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