In a recent discovery, the James Webb Space Telescope (JWST) detected a massive stellar eruption measuring eight light-years across. This outflow, known as Sharpless 2-284 (Sh2-284 for short), is located about 15,000 light-years away in the outer reaches of the Milky Way. Based on an analysis by an international team of astronomers, this outflow appears to be coming from a newly forming star (a proto-star). It is moving at relativistic speeds (hundreds of thousands of kilometers per hour), and its size and strength indicate it is a rare phenomenon.
The discovery occurred while the team conducted their Low-Metallicity Star Formation Survey (LZ-STAR), a comprehensive observational program studying star formation in the low-metallicity environment. This survey has been examining Sh2-284, a star-forming region in our galaxy with very low metal content, to learn more about the formation and early evolution of stars. The survey relies on multiple observatories, including space-based telescopes like the JWST, Hubble, and Chandra, and ground-based observatories like the Atacama Large Millimeter-submillimeter Array (ALMA) and the Gemini Observatory.
The research is being led by Yu Cheng, a researcher with the National Astronomical Observatory of Japan (NOAJ), who planned and proposed the observations while earning his Ph.D. in astronomy at the University of Virginia (UVA). He was joined by researchers from the UVA, Chalmers University of Technology, the European Southern Observatory (ESO), the Instituto de Astrofísica de Andalucía (IAA), Shanghai Jiao Tong University, and the Space Telescope Science Institute (STScI). Their initial findings, presented in a series of publications, show evidence of ordered massive star formation with a stable, bipolar outflow from a massive protostar.
The most recent, "Ordered Massive Star Formation in the Outer Galaxy," has been accepted for publication in The Astrophysical Journal. As they explained, the jet observed belongs to a unique class of phenomena known as Herbig-Haro (HH) objects, referring to highly collimated jets of plasma extending from newly forming stars. Some of the gas building up around the central proto-star, likely under the influence of magnetic fields, was observed being blasted along the star's polar axis.
Over 300 HH objects, mainly emanating from low-mass stars, have been observed to date. Astronomers are interested in these jets because they offer clues into the nature of newly forming stars. Like other star-forming regions in the galactic hinterlands, Sh2-284 is deficient in heavier elements. This gradually changes as stars reach the end of their lifespan and expel the heavier elements forged in their interiors through supernovae and stellar wind. This makes clusters like Sh2-284 analogs for the early Universe, where stellar environments were also deficient in heavier elements. Said Chen in an ESA press release:
We didn't really know there was a massive star with this kind of super-jet out there before the observation. Such a spectacular outflow of molecular hydrogen from a massive star is rare in other regions of our galaxy. Massive stars, like the one found inside this cluster, have very important influences on the evolution of galaxies. Our discovery is shedding light on the formation mechanism of massive stars in low-metallicity environments, so we can use this massive star as a laboratory to study what was going on in earlier cosmic history.
Webb's resolution and advanced infrared optics captured the jet's detailed filamentary structure, caused by the jet pushing through interstellar dust and gas. Webb's data also allowed the team to test theories of massive star formation, which has been debated for over 30 years. Some believed that massive stars undergo a chaotic process known as competitive accretion. In this model, material falls in from many different directions, causing the orientation of the disc to change over time. The outflow is launched from above and below the disc, causing it to appear twisted and turning in different directions.
However, thanks to the detailed imagery Webb provided, the team saw that the opposite sides of the jets are nearly 180 degrees apart from each other. Said co-author Jonathan Tan of the UVA and Chalmers University of Technology:
That tells us that this central disc is held steady and validates a prediction of the core accretion theory. Originally, the material was close into the star, but over 100,000 years, the tips were propagating out, and then the stuff behind is a younger outflow. Webb's new images are telling us that the formation of massive stars in such environments could proceed via a relatively stable disc around the star that is expected in theoretical models of star formation known as core accretion.
In response, the team developed new theoretical core accretion models to tell them what kind of star is in the center. These models revealed that the star is about 10 times the mass of the Sun and is still growing and powering this outflow. This effectively demonstrates that HH jets scale up with the star's mass and are powered by the gravitational energy they release. As such, the jets are a record of the formation history of the protostar. The team is now looking through the survey data for other massive stars in the outer regions of the Milky Way, which could provide additional clues about early stellar formation.
Further Reading: ESA, The Astrophysical Journal, University of Virginia
