For the first time, spiral arms of gas and dust have been seen within a disc swirling around a massive protostar, feeding it with irregular bursts of material. The observations, made by a network of 25 radio telescopes across 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 about 22,000 light–years away. The young star’s mass is about eight times that of the Sun and growing. At this mass and greater, a star will explode as a supernova at the end of its life. However, only 1% of stars are considered high mass and why so few massive stars form is a longstanding puzzle of astrophysics.
“There has been a school of thought that high-mass star formation must be totally different to low-mass stars,” Ross Burns, of the National Astronomical Observatory of Japan (NAOJ), tells Physics World. “But what we’re generally finding over time is that there’s not much difference.”
A manifestation of masers
Massive protostars can be difficult to observe because they are often hidden by dense agglomerations of gas and dust at the heart of the most intense star-forming regions. However, in January 2019 increased microwave emission was detected coming from G358.93-0.03-MM1 in the form of methanol masers. These are naturally occurring microwave equivalents of lasers.
Burns leads the Maser Monitoring Organisation, which is an international association of astronomers who study masers. Burns and colleagues assembled an impressive array of radio telescopes to observe G358.93-0.03-MM1’s maser activity.
The network imaged the protostar six times over the course of 2019. In a 2020 paper, Burn’s team released the 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 onto the growing protostar. This had ignited thermal pulses that radiated through the surrounding accretion disc, exciting the masers at increasing distance from the star. Burns describes this as “heat wave mapping”.
Now, having analysed data from the other four sets of observations taken between March and September 2019, Burns’ team have published a new paper that shows that the methanol masers are embedded within a pattern of spiral arms within the accretion disc, extending from a distance of 50 AU from the star out to 900 AU (135 billion km).
Spiral arms in accretion discs around massive protostars had previously been suggested because they solve several problems, says Burns.
Pushing against accretion
“The main difference between high-mass and low-mass star formation is that the high-mass stars produce a lot more radiation, they’re a lot hotter, so they typically push against accretion,” he says.
Above eight solar masses, this outward radiation pressure should oppose any further accretion and prevent the protostar from acquiring anymore mass. However, astronomers have observed massive stars up to several hundred times the mass of the Sun, so clearly something can override the outward radiation pressure and allow growth to continue.
Accretion from a disc, rather than material falling on the star from all directions, can counter this outward pressure, but spinning discs tend to contain a lot of angular momentum that needs to be shed in order for accretion to take place. Spiral arms can remove this excess angular momentum, but while spiral arms have been seen in star-forming discs around low-mass protostars before, they had 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 a galaxy’s spiral arms, the arms are probably formed by the destabilization of the disc through the self-gravity of denser pockets of material. The arms channel clumps of material towards the protostar, where they accrete onto it, prompting a blast of heat like the one that sparked the masers into activity.
Initial mass function
A greater understanding of high-mass star formation could ultimately help solve the mystery of why massive stars are so rare. The most common stars in the universe are the smallest, which are the M-dwarfs, and the more massive a star is, the fewer in number they seem to be. Astronomers call this distribution of stellar masses the initial mass function (IMF), but why it is skewed so much towards smaller stars remains a puzzle.
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Even more intriguing is that the IMF may have been different in the past. The JWST’s observations of very old galaxies shows then to be more luminous than expected. One explanation is that the IMF may have been different 13.5 billion years ago, with conditions somehow more favourable towards forming massive stars that are intrinsically more luminous. Therefore, understanding the process by which massive stars form today, and the environments in which they form, could help us better understand the IFM whether it may have been different in the early universe.
Burns is keen to point out that the research was done by astronomers in 21 countries, including some currently at war with each other or on opposite sides of diplomatic relations.“Given the geopolitical climate, I think it is great that we are showing that academic research is continuing through groups of people of so many nations”.
The research is described in Nature Astronomy.