The 2023 outburst of the Gaia alerted EXor Gaia23bab

TL;DR

This paper investigates the 2023 outburst of Gaia23bab, an EXor young star, using multi-wavelength observations to analyze its accretion properties, line emissions, and physical conditions, confirming its classification as a typical EXor.

Contribution

The study provides detailed spectroscopic analysis of Gaia23bab's outburst, measuring accretion rates and physical conditions, and confirms its classification as a prototypical EXor based on emission line diagnostics.

Findings

Accretion rate of approximately 2.0 x 10^-7 M_sun/yr during outburst

Excitation temperatures of 5000-10000 K and electron densities of 10^9-10^10 cm^-3 from hydrogen lines

Gaia23bab's physical properties align with typical EXor characteristics

Abstract

Episodic accretion is a fundamental process in the build-up of the stellar mass. EX Lupi-type eruptive young stars (EXors) represent one of the main types of episodic accretion. We study the recently discovered EXor Gaia23bab during its 2023 outburst. We obtained optical and near-infrared photometry and spectroscopy to probe the variation of the physical properties of Gaia23bab during its recent outburst. We also collected archival photometry to study a previous outburst of the star. We used several accretion tracers, including the Ca II triplet, He I, and various hydrogen lines from the Paschen and Brackett series, to measure the accretion rate during the outburst. The accretion rate is consistent with $\sim 2.0 \times 10^{-7} M_\odot$ $\rm{yr}^{-1}$. Comparing the line fluxes of the hydrogen Brackett series to predictions of Case B theory suggests excitation temperatures of 5000 - 10000 K and electron densities of $10^9$-$10^{10}$ cm$^{-3}$. Comparison to the predictions of a model for T Tauri stars revealed that the fluxes of the Balmer series are consistent with temperatures of 5000 - 12500 K and a hydrogen density of $10^8$ cm$^{-3}$, while the fluxes of the Paschen series are consistent with temperatures in the range between 10000 and 12500 K and a hydrogen density of $10^{11}$ cm$^{-3}$. The derived temperatures and densities confirm that Gaia23bab is a prototypical EXor, not only due to its accretion rate, but also based on the best fit temperatures and densities revealed by the detected hydrogen lines.