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.

Ole J. Knudsen

 

Source: Stellar Astrophysics Centre - Aarhus University

Radio Telescopes Settle Controversy Over Distance to Pleiades

With parallax technique, astronomers observe object at opposite ends of Earth's orbit around the Sun to precisely measure its distance.

Credit: Alexandra Angelich, NRAO/AUI/NSF

Optical image of the Pleiades

Credit: NOAO/AURA/NSF

 

Astronomers have used a worldwide network of radio telescopes to resolve a controversy over the distance to a famous star cluster -- a controversy that posed a potential challenge to scientists' basic understanding of how stars form and evolve. The new work shows that the measurement made by a cosmic-mapping research satellite was wrong.

The astronomers studied the Pleiades, the famous "Seven Sisters" star cluster in the constellation Taurus, easily seen in the winter sky. The cluster includes hundreds of young, hot stars formed about 100 million years ago. As a nearby example of such young clusters, the Pleiades have served as a key "cosmic laboratory" for refining scientists' understanding of how similar clusters form. In addition, astronomers have used the measured physical characteristics of Pleiades stars as a tool for estimating the distance to other, more distant, clusters.

Until the 1990s, the consensus was that the Pleiades are about 430 light-years from Earth. However, the European satellite Hipparcos, launched in 1989 to precisely measure the positions and distances of thousands of stars, produced a distance measurement of only about 390 light-years.

"That may not seem like a huge difference, but, in order to fit the physical characteristics of the Pleiades stars, it challenged our general understanding of how stars form and evolve," said Carl Melis, of the University of California, San Diego. "To fit the Hipparcos distance measurement, some astronomers even suggested that some type of new and unknown physics had to be at work in such young stars," he added.

To solve the problem, Melis and his colleagues used a global network of radio telescopes to make the most accurate possible distance measurement. The network included the Very Long Baseline Array (VLBA), a system of 10 radio telescopes ranging from Hawaii to the Virgin Islands; the Robert C. Byrd Green Bank Telescope in West Virginia; the 1,000-foot-diameter William E. Gordon Telescope of the Arecibo Observatory in Puerto Rico; and the Effelsberg Radio Telescope in Germany.

"Using these telescopes working together, we had the equivalent of a telescope the size of the Earth," said Amy Miouduszewski, of the National Radio Astronomy Observatory (NRAO). "That gave us the ability to make extremely accurate position measurements -- the equivalent of measuring the thickness of a quarter in Los Angeles as seen from New York," she added.

The astronomers used this system to observe several Pleiades stars over about a year and a half to precisely measure the apparent shift in each star's position caused by the Earth's rotation around the Sun. Seen at opposite ends of the Earth's orbit, a star appears to move slightly against the backdrop of more-distant cosmic objects. Called parallax, the technique is the most accurate distance-measuring method astronomers have, and relies on simple trigonometry.

The result of their work is a distance to the Pleiades of 443 light-years, accurate, the astronomers said, to within one percent. This is the most accurate and precise measurement yet made of the Pleiades distance.

"This is a relief," Melis said, because the newly-measured distance is close enough to the pre-Hipparcos distance that the standard scientific models of star formation accurately represent the stars in the Pleiades.

"The question now is what happened to Hipparcos?" Melis said. Over four years of operation, the spacecraft measured distances to 118,000 stars. The cause of its error in measuring the distance to the Pleiades is unknown. Another spacecraft, Gaia, launched in December of 2013, will use similar technology to measure distances of about one billion stars.

"Radio-telescope systems such as the one we used for the Pleiades will provide a crucial cross-check to insure the accuracy of Gaia's measurements," said Mark Reid, of the Harvard-Smithsonian Center for Astrophysics.

Many ancient cultures, including Native Americans, used the Pleiades as a test of vision. The more Pleiades stars one can discern -- typically five to nine -- the better one's vision.

"Now we've used a system that provides modern astronomy's sharpest 'vision' to solve a longstanding scientific debate about the Pleiades themselves," said Melis.

Melis, Miouduszewski, and Reid worked with John Stauffer of the Spitzer Science Center, and Geoffrey Bower of the Academia Sinica Institute of Astronomy and Astrophysics. The scientists published their findings in the 29 August issue of the journal Science.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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Dave Finley, Public Information Officer
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dfinley@nrao.edu

Source: NRAO - National Radio Astronomy Observatory