by Zhen Guo

Numerous eruptive variable YSOs, mostly Class I systems, were recently detected by the near-infared (NIR) Vista Variables in the Via Lactea (VVV) survey. By combing photometric and spectroscopic features, variable YSOs are categorised into different subgroups with different physical mechanisms. In this paper, we present an exploratory multi-epoch NIR spectroscopic follow-up study of 14 eruptive YSO systems. We sampled spectroscopic variabilities over 1-day and 1 to 2-year intervals and analyse them in combination with VVV $K_s$-band lightcurves. Within the 1-day timescale, most sources have stable emission lines, except two objects have significant variations in H$_2$ (2.12 $\mu$m) and another two in Br~$\gamma$. Over year-long timescales, H$_2$ fluxes are typically stable whilst up to 1.8 mag variations in NIR flux occur, causing apparent changes in equivalent width. This indicates that the observed emission from the molecular outflow arises further from the protostar and is unaffected by relatively large changes in accretion rate on timescales of years .
CO overtone absorption features are observed on 3 objects, and all of them have deeper absorption during the photometric brighter stage. Two objects show signs of on/off magnetospheric accretion based on the existence of Br~$\gamma$ emission. In addition, A rapid changing of H$_2$ outflow is detected on one object.