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This artist’s print shows the first interstellar object discovered in the solar system, `Oumuamua. Credit: ESA / Hubble, NASA, ESO, M. Kornmesser
Scientists determine that ‘Oumuamua is not made from molecular hydrogen ice after all
The debate on the origins and molecular structure of ‘Oumuamua continued with an announcement in Letters from the astrophysical journal that despite previous promising claims, the interstellar object isn’t made of molecular hydrogen ice after all.
The previous study, published by Seligman & Laughlin in 2020 – after observations from the Spitzer Space Telescope set strict limits on the outgassing of carbon-based molecules – suggested that if “ Oumuamua was a hydrogen iceberg, then l The pure hydrogen that gives it its A rocket-like thrust would have escaped detection. But scientists at the Center for Astrophysics | Harvard and Smithsonian (CfA) and the Korea Institute of Astronomy and Space Science (KASI) were curious whether a hydrogen-based object could actually have made the journey from interstellar space to our solar system.
“Seligman and Laughlin’s proposal seemed promising because it could explain the extremely elongated shape of ‘Oumuamua as well as the non-gravitational acceleration. However, their theory is based on the hypothesis that H2 ice could form in dense molecular clouds. If this is true, H2 ice objects could be abundant in the universe, and therefore have far-reaching implications. H2 ice has also been proposed to explain dark matter, a mystery of modern astrophysics, ”said Dr. Thiem Hoang, senior researcher in KASI’s theoretical astrophysics group and lead author of the article. “We wanted to not only test the hypotheses of the theory, but also the dark matter proposition.” Dr Avi Loeb, Frank B. Baird science professor at Harvard and co-author of the article, added: “We suspected that the hydrogen icebergs could not survive the trip – which will likely take hundreds of millions of people. years – because they evaporate too quickly and wonder if they could form in molecular clouds. “
An illustration of ‘Oumuamua, the first object we have ever seen pass through our own solar system which has interstellar origins. Credit: ESO / M. Kornmesser
Traveling at a blazing speed of 196,000 mph in 2017, “ Oumuamua was first classified as an asteroid, and when it accelerated later it was found to have properties closer to comets. But the interstellar object with a radius of 0.2 km also did not fit into this category, and its point of origin has remained a mystery. The researchers focused on the giant molecular cloud (GMC) W51 – one of the closest GMCs to Earth just 17,000 light years away – as a potential point of origin for ‘Oumuamua, but emitted the ‘assumption that he just couldn’t have made the trip intact. “The most likely place to make hydrogen icebergs is in the densest environments of the interstellar medium. They are giant molecular clouds, ”Loeb said, confirming that these environments are both too far away and not conducive to the development of hydrogen icebergs.
An accepted astrophysical origin for solid objects is growth by sticky collisions of dust, but in the case of a hydrogen iceberg, this theory could not hold together. “One accepted way to form an object the size of a km is to first form grains of the order of a micron, and then these grains develop in sticky collisions,” Hoang said. “However, in regions with high gas density, gas collision collision heating can quickly sublimate the hydrogen mantle on the grains, preventing them from growing further.
Although the study explored the destruction of H2 ice by multiple mechanisms, including interstellar radiation, cosmic rays, and interstellar gas, sublimation due to heating by starlight has the most destructive effect. and according to Loeb, “thermal sublimation by collisional heating in GMCs could destroy molecules in ‘Oumuamua-sized hydrogen icebergs before they escape into the interstellar medium. This conclusion excludes the theory that “Oumuamua visited our solar system from a GMC, and further excludes the proposition of primordial snowballs as dark matter. Evaporative cooling in these situations does not reduce the role of starlight thermal sublimation in destroying H2 ice objects.
“ Oumuamua first gained notoriety in 2017 when it was discovered howling in space by observers from the Haleakalā Observatory, and has since been the subject of ongoing studies. “This object is mysterious and difficult to understand because it exhibits special properties that we have never seen from comets and asteroids in our solar system,” Hoang said.
While the nature of the interstellar traveler is currently an unsolved mystery, Loeb suggests he won’t stay that way for long, especially if he’s not alone. “If ‘Oumuamua is part of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which is expected to have its first light next year, should detect about one object of type’ Oumuamua per month. We will all look forward to seeing what he finds. “
Reference: “Destruction of Molecular Hydrogen Ice and Implications for 1I / 2017 U1 (‘Oumuamua)” by Thiem Hoang and Abraham Loeb, August 17, 2020, Letters from the astrophysical journal.
DOI: 10.3847 / 2041-8213 / abab0c
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