Abstract
ABSTRACT The decade-long Vista Variables in the Via Lactea (VVV) survey has detected numerous highly variable young stellar objects (YSOs). We present a study of 61 highly variable VVV YSOs (ΔKs = 1–5 mag), combining near-infrared spectra from Magellan and ESO Very Large Telescope with VVV and NEOWISE light curves to investigate physical mechanisms behind eruptive events. Most sources are spectroscopically confirmed as eruptive variables (typically Class I YSOs) but variable extinction is also seen. Among them, magnetically controlled accretion, identified by H i recombination emission (usually accompanied by CO emission), is observed in 46 YSOs. Boundary layer accretion, associated with FU Ori-like outbursts identified by CO overtone and H2O absorption, is observed only in longer duration events (≥5 yr total duration). However, even in long duration events, the magnetically controlled accretion mode predominates, with amplitudes similar to the boundary layer mode. Shorter (100–700 d) eruptive events usually have lower amplitudes and these events are generally either periodic accretors or multiple time-scale events, wherein large photometric changes occur on time-scales of weeks and years. We find that the ratio of amplitudes in Ks and W2 can distinguish between variable accretion and variable extinction. Several YSOs are periodic or quasi-periodic variables. We identify examples of periodic accretors and extinction-driven periodicity among them (with periods up to 5 yr) though more data are needed to classify some cases. The data suggest that dynamic interactions with a companion may control the accretion rate in a substantial proportion of eruptive systems, although star–disc interactions should also be considered.
Highlights
Pre-main-sequence (PMS) stellar evolution is shaped by the mass accretion process
The spectroscopic features of young stellar objects (YSOs) are shaped by mass accretion process (Hartmann & Kenyon 1996; Hartmann et al 2016)
Magnetospheric accretion is widely observed on disc-bearing YSOs
Summary
Pre-main-sequence (PMS) stellar evolution is shaped by the mass accretion process (see review by Hartmann et al 2016). EXors have amplitudes up to ΔV = 5 mag (Lorenzetti et al 2009) on timescales of a few hundred days, and some of them have repeated outbursts (Herbig 1989, 2008; Audard et al 2010). Most FUor outbursts have higher luminosity than EXors with high mass accretion rate (10−5 – 10−4 M yr−1). The mass accretion process of FUors is not controlled by the stellar magnetic field. As a consequence, their near infrared spectra are dominated by broad H2O and CO absorption bands; Brγ emission is not seen and the Paschen lines are often detected in absorption (Connelley & Reipurth 2018). As the prototype of FUors, an extended self-luminous disc is found around FU Ori, as a consequence of inefficient radiative transfer throughout the geometrically thick disc (Zhu et al 2007, 2009a; Hartmann et al 2011)
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