Two young massive stars may have become a pair after a close encounter, rather than being born together.
The system studied, named IRAS 07299−1651, contains two stars still in formation. They are currently separated by about 200 astronomical units, or 200 times the average distance between Earth and the Sun. Massive stars very often live in pairs, but their birth remains difficult to observe because it takes place deep inside dense clouds of gas and dust.
The star-forming region IRAS 07299−1651 observed by ALMA. The inset shows the two young massive stars at the center of the system and their relative motion.
Credit: ALMA (ESO/NAOJ/NRAO); RIKEN, Zhang et al. — CC BY 3.0
To understand their history, the researchers observed the system repeatedly with the ALMA and JVLA radio telescope arrays. They supplemented these data with infrared images from the James Webb Space Telescope and the Very Large Telescope. This set of observations spans nearly eight years and makes it possible to track the movement of the two objects.
The astronomers thus reconstructed their motion in the three dimensions of space. It is this trajectory that holds the surprise. Instead of following an almost circular orbit, the two young stars follow a highly elongated trajectory, similar to one produced during an initially nearly unbound encounter between two objects.
Another clue comes from the disks of material surrounding each star. These disks are still feeding gas to the young stars. Their orientation, however, differs greatly from that of the pair's orbital motion. A common birth in the same disk would more naturally have produced aligned orientations.
The researchers therefore propose another scenario. The two stars may have begun forming separately, in two distinct concentrations of gas. These would then have moved closer together under the influence of gravity. A sufficiently close encounter could have dissipated energy and left the two young stars bound to each other.
This mechanism is called core merger, with “core” here referring to a dense region of a cloud where a star forms. The two stars remain clearly distinct, but their initially separate formation regions came together.
It remains to be determined whether IRAS 07299−1651 represents a rare case or a common formation pathway. The authors indicate that this type of encounter could play an important role in the birth of highly elongated massive-star pairs. Comparable observations of other young systems will make it possible to measure the true frequency of this mechanism.
Many massive stars end their lives as supernovae and subsequently give birth to compact objects, such as neutron stars or black holes.