Scientists determine ‘Oumuamua is probably not made of molecular hydrogen ice

The debate over the origin and molecular structure of ‘Oumuamua continued today with an announcement in The Astrophysical Magazine Letters that despite previous promising claims, the interstellar object is apparently not made of molecular hydrogen ice.

The previous study, published in 2020 by Seligman & Laughlin in 2020 – after observations by the Spitzer Space Telescope set strict limits for the degassing of carbon-based molecules – suggested that if ‘Oumuamua was a hydrogen iceberg, it would be pure hydrogen gas that it rocket-like push would be discovered. But scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA) and the Korean Institute of Astronomy and Space Science (KASI) were wondering if a hydrogen-based object could have made the journey from interstellar space to our solar system.

“Seligman and Laughlin’s proposal appeared promising because it could explain the extremely elongated form of ‘Oumuamua’, as well as the non-gravitational acceleration. However, their theory is based on an assumption that H2 ice could form in dense molecules. “If this were true, H2 ice objects could be abundant in the universe, and so they would have far-reaching implications. H2 ice was also proposed to explain dark matter, a mystery of modern astrophysics,” said Dr. Thiem Hoang, senior researcher in the theoretical astrophysics group at KASI and lead author on paper. “We wanted to test not only the assumptions in theory, but also the theorem of dark matter.” Dr. Avi Loeb, Frank B. Baird professor of science at Harvard and co-author on paper, added, “We suspected that hydrogen icebergs could not survive the journey – which probably takes hundreds of millions of years – because they evaporate too fast, and whether they could form in molecular clouds. “

Traveling in 2017 at a blistering speed of 196,000 km / h, ‘Oumuamua was first classified as an asteroid, and when it later emerged, it was found to have properties more similar to comets. But the interstellar object of 0.2 km radius also did not fall into that category, and the point of origin has remained a mystery. Researchers have focused on the giant molecular cloud (GMC) W51 – one of the closest GMCs to Earth at just 17,000 light-years away – as a potential origin for Oumuamua, but hypothesized that it could not have made the journey intact. “The most likely place to make hydrogen icebergs is in the immediate vicinity of the interstellar medium. These are giant molecular clouds,” Loeb said, confirming that these environments are both too far away and not conducive to hydrogen evolution. icebergs.

An accepted astrophysical origin for solid objects is growth due to sticky collisions of dust, but in the case of a hydrogen iceberg this theory could not hold together. “An accepted route to form a km-sized object is to first form micron-sized grains, then such grains grow by sticky collisions,” Hoang said. “However, in regions with high gas density, collision heating due to gas collisions can quickly sublimate the hydrogen mantle on the grains, preventing them from growing further.”

Although the study destroyed H2 ice by multiple mechanisms including interstellar radiation, cosmic rays, and interstellar gas, sublimation due to heating by starlight has the most devastating effect, and according to Loeb, “Thermal sublimation due to collision heating in GMCs could destroy molecular fluid. icebergs of ‘Oumuamua size before escaping into the interstellar medium.’ This conclusion refutes the theory that ‘Oumuamua traveled to our solar system from a GMC, performing the representation of primordial snowballs as dark matter. Evaporative cooling in these situations does not reduce the role of thermal sublimation by starlight in destroying H2 ice objects.

‘Oumuamua first gained notoriety in 2017 when it was discovered to be writing through space by observers at Haleakalā Observatory, and has since been the subject of ongoing studies. “This object is mysterious and difficult to understand because it exhibits peculiar properties we have never seen from comets and asteroids in our solar system,” Hoang said.

Although the nature of the interstellar traveler is currently an unsolved mystery, Loeb suggests that it will not stay that long, especially if it is not alone. “If ‘Oumuamua’s a member of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which is scheduled to have its first light next year, should detect about one’ Oumuamua-like object per month. “We will all be waiting with anticipation to see what it will find.”