ALMA Captures Two Massive Stars Forming a Binary Star System

Astronomers have caught two massive stars as they become a binary system. The pair is still forming within its natal cloud, and researchers have already measured how the stars orbit each other. The system, called IRAS 07299−1651, lies about 5,300 light-years from Earth.

Astronomers have been following the two young stars for almost eight years with the Atacama Large Millimeter/submillimeter Array (ALMA). Recently, along with ALMA’s data, they have used images from the Very Large Array (VLA), the James Webb Space Telescope (JWST), and the Very Large Telescope (VLT). The observations reveal that the stars follow a highly stretched orbit, while the disks around them point in very different directions.

A massive binary system forming

Massive stars are rarely loners. At least 90 percent are thought to belong to binary or higher-order multiple systems. Their companions can have a major effect on how they evolve, particularly when both stars are massive enough to produce strong winds, radiation, and eventually supernova explosions.

IRAS 07299−1651 is somewhat different because its two members are still protostars. They are embedded in their birth environment and continue to gather material.

The system had already attracted attention before the latest study. ALMA observations in 2019 gave researchers the first direct dynamical constraints on the pair. At that stage, the observations could still be explained by a familiar model in which two stars form from the fragmentation of a large rotating disk.

The new observations have now added almost eight years of positional measurements. That longer record has changed the understanding considerably. Rather than seeing two stars that grew up together in one structure, astronomers found evidence for an orbit that looks more like the aftermath of a close encounter.

ALMA measures the motion of stars still being born

IRAS 07299−1651 is so far away that the two protostars barely shift across the sky from one observation to the next. ALMA had to measure the tiny changes. By comparing observations made over nearly eight years, the researchers could establish how the stars move around their common center of mass.

The result is a highly eccentric orbit. Unlike a near-circular binary, where the stars maintain roughly the same separation, this pair follows a much more elongated path. The preferred orbital solutions lie close to a parabolic trajectory.

A highly eccentric orbit can result from a close interaction between young stellar objects. It can preserve some evidence of the event long after the encounter has taken place. In this case, the orbital shape also fits the other clues found around the stars.

The two protostars are currently separated by about 200 astronomical units. An astronomical unit is the average distance between Earth and the Sun, so the stars are roughly 200 times farther apart than Earth and the Sun are.

JWST, VLA and the VLT fill in the missing pieces

ALMA’s long-term monitoring provided the measurements needed to reconstruct the orbit, but the researchers required observations at other wavelengths to understand what surrounded the stars.

The National Science Foundation’s Very Large Array (VLA) supplied radio observations that helped trace the jets produced by the protostars. These outflows extend away from the stars and provide useful information about their orientation.

The James Webb Space Telescope observed the region in infrared light. That is useful for a system such as IRAS 07299−1651, which remains buried inside a dense cloud of gas and dust. Much of the visible light from such an environment can be blocked, while infrared wavelengths can reveal structures hidden behind the dust.

The European Southern Observatory’s(ESO) Very Large Telescope added further infrared observations and helped trace the jets.

“Each telescope revealed a different piece of the puzzle. The combination of radio and infrared observations provides the most exquisite detail on the formation of this massive protobinary system.”

– Ruben Fedriani, Astronomer at the Instituto de Astrofísica de Andalucía (IAA-CSIC), co-author of the paper.

Clear skies!

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