Ross A. Burns, Yuri Uno, Nobuyuki Sakai, et al., “A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar”, 2023, Nature Astronomy, digital object identifier (DOI):10.1038/s41550-023-01899-w: https://www.nature.com/articles/s41550-023-01899-w
High-mass stars are 8 or more times more massive than the Sun. They act like atomic factories to generate many of the necessary building blocks for life in the universe and they alter the appearance and evolution of galaxies. The most massive stars become enigmatic black holes when they die, and exploded as super nova explosion at the end of their life with spreading heavy chemical elements and contributing to form stars in the next generation. Despite their importance in the Universe, the process by which high-mass stars are born has been a mystery for many decades. It has recently become known that they form at the hearts of rotating disks of gas and dust, known as protostellar disks where a baby is born at the center, in which the sizes are ~1000 AU radius (AU: the distance at which the Earth orbits the Sun).
One theory which is emerging as a front runner in high-mass star formation research is the idea of ‘episodic accretion’ where by clumps of dusty gas occasionally fall from the protostellar disk onto the growing star, or ‘protostar’ (baby of a star), at the center, resulting in releasing a drastic energy and triggering a “burst” phenomenon that causes its luminosity brighter than usual with more than one order of magnitude. These growth bursts supply the protostar with more than half of the mass it gains during its growth stage. However, these growth bursts occur on timescales of hundreds to thousands of years, and, lasting only a few months to years, are very rare events to witness. To date, astronomers have only witnessed a few growth bursts in high-mass protostars. The most recent, and the most intensely investigated was the 2019 growth burst in high-mass protostar G358-MM1.
The episodic accretion theory proposes that protostellar disks are clumpy and that spiral arms may emerge in the disk due to it experiencing the pull of its own self-gravity.
Observing protostellar disks around high-mass protostars, let alone any spiral structure or clumpiness, has been a challenge for astronomers. Such disks, and their high-mass protostars, form inside dense clouds of gas and dust in turbulent stellar nurseries, which are for the most part invisible to conventional optical telescopes.
However, in a new Nature Astronomy publication, a team of astronomers who specialise in ‘maser’ emission - which is a microwave wavelength laser - were able to map a high-mass protostellar disk in higher detail than was achieved before: Ross A. Burns, et al. 2023 1
Using Very Long Baseline Interferometry (VLBI) arrays from around the world in an international collaboration, the team was able to discover the obvious spatial structure of spiral arms in the rotating disk of high-mass protostar, G358-MM1 (see the left-panel of figure below as an artist’s impression of the four-arm spiral disk, which was depicted on the basis of this observations result shown in the right-panel). This is the very same protostar which was seen to experience a growth burst in 2019.
Fig. (Left) Artist’s impression of the 4-arm spiral disk which houses the episodically accreting high-mass protostar G358-MM1 (Credit: Charlie Willmott, Ross A. Burns). (Right) Map of the 6.7 GHz methanol maser emission in G358-MM1, imaged using heatwave mapping (see the main text for details). The center represents the location of the high-mass protostar determined in millimetre wavelength interferometric imaging. Spiral structures can be seen, wrapping around the protostar in a counter-clockwise direction, along the four grey lanes. Colours show the velocity of gas along the line of sight. Regions in blue are moving toward us while red regions show gas that is moving away. Overall, the colours indicate that the system is rotating, in the form of a Keplerian disk, around G358-MM1.
The team used a new technique called 'heat-wave mapping' where the entire region of a high-mass protostellar disk is mapped by multi-epoch VLBI observations of masers from methanol molecules. Methanol maser emission at a radio frequency of 6.7 GHz traces a heat wave which propagates outward from a high-mass protostar. The heat-wave mapping resembles 'CT scans' which reveal the inside of the human body by multi X-ray scans. In total, 24 radio telescopes were used, from Oceania, Asia, Europe and America. All data were combined to produce an image of the G358-MM1 spiral disk with milliarcsecond (1/3600000th of a degree) spatial angular resolution (or eye sight).
The discovery brings together evidence of several of the aspects of episodic accretion theory: a rotating disk (see the right-panel of the figure as the observational evidence of rotation in the form of a Keplerian disk shown in the distribution and its colours), growth bursts, and spiral structure which helps to feed the growing high-mass protostar.
The team will continue to search for growth bursts in high-mass protostars, using a global cooperative of traditional radio telescopes, called the Maser Monitoring Organisation (M2O) 2. So far, only 3 high-mass protostellar bursts have been seen, the team hopes to find many more to explore more growth bursts in other high-mass protostars.
G358-MM1 has four spiral arms which wrap beautifully around the protostar. The spiral arms help to feed disk material down to the center of the system where it can reach the protostar and feed it. If more spiral systems and growth bursts are discovered in other high-mass protostars, either using heat-wave mapping or other observational techniques, then astronomers will be able to provide a better understanding of the births of high-mass stars, which are the energetic progenitors of life in the Universe.
Dr. Nobuyuki Sakai who is a research associate at NARIT and one of main contributors to this paper and Dr. Koichiro Sugiyama who is also a research associate at NARIT and a co-author said, “Our radio data with unprecedented magnificent eye sight provided unprecedented view and the result via carefully analyzing by researchers using different statistical techniques, and we unveiled the existence of symmetric spiral arms in a protostellar disk. This discovery allows us to much better understand how a high-mass star is born clearly. Enlarging the number of an observational sample and results of this kind of spiral system will enable us to reach the ultimate goal of understanding an entire view of the high-mass star formation and the evolution completely with the magnificent statistical sample and the research.”
URL related to this research activities:
- A. Burns, Y. Uno, N. Sakai, et al., “A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar”, 2023, Nature Astronomy, doi:10.1038/s41550-023-01899-w - https://www.nature.com/articles/s41550-023-01899-w
- M2O - https://www.masermonitoring.com/
- Press-release through the webpage of National Astronomical Observatory of Japan - https://www.nao.ac.jp/en/news/science/2023/20230228-dos.html
Author affiliations (21 countries; 34 affiliations):
- National Astronomical Research Institute of Thailand (Public Organization), 260 Moo 4, T. Donkaew, A. Maerim, Chiangmai 50180, Thailand
- Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
- Department of Science, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
- Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
- Department of Physics, National Chung Hsing University, No. 145, Xingda Rd., South Dist., Taichung 40227, Taiwan
- National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Rd., Socorro, NM 87801, USA
- Department of Physics, Faculty of Science, University of Malaya, 50603, Kuala Lumpur, Malaysia
- Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
- Center for Astronomy, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
- Department of Astronomical Sciences, SOKENDAI (The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka-shi, Tokyo 181-8588, Japan
- University of Science and Technology, Korea (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
- Ventspils International Radio Astronomy Center", Ventspils University of Applied Sciences, Inzenieru Str. 101, Ventspils, LV-3601, Latvia
- Radio Astronomy and Geodynamics Department of Crimean Astrophysical Observatory, Katsively, RT-22 Crimea Ukraine
- Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
- INAF-Osservatorio Astronomico di Capodimonte Napoli, Salita Moiariello 16, 80131 - Naples, Italy
- Astronomical Observatory, Ural Federal University, 19 Mira Street, 620002 Ekaterinburg, Russia
- Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
- National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA, 22903, USA
- Australia Telescope National Facility, CSIRO, PO Box 76, Epping NSW 1710, Australia
- NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Rd, Victoria, BC, V9E 2E7, Canada
- INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
- Department of Physical Sciences, The Open University of Tanzania, P.O. Box 23409, Dar-Es-Salaam, Tanzania
- Hartebeesthoek Radio Astronomy Observatory, PO Box 443, Krugersdorp, 1741, South Africa
- Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
- Space Research Unit, Physics Department, North West University, Potchefstroom 2520, South Africa
- Department of Physics and Astronomy, Faculty of Physical Sciences, University of Nigeria, Carver Building, 1 University Road, Nsukka, Nigeria
- INAF - Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
- School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, Tasmania 7001, Australia
- Departamento de Astronomía, Universidad de Guanajuato, A.P. 144, 36000 Guanajuato, Gto., Mexico
- Space Radio-Diagnostic Research Center, Faculty of Geoengineering, University of Warmia and Mazury Oczapowskiego 2, PL-10-719 Olsztyn, Poland
- INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
- SKA Observatory, Jodrell Bank, SK11 9FT, UK
- Center for Astrophysics, GuangZhou University, Guangzhou, China
- Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China