Young stars brighten considerably when they go through phases of rapid growth. Such episodes are known as accretion bursts. Researchers at the Thuringian State Observatory identified the most energetic accretion burst of a massive young star yet discovered. For the first time, they modeled how the thermal radiation of the dust in the vicinity of the young star changes during such an event. Such time-dependent simulations enable astronomers to better analyse accretion outbursts of young stars.
Researchers at the Thuringian State Observatory (TLS) in Tautenburg found proof of the most energetic accretion burst of a massive young star yet discovered. Stars form in dense clouds of gas and dust. Young stars, known as protostars, grow when matter from their surroundings falls onto them. However, this is not a steady process. Sometimes protostars gain a particularly large amount of mass within a short time.
Such growth spurts in young stars are called accretion bursts. If an accretion burst occurs, astronomers want to know how long it lasts and how much energy is released. The answer to these questions gives them a better understanding of such bursts.
Increase in microwave radiation points to an accretion burst
In 2019, radio astronomers reported a sudden increase in methanol maser emission, a specific type of microwave radiation, in the star-forming region G323.46-0.08 (G323 for short). This region is located in the constellation Circinus on the southern sky. They suspected that a young massive star in this region was undergoing an accretion burst. Stars with more than eight solar masses are called massive.
Dr. Verena Wolf and Dr. Bringfried Stecklum, scientists at the Thuringian State Observatory, investigate how stars form. Together with an international team they wanted to clarify the cause of the increase in the microwave radiation. Was there really an accretion burst?
Therefore, they searched for images of G323 and found what they were looking for in the archive of the VISTA telescope (Visible and Infrared Survey Telescope for Astronomy) of the European Southern Observatory (ESO). VISTA mapped the southern Milky Way in the near infrared wavelength range.
The star-forming region G323 imaged with the VISTA telescope of the European Southern Observatory in the near infrared wavelength range. The image on the right from 2015 shows that the region shines much brighter than in 2010, proving the presence of an accretion burst. (The cyan-colored area in the center of the image marks a spot where the object was too bright for the detector.) Credit: TLS / Wolf et al. 2024
Infrared images confirm the growth spurt
"With the VISTA images we were able to confirm the accretion burst," says Dr. Stecklum. Numerous images of the star-forming region G323 at different times allowed the team to measure the temporal brightness change during the event. The burst lasted around eight years, from 2012 to 2020. It is the sixth confirmed accretion burst of a massive protostar.
Moreover, Wolf, Stecklum, and their team successfully modeled the progression of the radiation burst. They utilized specialized software, which was developed by their colleague, Professor Dr. Tim Harries, who works at the University of Exeter, UK.
In a first step, they simulated the temperature change of the dust at different times during and after the burst. Then, they calculated how this temperature change affects the thermal radiation of the dust. The simulation predicted that the afterglow of the accretion event could still be detectable in the far infrared wavelength range in 2022, even though the burst already finished in 2020. This created the opportunity to narrow down the burst energy with far infrared measurements.
Images taken with the SOFIA telescope confirm the model
SOFIA plane and telescope, Credit: NASA / Jim Ross
In order to obtain images of G323 in the far infrared wavelength range, the research team applied for observations with the airborne observatory SOFIA (Stratospheric Observatory For Infrared Astronomy). The SOFIA aircraft and telescope have been operated by the US space agency NASA and the German Aerospace Center (DLR) until December 2022. The observations in the far infrared showed a slight increase in brightness in accordance with the simulation.
More precise estimation of the accreted mass
With the combination of the VISTA data and the SOFIA data, Wolf, Stecklum and the team achieved a breakthrough: "This allowed us to accurately determine the energy released during the accretion burst and to estimate the accreted mass," says Dr. Wolf.
They found that the accretion burst of G323 is likely the strongest burst ever observed in a massive young star. In eight years, as much energy was released as the Sun emits in 740,000 years. Dr. Wolf explains the exceptional event: "Presumably, a huge clump with approximately seven times the mass of Jupiter fell onto the star.”
The combination of observational data across various wavelength regimes and the computer simulation allowed the research team, for the first time, to precisely examine the interplay between the local dust distribution and the protostellar accretion burst.
Time-dependent simulations like these are not restricted to protostars, but they can be used to better describe dust-enshrouded variable objects. These include, for example, active galactic nuclei and evolved pulsating stars.
The team led by Verena Wolf and Bringfried Stecklum has published the results of their research in the scientific article "The accretion burst of the massive young stellar object G323.46-0.08" in the journal "Astronomy & Astrophysics" (Link: www.aanda.org/10.1051/0004-6361/202449891) Verena Wolf’s research work was supported by the German Aerospace Center (DLR) under grant number 50OR1718.
The Research Team
The members of the research team are:
Bringfried Stecklum, Verena Wolf, Jochen Eislöffel (Thüringer Landessternwarte Tautenburg, Germany),
Paul Boley, Pierre Cruzalebes, Alexis Matter (Université Côte d'Azur, Observatoire de la Côte d'Azur, Nice, France),
Alessio Caratti o Garatti (INAF, Osservatorio Astronomico di Capodimonte, Italy),
Christian Fischer (German SOFIA Institute, University of Stuttgart, Germany),
Tim J. Harries (Department of Physics and Astronomy, University of Exeter, UK),
Hendrik Linz (Max Planck Institute for Astronomy, Heidelberg, Germany),
Aida Ahmadi (ASTRON, Netherlands Institute for Radio Astronomy, Dwingeloo, The Netherlands),
Julia Kobus (Institute of Theoretical Physics and Astrophysics, University of Kiel, Germany),
Xavier Haubois (European Organisation for Astronomical Research in the Southern Hemisphere, Santiago, Chile)
Background: How do stars form?
Stars are formed in the central regions of dense clouds of molecular gas and dust that collapse under their own gravity. Even a very small initial rotation of these clouds leads to a flattening of the collapsing core. Thereby, a protoplanetary disk of gas and dust forms which regulates the flow of matter onto the young star and may become the birthplace of planets.
Initially, matter falls from the envelope, in which the protostar is embedded, onto the disk. Due to mutual friction, it slows down and it moves towards the young star. The result is an unsteady flow of matter from the disk onto the protostar.
The protostar undergoes an accretion burst, if a particularly large amount of mass falls onto it within a brief period. The potential energy released from this infall causes intense heating. The strength and duration of the burst depends on the mass and compactness of the infalling object.
Weaker bursts occur frequently. Strong ones, such as the one observed in G323, are rare. Nevertheless, they play an important role. Young stars may acquire about half of their final mass through strong growth spurts.
Accretion bursts provide a unique opportunity to witness the formation of young stars. This is especially true for massive stars. By observing these events, scientists hope to answer questions like, how much does the luminosity change? What is the burst duration? How much energy is released? How much mass fell onto the protostar? Furthermore, they want to learn how the protoplanetary disk is structured and how it loses its mass.
Since 1939, several hundred accretion bursts have been observed in low-mass stars. But for massive protostars, only six bursts are known. This is because young high-mass stars are much rarer, evolve more quickly, and they are situated in much denser regions, compared to their low-mass counterparts.
The Thuringian State Observatory
The Thuringian State Observatory Tautenburg (TLS) is a research institute funded by the Free State of Thuringia, Germany. It conducts basic research in astrophysics. Researchers at TLS use various telescopes throughout the world for their observations of galaxies, stars, the sun, gamma ray bursts, and extrasolar planets.
The Thuringian State Observatory uses and operates the 2-meter Alfred Jensch Telescope for observations in the optical spectral range and a station of the European Low Frequency Array Radio Telescope (LOFAR). It is also building a solar lab to develop a prototype of an automated telescope for the continuous observation of the sun.
Contact
We are happy to answer your questions:
Dr. Verena Wolf
Telefon: +49 36427 863 754
Dr. Bringfried Stecklum
Telefon: +49 36427 863 755
Thüringer Landessternwarte
Sternwarte 5, 07778 Tautenburg
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