Astronomers observing the young protostar MM 1a have discovered an unusual method of binary star formation. While binary stars are not uncommon — they may account for half or more of all visible stars — the mechanics at work in the case of MM 1a are quite different than what we’d ordinarily expect.

A Star Is Formed

Stars are thought to form inside giant molecular clouds of gas, sometimes with diameters hundreds of light years across, with a combined mass equivalent to millions of solar masses. As these clouds collapse, they form fragments. Said fragments are thought to contain the mass that will become both the star and its protoplanetary disc, though there are opportunities for interactions with other nearby clouds. If an aged star near a stellar nursery goes supernova, for example, heavier elements created by the core collapse may form part of the new stars. A nearby supernova can even trigger the formation of new stars when its shock wave sharply compresses the molecular gas cloud.

Planets, in contrast, coalesce out of the grains of ice and dust embedded within the protoplanetary disc surrounding a young star. Stars, in other words, are formed by the compression of gas (mostly hydrogen) in molecular clouds that are often extremely cold, with an average temperature as low as 10K. Planets are formed from grains of ice and dust suspended in the protoplanetary discs of young stars, and begin to accrete into protoplanets and planetesimals as these grains bind together and begin to attract one another. The temperature and physical conditions in the protoplanetary disc are different from those in the molecular clouds that form stellar nurseries, and it’s therefore rather surprising to see a star bulking up, planet-style.

In this case, the difference is likely caused by the extreme discrepancy in size between the two stellar bodies. MM 1a is roughly 40x the mass of our sun and is known as a proto-O-type star. An O-type star is a massive blue-white star. A proportional comparison of our own Sun (G-type) with an O-type star is shown below:

O-type stars are rare, incredibly bright, and don’t tend to survive for more than a few million years before going supernova. In this case, the molecular cloud that formed the star was dense enough to leave enough material in a fragmented disc that could still coalesce into a second star. While MM 1a is massive, at more than 40x the Sun’s mass, the smaller MM 1b is less than half of our own star’s weight.

“Many older massive stars are found with nearby companions,” said Dr. Ilee, lead author of the study. “But binary stars are often very equal in mass, and so likely formed together as siblings. Finding a young binary system with a mass ratio of 80:1 is very unusual, and suggests an entirely different formation process for both objects.”

It’s even possible that MM 1b has its own protoplanetary disc as well, though any planetary formation is likely to be cut short. “Stars as massive as MM 1a only live for around a million years before exploding as powerful supernovae,” Dr. Ilee said, “so while MM 1b may have the potential to form its own planetary system in the future, it won’t be around for long.”

Feature image credit: J. D. Ilee, University of Leeds

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