A team of astronomers from Niigata University and The University of Tokyo investigated two enigmatic interstellar objects using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Initially identified in 2021 by Japan's infrared satellite AKARI, these objects were known to harbor interstellar ices rich in water and organic molecules, but their precise nature remained elusive. Unlike typical icy objects found in dense, star-forming clouds, these two entities exist outside any known stellar nursery.
To analyze these objects, the researchers utilized ALMA's 0.9 mm wavelength capabilities. Infrared observations excel in detecting solid materials, whereas radio wavelengths provide insights into the movement and composition of gases. If these objects were forming stars, ALMA's high-resolution imaging would have detected molecular emissions associated with star formation. Similarly, the presence of an undiscovered molecular cloud would have been visible through widespread carbon monoxide emissions.
Instead, the ALMA data defied expectations. Only molecular emission lines of carbon monoxide and silicon monoxide were detected, each exhibiting a remarkably compact spatial distribution of less than one arcsecond. The team leveraged this data to determine the objects' distances, motion, sizes, and chemical compositions.
Based on their line-of-sight velocities, researchers estimated that the two objects lie approximately 30,000 to 40,000 light-years from Earth. Despite their proximity of only 3 arcminutes on the celestial sphere and their shared spectral characteristics, their vastly different velocities suggest they are unrelated and located at different depths in space.
Interstellar ices typically exist within dust-rich environments, causing them to emit strongly in far-infrared to submillimeter wavelengths. However, ALMA failed to detect submillimeter radiation from these two objects, revealing an anomalous energy distribution that diverges from previously observed interstellar icy bodies.
Moreover, the ALMA data showed an unusually high ratio of silicon monoxide to carbon monoxide, a condition typically found only where intense shock waves disrupt interstellar dust. This suggests that the objects are associated with an unknown energetic phenomenon that is significantly disturbing the surrounding gas.
The distinct characteristics of these objects challenge all known classifications of interstellar icy bodies, including newly forming stars, young stellar disks, evolved stars undergoing mass loss, or bright stars seen through dense molecular clouds.
"They may represent a new class of interstellar objects that provide an environment conducive to the formation of ices and organic molecules," said Takashi Shimonishi, an astronomer at Niigata University and lead author of the study. "Future high-resolution observations of the associated gas using ALMA, coupled with more detailed studies of ices and dust via the James Webb Space Telescope, could offer deeper insight into these mysterious icy objects," he added.
Research Report:ALMA Observations of Peculiar Embedded Icy Objects
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Niigata University
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