Extreme variables in VVV: the complete sample with DeltaKs>4

by Philip Lucas

The VVV and VVVX surveys are now in their 10th year of observations.
I present a complete sample of 195 variables with amplitudes > 4 mag,
drawn from the initial version of the 8 year VIRAC2 light curve and astrometry database of Leigh Smith. This uses PSF photometry from Javier Alonso Garcia's VVV-optimised version of the DoPhot code. The sample is dominated by YSOs and classical novae, with slightly smaller numbers of microlenses and LPVs. There are also a few unusual events that are hard to classify. While the initial sample of variable YSOs in Contreras Pena et al.2017 had typical amplitudes of 1 to 3 mag, we now have a large sample with higher amplitudes and longer duration light curves that enable clearer analysis. Most of the YSOs appear to be eruptive variables powered by episodic accretion. Classical FU Orionis-type and EX Lupi-type light curves are a small minority, Instead the outbursts typically have a rise time of 2 or 3 years and a duration in excess of 5 years. This is somewhat longer than was initially found in the lower amplitude sample of Contreras Pena et al. The diversity of the light curves suggests that multiple processes are responsible for triggering episodic accretion. I also discuss some of the more unusual transient light curves, 9 of which occur in star forming regions. I consider the possibility that some of these are protostellar collisions/mergers, similar to the BN/KL explosion but involving low mass YSOs. However, at present a classical nova interpretation appears more likely.