An international astronomers group including three scientists affiliated with Nicolaus Copernicus University has been successful in mapping the protostellar disk with the greatest accuracy yet.
The breakthrough offers proof hypothesized by the theory of episodic accretion.
The study was published in the journal Nature Astronomy, titled: “A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar.”
Among the authors, an international team of astronomers specializing in observations of maser emissions, were three scientists who have been affiliated with the Institute of Astronomy at the NCU Faculty of Physics, Astronomy and Informatics: Dr habil. Anna Bartkiewicz, NCU Prof., Mgr Michal Durjasz, and Dr. Mateusz Olech, who defended his doctorate at the Faculty and is currently part of the team of the Space Radio Diagnostics Center at the University of Warmia and Mazury in Olsztyn.
In her scientific work, Dr. Habil. Anna Bartkiewicz, an NCU Professor, is concerned with the study of star-forming objects displaying the methanol maser’s ring structure. Using the European VLBI Network, Bartkiewicz accurately identifies the natural motions of the maser clouds at the level of just a few kilometers per second.
At present, Bartkiewicz is leading the Opus grant of the National Science Centre, studying “Space masers as a tool for identifying accretion explosions of massive protostars”. Michal Durjasz, a Ph.D. student at the Nicolaus Copernicus University Doctoral School of Exact and Natural Sciences, has also made a significant contribution to the study, working on areas of huge stars causing sudden changes in methanol maser emission.
The article, and particularly the study that preceded it, is the outcome of a successful collaboration among specialists from around the world.
Observational data acquired from 24 radio telescopes from around the world contributed to our discovery, which was then carefully correlated by teams at three centers on three different continents, explains dr Ross Burns of the National Astronomical Observatory of Japan, first author of the paper-Around 150 people were involved, and we would like to express our gratitude to them for their efforts, hoping for further collaboration in the future.
Dr. Habil Anna Bartkiewicz, Professor, Nicolaus Copernicus University in Torun
Mysteries of the Stars
The scientists concentrated on observations of huge stars - those with masses more than eight times that of our Sun. They play a major role in the production of the elements that are essential to building life in the Universe and also impact the formation and progress of galaxies. The greatest stars die and turn into enigmatic black holes.
In spite of their significance in the Universe, the process of massive star formation has been shrouded in mystery for several decades.
So far, there has been no single theory accepted by the entire scientific community to explain their formation. It has only recently been confirmed that massive stars are born in the centers of rotating disks composed of gas and dust.
Anna Bartkiewicz, Professor, Nicolaus Copernicus University in Torun
Bartkiewicz added, “Protostellar disks, as they are called, have a radius of about one thousand astronomical units, i.e., the average distance between the Earth and the Sun multiplied by one thousand [the astronomical unit, or conventional measure of distance used in astronomy, is the average distance between the Earth and the Sun; i.e., 149597870.7 km].”
One theory of huge star formation that is turning out to be popular among scientists is episodic accretion.
Accretion, or the deposition of matter on a star. It involves clouds of dusty gas occasionally 'breaking off' and dropping from the disk onto a growing protostar, i.e., a young star located in the center.
Mgr Michal Durjasz, Professor, Nicolaus Copernicus University in Torun
Durjasz added, “During such surges in the rate of matter accretion, the star accumulates more than half of the mass it gains during the formation stage. These accretion rate surges, or episodic accretion, are very rare events: they occur every hundreds or thousands of years and last from a few months to a few years.”
To date, the most elaborately studied star formation was the quick accretion of the huge protostar G358-MM1 (the name is connected to the coordinates of the object in the sky) in 2019.
The episodic accretion theory indicates that protostellar disks are huge and inhomogeneous. As a result of the impact of their own gravity, spiral arms might appear in them, added Dr. Mateusz Olech.
The high observation of protostellar disks in the areas of birth of huge stars is a difficulty for astronomers. They are developed in thick molecular clouds that are impenetrable to traditional optical astronomy. Noting possible spiral arms is even more difficult.
In the latest study in the journal Nature Astronomy, an international group of astronomers who specialized in observations of maser emission—which is the naturally-occurring cosmic counterpart of a laser at microwave radio wavelengths—was successful in achieving maps of the protostellar disk with the greatest precision yet.
Making use of a network of radio telescopes and very long baseline interferometry (VLBI), the researchers found spiral arms in the rotating disk of a high-mass protostar—named G358-MM1. This is the same protostar that experienced quick accretion in 2019.
The team used a new technique called 'heat-wave mapping', which produces a set of maps of the brightened masers of methanol molecules at different stages of an event. A total of 24 radio telescopes from Oceania, Asia, Europe, and America were used.
Anna Bartkiewicz, Professor, Nicolaus Copernicus University in Torun
Bartkiewicz continued, “This allowed us to image the G358-MM1 spiral disk with a resolution of one angular millisecond, i.e., 1/3600000th of a degree, explains Prof Bartkiewicz.—G358-MM1 has four spiral arms that wrap around the protostar.”
“This help to carry material from the disk to the center of the system, where it can reach the protostar. If more spiral systems and this type of brightness are discovered, astronomers will be able to understand better the processes that accompany the birth of high-mass stars, which are the true cradle of life in the Universe,” added Bartkiewicz.
The breakthrough offers observational proof for various aspects anticipated by episodic accretion theory: a rotating disk, quick brightening, and a spiral structure that helps “feed” an increasing high-mass protostar.
The research group will keep searching for such bursts of maser emission with the help of a global collaboration of conventional radio telescopes known as the Maser Monitoring Organisation. Until now, only three quick brightening high-mass protostars have been noted. However, scientists believe in finding many more.
Burns, R. A., et al. (2023) A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar. Nature Astronomy. doi.org/10.1038/s41550-023-01899-w.