The eruptive young star IRAS 21204+4913

TL;DR

This study documents a significant brightening event in the young star IRAS 21204+4913, revealing increased accretion activity, dust scattering, and unusual spectral features, indicating complex circumstellar processes.

Contribution

It provides detailed photometric, polarimetric, and spectroscopic analysis of IRAS 21204+4913's outburst, highlighting its unique spectral and circumstellar characteristics.

Findings

Star's brightness increased by approximately 5 magnitudes since October 2025.

Polarization degree increased to about 16% in the I-band, indicating dust scattering.

Outburst likely caused by increased accretion rate of disk material onto the star.

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 $0.36 - 0.75 \mu$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$\alpha$ 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.