For the first time, spiral arms of gas and dust within a disk have been seen circling a massive protostar, feeding it irregular bursts of material. The observations, made by a network of 25 radio telescopes in ten different countries, provide new insights into how the most massive stars form.
The protostar is called G358.93-0.03-MM1 and is located in a star-forming region in the Milky Way, some 22,000 light-years away. The mass of the young star is about eight times that of the Sun and it continues to grow. With this mass and more, a star will explode as a supernova at the end of its life. However, only 1% of stars are considered to be of high mass and why so few massive stars form is a long-standing puzzle of astrophysics.
For the first time, spiral arms of gas and dust within a disk have been seen circling a massive protostar, feeding it irregular bursts of material. The observations, made by a network of 25 radio telescopes in ten different countries, provide new insights into how the most massive stars form.
The protostar is called G358.93-0.03-MM1 and is located in a star-forming region in the Milky Way, some 22,000 light-years away. The mass of the young star is about eight times that of the Sun and it continues to grow. With this mass and more, a star will explode as a supernova at the end of its life. However, only 1% of stars are considered to be of high mass and why so few massive stars form is a long-standing puzzle of astrophysics.
Burns leads the Maser Monitoring Organization, which is an international association of astronomers who study masers. Burns and his colleagues have assembled an impressive array of radio telescopes to observe the maser activity of G358.93-0.03-MM1.
The network imaged the protostar six times over the course of 2019. In a 2020 paper, Burn’s team published preliminary results from the first two observations, made in February 2019, which showed that an accretion burst had occurred. in which a large amount of gas had fallen on the growing protostar. This ignited thermal pulses that radiated through the surrounding accretion disk, exciting masers at ever greater distances from the star. Burns describes this as “heat wave mapping.”
Now, after analyzing data from the other four sets of observations taken between March and September 2019, Burns’s team published a new paper showing that the methanol masers are embedded within a pattern of spiral arms within the disk. of accretion, extending from a distance. from 50 AU from the star to 900 AU (135 billion km).
Spiral arms in accretion disks around massive protostars have previously been suggested because they solve several problems, Burns says.
Pushing against the buildup
“The main difference between high-mass and low-mass star formation is that high-mass stars produce much more radiation, are much hotter, so they normally push against accretion,” he says.
Above eight solar masses, this outward radiation pressure should oppose any further buildup and prevent the protostar from gaining more mass. However, astronomers have observed massive stars several hundred times the mass of the Sun, so clearly something can nullify the outward radiation pressure and allow growth to continue.
Accretion from a disk, rather than material falling on the star from all directions, can counteract this outward pressure, but spinning disks tend to contain a large amount of angular momentum that must be removed for accretion to occur. . Spiral arms can remove this excess angular momentum, but while spiral arms have been seen before in star-forming disks around low-mass protostars, they have never been seen around high-mass protostars.
“We’ve always assumed that the spiral arms are there, but there has never been an observational approach capable of revealing them, until now,” says Burns.
Like the spiral arms of a galaxy, the arms probably form from the destabilization of the disk through the self-gravity of denser pockets of material. The arms funnel clumps of material toward the protostar, where they accumulate above it, causing a blast of heat like the one that ignited the maser activity.
Initipal mass function
Further understanding of high-mass star formation could help solve the mystery of why massive stars are so rare. The most common stars in the universe are the smallest, which are M dwarfs, and the more massive a star is, the fewer in number they appear to be. Astronomers call this distribution of stellar masses the initial mass function (IMF), but why it is so skewed toward smaller stars remains a puzzle.
Even more intriguing is that the IMF may have been different in the past. JWST observations of very old galaxies show that they are more luminous than expected. One explanation is that the IMF may have been different 13.5 million years ago, with conditions somewhat more favorable for the formation of massive stars that are inherently more luminous. Therefore, understanding the process by which massive stars form today, and the environments in which they form, could help us better understand whether the IFM might have been different in the early universe.
Burns wishes to point out that the research was conducted by astronomers in 21 countries, including some that are currently at war with each other or on opposite sides of diplomatic relations. “Given the geopolitical climate, I think it’s great that we’re showing that academia research continues across groups of people from so many nations.”
The research is described in Nature Astronomy