates with a ten day period.”
“Sixty years ago, astronomers had thought they could see variability in Maia with periods of a few hours and suggested this was the first of a whole new class of variable stars they called ‘Maia Variables’,” Dr. White said.
“But our new observations show that Maia is not itself a Maia Variable!”
“No signs of exoplanetary transits were detected in this study,” the astronomers said.
Their work is published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
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T.R. White et al. 2017. Beyond the Kepler/K2 bright limit: variability in the seven brightest members of the Pleiades. MNRAS 471 (3): 2882-2901; doi: 10.1093/mnras/stx1050
