Russian astronomers observe the eruptive behavior of a young star

Russian astronomers from Moscow State University have performed photometric, polarimetric, and spectroscopic observations of a young star designated IRAS 21204+4913. Results of the new observations, which were published February 16 on the arXiv preprint server, reveal the eruptive behavior of this star and shed more light on its nature.

When young stars erupt

Young stellar objects (YSOs) are stars in the early stages of evolution; in particular, protostars and pre-main sequence (PMS) stars. They are usually observed embedded in dense molecular clumps, environments containing plenty of molecular gas and interstellar dust.

Given that episodic accretion processes occur in YSOs, these objects may experience accretion-driven outbursts. Astronomers usually divide such events into EX Lup (also known as EXors) and FU Ori outbursts (or FUors). EXors are a few magnitudes in amplitude, and last from a few months to one or two years; FUors are more extreme and rare. They can be up to 5 to 6 magnitudes in amplitude and last from decades to even centuries.

A transitional YSO

IRAS 21204+4913 is an YSO assumed to be in the transitional evolutionary stage between a protostar and a PMS star. The star is located some 1,600 light years away in the relatively compact dark nebula D 2944. On October 25, 2025, an outburst of IRAS 21204+4913 occurred, which reached a brightness of 12.4 mag by late November.

Hoping to shed more light on the nature of IRAS 21204+4913, a group of astronomers led by Moscow State University’s Marina Burlak began to monitor the star with the Caucasian Mountain Observatory (CMO), starting from early December 2025. Their study was complemented by analysis of archival data from various telescopes.

Unraveling the eruptive behavior

The observations found that the outburst of IRAS 21204+4913 had an amplitude of more than 6.0 mag, thus resembling FUor-type eruptions. Such large amplitude corresponds to an accretion rate of at least 0.00003 solar masses per year.

It turned out that the absorption spectrum of IRAS 21204+4913 resembles those of giant and supergiant stars of spectral types A-F. This suggests that the spectral energy distribution (SED) in the optical band should also be consistent with stars of these spectral types. Moreover, the observations detected several diffuse interstellar bands (DIBs) in the spectrum of IRAS 21204+4913 and their presence is assumed to be due to D 2944, which partially obscures the star.

Besides the inspection of the outburst, the study has identified several distinctive features in IRAS 21204+4913. The astronomers note that as the star brightened, its radiation became increasingly polarized, reaching about 16% in the I band by the end of their observations. This can be attributed to the presence of a dusty wind.

By analyzing the archival data, Burlak’s team found that an outburst of IRAS 21204+4913, of comparable amplitude to that in 2025, also occurred in 1948. However, the researchers underline that it is difficult to estimate how similar the 1948 and 2025 outbursts are as we do not know what the star’s spectrum was during the first outburst, how quickly its brightness increased in 1948, and how long it will continue to decline after the second outburst.

FUor or not

Summing up the results, the authors of the paper emphasize that it is still too early to conclusively classify IRAS 21204+4913 as a “bona fide” FUor, given that this group includes only objects whose brightness fades significantly over timescales of years to decades after maximum light. Further monitoring of IRAS 21204+4913 is therefore required to achieve this.

“It would be highly desirable not only to repeat the observations presented here, but also to obtain spectropolarimetric data and to monitor the evolution of its SED at wavelengths beyond 5 µm,” the scientists explain.

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