This impression of this artist shows the first interstellar object discovered in the solar system, `Oumuamua. Credit: ESA / Hubble, NASA, ESO, M. Grain Knives
Scientists determine ‘Oumuamua is probably not made of molecular hydrogen
The debate over the origins and molecular structure of ‘Oumuamua continued with an announcement in The Astrophysical Journal 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 Korea Astronomy and Space Science Institute (KASI) were envious of whether an object based on hydrogen could make the journey from interstellar space to our solar system.
“Seligman and Laughlin’s proposal was promising because it could explain the extremely elongated form of ‘Oumuamua’, such as acceleration without gravity. However, their theory is based on the assumption that H2 ice could form in dense molecular clouds. If this were true, H2 ice objects could be abundant in the universe, and thus it 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 of the paper, added, “We suspected that hydrogen icebergs could not survive the journey – which would probably take hundreds of millions of years – because they evaporated too quickly, and whether they can form in molecular clouds. ”
An illustration of ‘Oumuamua, the first object we’ve ever seen running through our own solar system that has interstellar origin. Credit: ESO / M. Grain Knives
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 create hydrogen icebergs is in the densest environments of the interstellar medium. These are gigantic molecular clouds, ”said Loeb, confirming that these environments are both too far away and not conducive to the evolution of hydrogen 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. “In regions with high gas density, however, collision heating due to gas collisions can quickly sublimate the hydrogen mantle on the grains, preventing them from growing further.”
Although the study investigating H2 ice destruction by multiple mechanisms, including interstellar radiation, cosmic rays, and interstellar gas, sublimation due to starlight heating has the most devastating effect, and according to Loeb, “Thermal sublimation due to collision heating in GMCs could destroy molecules hydrogen 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 is a member of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which has its first light next year, would have to track down about one’ Oumuamua-like object per month. We will all be waiting with anticipation to see what it will find. ”
Reference: “Destruction of Molecular Hydrogen Ice and Implications for 1I / 2017 U1 (‘Oumuamua)’ by Thiem Hoang and Abraham Loeb, 17 August 2020, The Astrophysical Journal Letters.
DOI: 10.3847 / 2041-8213 / abab0c
