Short and long term near-infrared spectroscopic variability of eruptive protostars from VVV

TL;DR

This study investigates near-infrared spectroscopic variability in 14 eruptive young stellar objects over short and long timescales, revealing insights into accretion processes, circumstellar disc behavior, and outflow activity.

Contribution

It provides the first detailed spectroscopic variability analysis of eruptive YSOs over multiple timescales, confirming FUor-like behavior and the slow recovery of stellar magnetic fields.

Findings

Deeper CO absorption correlates with brighter states in FUor-like objects.

H$_2$ emission shows modest variability over years, unaffected by large accretion changes.

Evidence of on/off magnetospheric accretion traced by Br$\gamma$ emission.

Abstract

Numerous eruptive variable young stellar objects (YSOs), mostly Class I systems, were recently detected by the near-infrared Vista Variables in the Via Lactea (VVV) survey. We present an exploratory near-infrared spectroscopic variability study of 14 eruptive YSOs. The variations were sampled over 1-day and 1 to 2-year intervals and analysed in combination with VVV light curves. CO overtone absorption features are observed on 3 objects with FUor-like spectra: all show deeper absorption when they are brighter. This implies stronger emission from the circumstellar disc with a steeper vertical temperature gradient when the accretion rate is higher. This confirms the nature of fast VVV FUor-like events, in line with the accepted picture for classical FUors. The absence of Br$\gamma$ emission in a FUor-like object declining to pre-outburst brightness suggests that reconstruction of the stellar magnetic field is a slow process. Within the 1-day timescale, 60% of H$_2$-emitting YSOs show significant but modest variation, and 2/6 sources have large variations in Br$\gamma$. Over year-long timescales, H$_2$ flux variations remain modest despite up to 1.8 mag variation in $K_s$. This indicates that emission from the molecular outflow usually arises further from the protostar and is unaffected by relatively large changes in accretion rate on year-long timescales. Two objects show signs of on/off magnetospheric accretion traced by Br$\gamma$ emission. In addition, a 60% inter-night brightening of the H$_2$ outflow is detected in one YSO.