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From: atel@astronomerstelegram.org
Subject: ATel  17823
Message-Id: <20260531070313.75BE128807E6@server.astronomerstelegram.org>
Date: Sun, 31 May 2026 03:03:13 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17823							     ATEL #17823

Title:	Infrared Spectroscopy Confirms the Transition of IRAS 21204+4913
		to an FU Ori Outburst
Author:	 E. Portnoi, L. A. Hillenbrand (Caltech), A. S. Carvalho (Harvard/Smithsonian)
Queries:	lah@astro.caltech.edu
Posted:	30 May 2026; 18:23 UT
Subjects:Infra-Red, Young Stellar Object

S. Kaneko first announced an   
<a HREF="https://www.astroarts.co.jp/photo-gallery/photo/129147">optical
transient</A>  
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  
<A HREF="https://ui.adsabs.harvard.edu/abs/2026arXiv260215157B/abstract">
Burlak et al. (2026)</A>, 
along with detailed analysis of the object.

Here, we present new infrared spectroscopy and use the spectral diagnostics
described in  
<A HREF="https://ui.adsabs.harvard.edu/abs/2026arXiv260512795P/abstract">
Portnoi et al. (2026)</A>  
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 &mu;m and
has signal-to-noise ratio above 100 at all wavelengths.

We normalize and then deredden the spectrum as described in  
<A HREF="https://ui.adsabs.harvard.edu/abs/2026arXiv260512795P/abstract">
Portnoi et al. (2026)</A>,  
by comparing it to the FU Ori disk model from  
<A HREF="https://ui.adsabs.harvard.edu/abs/2024ApJ...973L..40C/abstract">
Carvalho et al. (2024)</a>.  
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  
<A HREF="https://ui.adsabs.harvard.edu/abs/2026arXiv260512795P/abstract">
Portnoi et al. (2026)</A>  
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 &mu;m) and H-band (1.74 &mu;m).  
We also see the characteristic CO feature in K-band (2.30 &mu;m),  
as well as a combined CO/FeH strong feature in H-band (1.62 &mu;m). 
For the atomic features, in J-band we see strong Si I (1.32 &mu;m). In
H-band, we see Si I (1.59 &mu;m) and weak Mg I (1.50 &mu;m). In K-band,
the metallic lines of Na I (2.21 &mu;m) and Ca I (2.26  &mu;m) 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, Pa&beta;,
Pa&gamma;,  Pa&delta; and Br&gamma;. 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]: https://sites.astro.caltech.edu/~lah/iras21204p4913/
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