Spectroscopic follow-up of Gaia alerted Young Stellar Object variables: the Large Binocular Telescope view

TL;DR

This study presents the first statistical view of Gaia-alerted YSOs through ten years of optical/near-IR spectroscopy with the Large Binocular Telescope, targeting 16 sources identified by Gaia as photometric variables. By combining multi-epoch photometry (Gaia, Pan-STARRS, ZTF, NEOWISE, etc.), distance estimates, SED fitting, and LBT MODS/LUCI spectra, the authors derive extinction, stellar, and accretion parameters across brightness phases. They find that all sources are accreting YSOs, with two light-curve categories: distinct bursts above a baseline and persistent variability; several sources reach EXor-like accretion levels during bursts, while some Class I objects exhibit unusually high accretion luminosities. The results support episodic accretion as a common mechanism across low- and intermediate-mass YSOs, and highlight the value of coordinated spectroscopic monitoring for interpreting Gaia-driven variability in the era of LSST.

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 ten-year light curves and photometric/spectroscopic features provide 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 (Lacc) and mass accretion rate (Macc) at different brightness phases. Only two sources showed variability primarily due to extinction. During quiescence, our sources exhibit Lacc and Macc values typical of T Tauri and Herbig Ae/Be (HAEBE) sources, supporting the hypothesis that any YSO may undergo episodic accretion. In bursts, the Lacc and Macc 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 Lacc value about ten times higher than typical EXor bursts of the same mass. In the other cases, Lacc and Macc align with variability seen in T Tauri and HAEBE sources.

Figures (32)

• Figure 1: Columns 1-3: Clusters from Hunt & Reffert (2024) close to the target YSOs. Different clusters are depicted with different colors and symbols, as indicated on top. Col. 1: Spatial distribution of cluster members. The target YSO is shown as a blue circle. Arrows indicate proper motions. Col. 2: Histogram of the distance distribution. The solid and dashed vertical line represents the distance and its uncertainties of the target YSO from Bailer-Jones et al. (2021). Col. 3: Proper motion distribution with uncertainties. The target YSO is shown as a blue circle. Column 4 : Extinction distribution along the line of sight toward the target. The solid vertical line shows the photogeometric distance from Bailer-Jones et al. (2021). Dashed vertical lines mark the distance of the largest extinction increase. See Section \ref{['sec:sec2.2']}. for details.
• Figure 2: Continuation of Figure \ref{['fig:fig1']}.
• Figure 3: Light curves of Gaia21bkw, Gaia22efa, Gaia22bvi, and Gaia22ehn. Symbols and colors identify data points from different surveys as reported in the legend. Black ticks indicate the date of January 1st of the year displayed on top. Vertical dashed lines mark the dates of the LBT observations (blue: MODS, red: LUCI, black: MODS/LUCI spectra taken in close dates). The green dot-dashed line marks the date of the Gaia alert. The red ticks on top indicate the dates of optical bursts (see also Table \ref{['tab:tab7']}).
• Figure 4: Light curves of Gaia22dbd, Gaia21arv, Gaia23bri, and Gaia21ebu. Symbols and colors are the same as in Figure \ref{['fig:fig3']}.
• Figure 5: Light curves of Gaia21aul, Gaia23bab, Gaia23dhi, and Gaia24afw. Symbols and colors are the same as in Figure \ref{['fig:fig3']}.