Scientists have discovered two mysterious interstellar objects containing water-rich ice and organic molecules, but their characteristics do not match any known objects. Are these cosmic relics or a new environment for the formation of living molecules?
Organic molecules, the basic building blocks of life, are thought to form in space. But where exactly they originate and how they get to planets is still a central question in astronomy and planetary science. A crucial piece of this puzzle is the presence of ice in interstellar space. In the cold, dense, and sheltered regions of the galaxy, atoms and molecules bind to tiny dust particles to form interstellar ice—a process similar to how snowflakes form in clouds on Earth.
To investigate this, Japanese scientists used the ALMA telescope array in Chile to observe two mysterious interstellar objects. These objects were first detected in 2021 by the Japanese space agency's Akari satellite and are known to contain interstellar ice rich in water and organic molecules. Unlike most interstellar ice, which is typically found in dense star-forming regions, these two objects exist outside any known star-forming regions, making them particularly intriguing.
Using ALMA, the research team observed objects at a wavelength of about 0.9 mm. Radio observations are useful when studying solid materials like ice, but radio observations, like those from ALMA, provide better insights into the motion and composition of the surrounding gases. If these objects were forming stars, ALMA's high-resolution imaging would reveal molecular emissions associated with star formation. Similarly, if a previously unknown molecular cloud were present near these objects, it would appear as an extended region of gaseous emission, particularly carbon monoxide.
But the observations revealed something different, different from these two expected things. In the locations of the two ice bodies, only molecular emission lines of carbon monoxide and silicon monoxide were detected, showing a very compact distribution of less than one arcsecond. Using the data from ALMA, the team analyzed the distance, motion, size and chemical composition of the molecular gas associated with these objects.
For example, analysis of their line-of-sight velocities suggests that the two objects are approximately 30,000 to 40,000 light-years from Earth. Additionally, the significant difference in their velocities suggests that the two objects are kinematically independent and at different distances, despite being separated by only three arcminutes in the celestial sphere and exhibiting similar colors, brightness, and interstellar ice properties.
Icy interstellar objects are typically surrounded by large amounts of dust, which causes them to glow brightly in the far-ultraviolet to submillimeter wavelength range. But the ALMA observations in this study did not detect submillimeter radiation from the icy object, revealing an unusual energy distribution that does not match the characteristics of known icy interstellar objects.
Additionally, ALMA observations revealed that the ratio of silicon monoxide to carbon monoxide in both objects is significantly higher than is typically observed in normal molecular clouds. Such abundances of silicon monoxide are usually only found in regions where interstellar dust is destroyed by powerful shock waves, suggesting that both objects are associated with an energetic source that is strongly dispersing the gas.
The unique properties of the mysterious ice cores discovered by ALMA cannot be explained by the properties of any known objects associated with interstellar ice, such as newly formed stars, young stars with protoplanetary disks, evolved stars showing intense mass loss, or bright stars located behind dense molecular clouds.
"They may be a new type of interstellar object that provides an environment conducive to the formation of ice and organic molecules," says Japanese astronomer Takeshi Shimonishi, the paper's lead author. "Further high-resolution observations of the associated gas using ALMA, along with more detailed studies of ice and dust using the Webb Space Telescope, will shed light on the nature of these mysterious icy bodies," Shimonishi hopes.
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