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“Intruder” caught redhanded in rarely detected stellar flyby event


An international team of astronomers, including Péter Ábrahám, Lei Chen, and Ágnes Kóspál from the Konkoly Observatory of the ELKH Research Centre for Astronomy and Earth Sciences (Budapest, Hungary), published their latest results on the Z Canis Majoris (Z CMa) pre-main sequence binary system in the prestigious Nature Astronomy journal. The team claims that their images obtained with three powerful telescopes (the Subaru Telescope in Hawaii, the Jansky Very Large Array in the US, and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile) imply that the circumstellar dust and gas disk was disturbed by an external body.

The disturbances in the disk are seen as extended gas and dust streams. The morphology of these streams, the fingerprints of the flyby, helped the scientists to identify and pinpoint the intruder. Using numerical simulations, they demonstrated that gravitational interaction between the faint point source discovered in the ALMA images and the Z CMa disk could be responsible for the observed structure.

The rare detection of a flyby event, long predicted in models of star formation, raises new questions for the future of forming stars and the baby planets being born in their disks, a process that so far has remained a mystery to scientists. An unusual feature of the Z CMa binary system is that both components undergo unpredictable large brightenings, known as accretion outbursts. Ágnes Kóspál, a co-author of the publication and the PI of a European ERC project studying the outburst of young stars, said that “stellar flybys may provide an explanation for such eruptions. The perturbation of the disk structure may push some disk material onto the protostar, increasing its brightness and possibly influencing the planetary embryos as well.”

Ruobing Dong, an astronomer at the University of Victoria in Canada and the principal investigator of this study, added that studying the evolution and growth of young star systems throughout the galaxy helps scientists better understand our own Solar System’s origin. "Studying these types of events gives a window into the past, including what might have happened in the early development of our own Solar System, critical evidence of which has long gone."

Studying planet and star formation is an acknowledged research field at the Konkoly Observatory. In the Z CMa research project, the Hungarian astronomers contributed to the preparation of the ALMA observations and refined the distance of the system based on data from ESA’s Gaia space telescope.

More details are available in a press release published by the National Radio Astronomy Observatory:

The Atacama Large Millimeter/submillimeter Array (ALMA) is located in Chile at an altitude of more than 5000 m. It is the most sensitive and sharpest instrument on Earth to map the microwave radiation of the Universe. Since circumstellar disks also emit at these wavelengths due to their low temperatures, ALMA is one of the most powerful instruments for revealing how stars and planets are born. (Credit: ESO/B. Tafreshi (
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Composite image from the Subaru Telescope, Jansky Very Large Array, and the Atacama Large Millimeter/submillimeter Array (ALMA). The intruder object (lower right in the composite image) was detected by ALMA at the end of the streamer point clearly seen in the image taken by the Subaru Telescope. Z CMa is a pre-main sequence binary system located at around 1100 pc in the constellation Canis Major. Both binary components undergo accretion outbursts, which may be facilitated by perturbations to the host disk by flybys. (Credit: ALMA (ESO/NAOJ/NRAO), S. Dagnello (NRAO/AUI/NSF), NAOJ)
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Artist’s impression of the perturber leaving Z CMa, pulling a long stream from the protoplanetary disk along with it. Its “visit” may have other as yet unknown impacts on the growth and development of planets in the star system. (Credit: ALMA (ESO/NAOJ/NRAO), B. Saxton (NRAO/AUI/NSF))