A recent study investigates two enigmatic interstellar objects discovered through observations with the ALMA telescope (Atacama Large Millimeter/submillimeter Array) and the AKARI satellite. These bodies, hidden behind dense clouds of gas and dust, could represent a previously unknown type of interstellar object. The research, led by astrophysicist Takashi Shimonishi and his team, raises fundamental questions about the chemistry and evolution of remote regions of our galaxy.

The objects, provisionally named “Object 1” and “Object 2”, were initially identified in a spectroscopic study conducted with the AKARI satellite, which detected strong ice and dust absorption features. These characteristics are common in objects associated with young stars in formation or stars located behind dense gas clouds. However, these bodies are located outside known star-forming regions and lack any evident association with cataloged dense clouds.

The team used ALMA to analyze specific molecular emissions, such as those from carbon monoxide (CO) and silicon monoxide (SiO), revealing surprising clues about their composition and environment. The nature of these objects remains a mystery, but their features could suggest a new type of interstellar body, the researchers note.

The study describes that both objects exhibit an extremely compact size and are surrounded by molecular gas with high SiO-to-CO ratios, a configuration typical of regions impacted by high-energy shocks. Additionally, the observations revealed broad line widths in the molecular emissions, indicating turbulent motions or non-thermal processes in their environment.

Interestelar objects ice
Both objects seen by ALMA telescope. Credit: T. Shimonishi et al. / Atacama Large Millimeter/submillimeter Array

Another intriguing fact is that, despite showing deep ice and dust absorption features indicative of a visual extinction (AV) near 100 magnitudes, no continuous emission was detected in the submillimeter wavelengths. This result suggests that the objects have a small physical size, estimated between 100 and 1,000 astronomical units, and are highly compacted.

Both objects are located in the direction of the Crux-Scutum arm of the Milky Way, a region where stars and interstellar gas are concentrated. However, the systemic velocities of these bodies, measured through the Doppler shifts of molecular emissions, kinematically separate them from the surrounding gas clouds. This suggests that they are isolated, floating in more remote regions of interstellar space.

Kinematic analysis also allowed for the estimation of distances of approximately 9.3 and 13.4 kiloparsecs for Objects 1 and 2, respectively, placing them in the outermost parts of our galaxy. At these distances, their bolometric luminosities are calculated to be between 500 and 750 times that of the Sun, which could suggest that they are remnants of intermediate-mass stars or entirely new phenomena.

The researchers compared the properties of the objects with those of other known sources of ice absorption in the cosmos, such as protostars, oxygen-rich evolved stars (OH/IR), and protoplanetary disks. However, the unique features of these bodies do not easily fit into any established category.

For instance, the objects exhibit energy spectra that peak around 5 microns but lack significant emission at longer wavelengths, an atypical behavior for protostars or accretion disks. Moreover, while they share some similarities with evolved stars, the detected molecular species, such as methanol (CH3OH) and carbon monoxide, are uncommon in those scenarios.

The discovery of these objects raises fundamental questions about interstellar chemistry and physical processes in extreme environments. They could be remnants of violent processes or new types of evolving systems. We still do not know how many more might exist, the researchers state. The findings also underscore the importance of future missions and telescopes to track similar objects and deepen our understanding of their nature.


SOURCES

Takashi Shimonishi, Takashi Onaka, Itsuki Sakon, ALMA Observations of Peculiar Embedded Icy Objects. arXiv:2501.05008 astro-ph.GA, doi.org/10.48550/arXiv.2501.05008


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