The eruptive young star IRAS 21204+4913

TL;DR

IRAS 21204+4913 is characterized as a low-mass (M_* ≲ 0.5 M_⊙) young stellar object undergoing a major accretion-driven outburst beginning in late 2025, with a luminosity rise to L_* ≈ 90 L_⊙ and a derived accretion rate of ≥3×10⁻⁵ M_⊙ yr⁻¹. The eruption displays FUor-like absorption-dominated spectra, but uniquely shows TiO emission bands and a dusty, accelerating wind with a P Cygni Hα profile and a high degree of polarization (p_I ≈ 16%), indicating strong scattering. High-resolution imaging reveals nearby HH objects and a jet aligned with the wind, supporting an outflow–driven geometry; a century-scale light curve confirms a prior 1948 outburst of comparable amplitude, suggesting recurrence. Extinction toward the object is modest (A_V ≲ 2 mag), with DIBs and multiwavelength SED constraints favoring a predominantly circumstellar origin for the observed polarization and spectral features. Overall, the object challenges simple FUor/EXor classifications and provides a valuable laboratory for studying episodic accretion, wind scattering, and disk–jet interactions in very young, low-mass stars.

Abstract

The results of photometric, polarimetric, and spectroscopic observations are presented for the young star IRAS 21204+4913, whose visible brightness has increased by $\approx 5^{\rm m}$ since October 2025. The star's absorption spectrum in the $0.36 - 0.75 μ$m range resembles those of A - F giants and supergiants, but it also exhibits molecular TiO bands. The brightening was accompanied by a significant increase in the degree of polarization of the stellar radiation (to $\approx 16 \%$ in the I-band), likely due to scattering by dust in an expanding circumstellar shell. The P Cygni profile of the H$α$ line implies a dusty wind velocity of $\approx 300$ km/s. We believe that the outburst of IRAS 21204+4913 is caused by an increase in the accretion rate of protoplanetary disk's matter onto the young star with a mass of $\lesssim 0.5$ M$_\odot$ to $\gtrsim 3\times 10^{-5}$ M$_\odot$ yr$^{-1}$. Furthermore, IRAS 21204+4913 displays several unusual features: the dependence of the width and radial velocity of absorption lines on the excitation potential, emission in the TiO molecular bands, and a comparably bright outburst that occured in 1948. Several T Tauri stars and a group of Herbig-Haro objects are found in the vicinity of the star.

Figures (12)

• Figure 1: Difference image of the IRAS 21204+4913 surroundings in the Halp and Halpbc filters. Objects 0, 1, and 2 mark IRAS 21204+4913, star S1, and star S2, respectively. The red line segment passing through the Herbig-Haro objects (bright spots) indicates the position of the TDS spectrograph slit used to obtain the spectrum shown in Fig. \ref{['fig:jet-sp']}. Further details are given in the text.
• Figure 2: 2D (upper panel) and 1D (lower panel) TDS spectra of the emission nebulae located along the northern boundary of the dark cloud D 2944. The red curve shows the spectrum of the background nebula within the region bounded by the red dashed lines, while the blue curve represents the spectrum of the brightest clump (within the blue dashed lines) with the background spectrum subtracted. Scattered black dots in the 2D image are cosmic-ray hits.
• Figure 3: Variation of the brightness and selected color indices of IRAS 21204+4913 in the optical and NIR bands. Data obtained prior to December 10, 2025 are taken from Kochkina-2025.
• Figure 4: Evolution of the degree of polarization $p$ and polarization angle $\theta$ of the radiation from IRAS 21204+4913. The gray dashed line shows the least-squares fit.
• Figure 5: Low-resolution spectra of IRAS 21204+4913 (black lines). The blue lines show the spectrum of an F0 II star reddened by $A_{V}=1.7\pm 0.3.$ In the red part of the IRAS 21204+4913 spectrum, broad emission-like "humps" are present, which can be interpreted as TiO bands in emission. For comparison, the green line shows the inverted and scaled spectrum of the M-type star HD 211029; the same spectral regions are displayed at an enlarged scale in the insert.