Spectroscopic Follow-up of Gaia Alerted Young Stellar Object Variables: The Large Binocular Telescope View

Abstract We analyzed optical/near-IR Large Binocular Telescope spectra of 16 sources alerted by Gaia between 2021 and 2024 due to significant photometric variability. Half of the spectra were taken during quiescence and the rest during a burst or at intermediate brightness. Our analysis of their 10 yr light curves and photometric/spectroscopic features provides evidence that all 16 sources are accreting young stellar objects (YSOs). One object, Gaia23bab, is a known EXor source. Other light curves either have peaks over a stable baseline or significant variability throughout the entire observation period, suggesting multiple contributing processes. All spectra exhibit emission lines from accretion columns, and over half of them show atomic forbidden lines as signatures of outflowing gas. We determined stellar parameters, accretion luminosity ( L acc ), and mass accretion rate ( M ̇ acc ) at different brightness phases. Only two sources showed variability primarily due to extinction. During quiescence, our sources exhibit L acc and M ̇ acc ​​​​​​ values typical of T Tauri and Herbig Ae/Be (HAeBe) sources, supporting the hypothesis that any YSO may undergo episodic accretion. In bursts, the L acc and M ̇ acc ​​​​​​ of sources with photometric variations exceeding 2 mag follow a shallower relation with stellar luminosity and mass, typical of known EXor sources. This group includes one Class I, one flat-spectrum, and two Class II sources. Notably, the other Class I source, Gaia24beh, shows an L acc value about 10 times higher than typical EXor bursts of the same mass. In the other cases, L acc and M ̇ acc ​​​​​​ align with variability seen in T Tauri and HAeBe sources.