Photometric and spectroscopic study of the burst-like brightening of two Gaia-alerted young stellar objects

TL;DR

This study investigates two young stellar objects that experienced significant brightness increases, analyzing their photometric and spectroscopic data to understand their accretion behavior and propose a potential new class of variable young stars.

Contribution

The paper presents detailed photometric and spectroscopic analysis of two young stars with unusual brightness outbursts, suggesting a new class of young stellar objects with intermediate outburst characteristics.

Findings

Both stars showed accretion rates similar to classical T Tauri stars.

Brightness outbursts had smaller amplitudes and different time-scales than typical eruptive stars.

The accretion rate of Gaia20fgx decreased significantly over six months.

Abstract

Young stars show variability on different time-scales from hours to decades, with a range of amplitudes. We studied two young stars, which triggered the Gaia Science Alerts system due to brightenings on a time-scale of a year. Gaia20bwa brightened by about half a magnitude, whereas Gaia20fgx brightened by about two and half magnitudes. We analyzed the Gaia light curves, additional photometry, and spectra taken with the Telescopio Nazionale Galileo and the Gran Telescopio Canarias. Several emission lines were detected toward Gaia20bwa, including hydrogen lines from H$\alpha$ to H$\delta$, Pa$\beta$, Br$\gamma$, and lines of Ca II, O I, and Na I. The H$\alpha$ and Br$\gamma$ lines were detected toward Gaia20fgx in emission in its bright state, with additional CO lines in absorption, and the Pa$\beta$ line with an inverse P Cygni profile during its fading. Based on the Br$\gamma$ lines the accretion rate was $(2.4-3.1)\times10^{-8}$ $M_\odot$ yr$^{-1}$ for Gaia20bwa and $(4.5-6.6)\times10^{-8}$ $M_\odot$ yr$^{-1}$ for Gaia20fgx during their bright state. The accretion rate of Gaia20fgx dropped by almost a factor of 10 on a time-scale of half a year. The accretion parameters of both stars were found to be similar to those of classical T Tauri stars, lower than those of young eruptive stars. However, the amplitude and time-scale of these brightenings place these stars to a region of the parameter space, which is rarely populated by young stars. This suggests a new class of young stars, which produce outbursts on a time-scale similar to young eruptive stars, but with smaller amplitudes.