Infrared Spectroscopy Confirms the Transition of IRAS 21204+4913 to an FU Ori Outburst

ATel #17823; E. Portnoi, L. A. Hillenbrand (Caltech), A. S. Carvalho (Harvard/Smithsonian)
on 30 May 2026; 18:23 UT
Credential Certification: Lynne Hillenbrand (lah@astro.caltech.edu)

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S. Kaneko first announced an optical transient that was associated by Grosso et al. (2025) in ATel #17546 with unstudied source IRAS 21204+4913. Kochkina et al. (2025) in ATel #17519 presented multi-epoch photometry and an optical spectrum, and advocated for an FU Ori outburst interpretation. The source properties were further discussed by Stecklum (2025) in ATel #17556. Additional optical photometric and spectroscopic observations were published in Burlak et al. (2026), along with detailed analysis of the object.

Here, we present new infrared spectroscopy and use the spectral diagnostics described in Portnoi et al. (2026) to spectroscopically confirm IRAS 21204+4913 as having transitioned to an FU Ori outburst.

Our follow-up spectra were obtained with NIRES, the Near-Infrared Echellette Spectrometer at Keck Observatory, on 8 May 2026 (UT), near dawn. The spectrum covers 0.98-2.45 μm and has signal-to-noise ratio above 100 at all wavelengths.

We normalize and then deredden the spectrum as described in Portnoi et al. (2026), by comparing it to the FU Ori disk model from Carvalho et al. (2024). This produces (see link below) an estimated Av=7.1 mag (+0.8/-0.7 mag), a value that is consistent with the extinction estimates in both ATel #17546 and ATel #17556. Detailed comparison of the IRAS 21204+4913 outburst spectrum (see link below) to the median FUOr from Portnoi et al. (2026) shows a good match. This includes the presence of several key molecules and many atomic features in absorption, that originate in an accretion disk, and H and He lines that originate in a wind.

Regarding the molecules, we clearly see broad absorption from VO in Y-band and J-band, H2O in J-band (1.34 μm) and H-band (1.74 μm). We also see the characteristic CO feature in K-band (2.30 μm), as well as a combined CO/FeH strong feature in H-band (1.62 μm). For the atomic features, in J-band we see strong Si I (1.32 μm). In H-band, we see Si I (1.59 μm) and weak Mg I (1.50 μm). In K-band, the metallic lines of Na I (2.21 μm) and Ca I (2.26 μm) are weak.

When molecular band strengths and atomic line equivalent widths are measured, IRAS 21204+4913 is consistently clustered with other FUOrs. Feature strengths are distinct from M-type supergiant, giant, and dwarf stars, which have similar infrared spectra.

Moreover, we see strong wind lines, particularly He I 10830 A, Paβ, Paγ, Paδ and Brγ. These often blueshifted absorption or P Cygni type lines are another common feature of FUOrs.

Our results are that IRAS 21204+4913 shares many spectral similarities in the near-infrared with the population of known FUOrs. Coupled with the prior ATels and papers that supply additional evidence, we conclude that IRAS 21204+4913 is a recently outbursting FUOr.

[link to infrared spectrum]