Finding by Notre Dame astrophysicist helps understanding of planetary formation

by William G. Gilroy

A Notre Dame astrophysicist is part of a team of researchers who have studied a “winking star” that may offer important clues to how planets are formed.p. Peter Garnavich, assistant professor of physics, and colleagues from the Harvard-Smithsonian Center for Astrophysics studied the star, discovered last year by astronomers at Wesleyan University and known as KH 15D. The star undergoes a regular, long-lasting (20 day) eclipse, or dimming, every 48 days, in effect “winking.”p. The Wesleyan researchers and Garnavich and his colleagues theorized that the star may be surrounded by a disk of dust and gas left over from its birth. That disk, known as a protoplanetary disk, is a possible source of the eclipse.p. A nearly three-week eclipse is difficult to explain by the usual intervention of a planet or companion star; the star’s face is totally hidden from view half of the time. The most plausible cause is a wide swath of disk material – a planet in formation – sliding in front of the star, thereby blocking most of the star’s light.p. “An eclipse that lasts nearly half the orbit makes this star unique, and the more we find out about this object the stranger it gets,” Garnavich said.p. Garnavich and the Harvard researchers examined the past behavior of KH 15D using sky photographs taken during the first half of the 20th century and stored in the Harvard Photographic Plate Collection archives. Notre Dame researchers also monitored KH 15D extensively with the Vatican Advanced Technology Telescope during the past year.p. The researchers discovered that the star did not always wink. Today’s very long eclipses were not happening several decades ago, meaning that the current eclipses are a recent phenomenon that began in the last few decades – a remarkably short time by astronomical standards.p. “There are very few cases where astronomers can see a significant change to a star over a single human lifetime,” Harvard researcher Joshua Winn said. “And if the eclipses are caused by material in a protoplanetary disk, as suspected, then that would give us the exciting opportunity to study planet formation on surprisingly short time scales.”p. If the winking star’s eclipses took place in the past just as they do today, approximately 16 photographs (40 percent of the total) would have shown a dim, eclipsed star. Instead, the astronomers found that none of the photographic plates definitively showed an eclipse.p. “Our observations show that the length of the eclipse is evolving rapidly,” Garnavich said. “In a few years, this strange star will spend more time faint than bright.”p. Garnavich and his colleagues plan to collaborate with additional astronomers to investigate other photographic plate archives for data from the second half of the 20th century. By studying when and how the eclipses began, they hope to gather additional clues about the winking star and, more importantly, the planet that is forming around it.p. Harvard’s Photographic Plate Collection contains a half-million plates spanning a century of research from the 1880s to 1989, making it the largest such archive in the world and an irreplaceable resource for astronomers studying time-varying celestial phenomena.p. The team’s research will be published in an upcoming issue of The Astrophysical Journal Letters.

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