Spectroscopic confirmation of high-amplitude eruptive YSOs and dipping giants from the VVV survey

During the pre-main-sequence (pre-MS) evolution stage of a star, significant amounts of stellar mass are accreted during episodic accretion events, such as multidecade FUor-type outbursts. Here, we present a near-infrared spectroscopic follow-up study of 33 high-amplitude (most with ΔKs > 4 mag) variable sources discovered by the Vista Variables in the Via Lactea (VVV) survey. Based on the spectral features, 25 sources are classified as eruptive young stellar objects (YSOs), including 15 newly identified FUors, six with long-lasting, but EXor-like bursts of magnetospheric accretion and four displaying outflow-dominated spectra. By examining the photometric behaviours of eruptive YSOs, we found most FUor-type outbursts have higher amplitudes (ΔKs and ΔW2), faster eruptive time-scales and bluer infrared colours than the other outburst types. In addition, we identified seven post-MS variables apparently associated with deep dipping events and an eruptive star with deep aluminium monoxide absorption bands resembling those seen in the V838 Mon stellar merger.