Thousands of light-years away, a red giant star “blinked,” fading noticeably for seven long years before returning to normal.
Long after it happened, astronomers discovered the remarkable, seven-year-long dimming of a distant Milky Way star. “Unfortunately, we missed the eclipse,” Anastasios Tzanidakis (University of Washington) told the 241st meeting of the American Astronomical Society in Seattle on Tuesday.
Between 2012 and 2019, a bloated red giant star some 26,000 light-years away in the constellation Sagitta, the Arrow, was more than 60 times (4.5 magnitudes) fainter than normal. “Stars typically don’t do this,” Tzanidakis says. He and his colleagues think that the extremely long and deep dimming happened when a slowly orbiting companion, surrounded by a huge disk of absorbing dust, blocked the red giant’s light.
The Gaia mission of the European Space Agency was the first to notice the star’s peculiar activity. Gaia saw how the star, now known as Gaia17bpp, started to brighten in 2017, four years after its launch. It went from magnitude 20.5 to magnitude 16 in 2019. Last year, Tzanidakis and his coworkers used data from other telescopes and an automated Gaia signal sent in 2017 to confirm that the long and profound eclipse had started in 2012. Gaia17bpp hasn’t exhibited any other unusual behavior since 1950, according to even more ancient recorded observations.
What then might make a red giant star dim more than 60 times during the course of seven years? The only plausible explanation, according to the scientists, is a massive disk of absorbing material, several hundred million kilometers across, enveloping a faint companion star in an extended orbit around the red giant. The disk could be a white dwarf star’s debris disk or it could be made up of dust that the big star blew into space and then its companion collected. Tzanidakis says, “We need to perform more modeling.
“This is a very interesting find,” says Guillermo Torres (Center for Astrophysics), “and the authors were clever to keep an eye on the Gaia alerts. You never know what you might find there.”
Last year, Torres and his colleague Kristy Sakano reported the discovery of a similar “dusty companion” orbiting the bright star Eta (η) Geminorum every 8.2 years. A much better-known example is Epsilon (ε) Aurigae, which experiences two-year-long eclipses every 27 years.
In the case of Gaia17bpp, however, the orbital period must be on the scale of centuries, given the event’s long duration. Tzanidakis notes that there could be many more similar systems out there that won’t be detectable for a very long time. “What is needed to discover more of these objects,” adds Torres, “are surveys of large portions of the sky, or even of the entire sky, carried out over long periods of time, such as the Vera Rubin Observatory and others.”
Catching eclipses as they occur will help identify the composition of the absorbing material, as it will leave a telltale spectroscopic fingerprint in the star’s light. Eventually, astronomers also hope to learn how these weird binary systems arise in the first place. Says Tzanidakis: “We’re witnessing the emergence of a whole new population of binary stars.”
Soucre: skyandtelescope.org
