A dim star passed through the Oort Cloud 70,000 years ago

A group of astronomers from the United States, Europe, Chile and South Africa have determined that 70,000 years ago a newly discovered dim star may have passed through the distant comet cloud of solar system, the Oort cloud. No other star is known to have ever approached our solar system so closely – five times closer than the current nearest star, Proxima Centauri.

In an article published in Letters from the Astrophysical Journallead author Eric Mamajek of the University of Rochester and collaborators have analyzed the speed and trajectory of a low-mass star system dubbed “Scholz’s Star.”

The star’s trajectory suggests that 70,000 years ago it passed about 52,000 astronomical units (or about 0.8 light-years away, which equals 8 trillion kilometers or 5 trillion miles). It’s astronomically close; our nearest neighboring star Proxima Centauri is 4.2 light years away. In fact, the astronomers explain in the paper that they are 98% certain that it passed through what is known as the “outer Oort cloud” – a region on the outskirts of the solar system filled with trillions of comets a mile or more across that are thought to give rise to long-period comets orbiting the Sun after their orbits have been disrupted.

The star originally caught Mamajek’s attention during a discussion with co-author Valentin D. Ivanov of the European Southern Observatory. Scholz’s Star had an unusual mix of characteristics: although it was quite close (“only” 20 light-years away), it showed very slow tangential motion, that is, movement across the sky . Radial velocity measurements taken by Ivanov and his collaborators, however, showed that the star was moving away almost directly from the solar system at a considerable speed.

“Most nearby stars show much greater tangential motion,” says Mamajek, an associate professor of physics and astronomy at the University of Rochester. “The small tangential motion and proximity initially indicated that the star was most likely heading for a future close encounter with the solar system, or that it had ‘recently’ moved closer to the solar system and away from it. Indeed, the radial velocity measurements were consistent with it moving away from the vicinity of the Sun – and we realized it must have had a close flyby in the past.”

To calculate its trajectory, astronomers needed two data, the tangential velocity and the radial velocity. Ivanov and his collaborators had characterized the newly discovered star by measuring its spectrum and radial velocity via Doppler shift. These measurements were made using spectrographs at large telescopes in South Africa and Chile: the Large South African Telescope (SALT) and the Magellan Telescope at Las Campanas Observatory, respectively.

Once the researchers gathered all the information, they figured out that Scholz’s Star was moving away from our solar system and traced it back in time to its position 70,000 years ago, when their models indicated that it came closest to our Sun. So far, the best candidate for the closest flyby of a star in the solar system has been the so-called “rogue star” HIP 85605, which was expected to close in on our solar system in 240,000 to 470,000 years. However, Mamajek et al also demonstrated that the initial distance to HIP 85605 was likely underestimated by a factor of ten. At its most likely distance – about 200 light-years away – HIP 85605’s newly calculated trajectory would not take it into the Oort Cloud.

Mamajek worked with former University of Rochester undergrad Scott Barenfeld (now a graduate student at Caltech) to simulate 10,000 orbits for the star, taking into account position, distance and speed. of the star, the gravitational field of the Milky Way galaxy and the statistical uncertainties in all these measurements. Of these 10,000 simulations, 98% of the simulations showed the star passing through the outer Oort cloud, but fortunately only one of the simulations brought the star into the inner Oort cloud, which could trigger ” comet showers”.

While the close flyby of Scholz’s Star likely had little impact on the Oort Cloud, Mamajek points out that “other dynamically important disturbers of the Oort Cloud may lurk among nearby stars.” The recently launched European Space Agency Gaia satellite should map the distances and measure the velocities of a billion stars. With the Gaia data, astronomers will be able to tell what other stars may have encountered us in the past or will in the distant future.

Currently, Scholz’s Star is a small, dark red dwarf in the constellation of Monoceros, about 20 light-years away. However, at its closest flyby of the solar system, Scholz’s Star would have been a 10th magnitude star – about 50 times dimmer than what can normally be seen with the naked eye at night. However, it is magnetically active, which can cause stars to “pop” and briefly become thousands of times brighter. It is therefore possible that Scholz’s Star was visible to the naked eye by our ancestors 70,000 years ago for minutes or hours at a time during rare flaring events.

The star is part of a binary star system: a low-mass red dwarf star (with a mass of about 8% that of the Sun) and a companion “brown dwarf” (with a mass of about 6 % of that of the Sun). Brown dwarfs are considered “failed stars”; their masses are too small to fuse hydrogen into their cores like a “star”, but they are still much more massive than gas giant planets like Jupiter.

The star’s official designation is “WISE J072003.20-084651.2”, but it has been dubbed “Scholz’s Star” to honor its discoverer – astronomer Ralf-Dieter Scholz of the Leibniz-Institut für Astrophysik Potsdam (AIP ) in Germany — which first reported the discovery of the nearby dim star in late 2013. The “WISE” part of the designation refers to NASA’s Wide-field Infrared Survey Explorer (WISE) mission, which mapped the entire sky in infrared light in 2010 and 2011, and the “J-number” part of the designation refers to the star’s celestial coordinates.

Arline J. Mercier