Variability in young very low mass stars: Two surprises from spectrophotometric monitoring

TL;DR

This study investigates the causes of brightness variability in young very low mass stars, revealing that different mechanisms like spots and circumstellar extinction drive variability even after disk dispersal.

Contribution

It provides simultaneous photometric and spectroscopic data showing variability mechanisms in young low-mass stars differ from those in more massive T Tauri stars.

Findings

Brightness variations are not due to veiling.

Star spots cause continuum variations in some stars.

Circumstellar extinction explains eclipse-like lightcurves in others.

Abstract

We present simultaneous photometric and spectroscopic observations of seven young and highly variable M dwarfs in star forming regions in Orion, conducted in 4 observing nights with FORS2 at ESO/VLT. All seven targets show significant photometric variability in the I-band, with amplitudes between 0.1-0.8 mag, The spectra, however, remain remarkably constant, with spectral type changes less than 0.5 subtypes. Thus, the brightness changes are not caused by veiling that 'fills in' absorption features. Three objects in the $\sigma$ Ori cluster (age $\sim$3 Myr) exhibit strong H$\alpha$ emission and H$\alpha$ variability, in addition to the continuum variations. Their behaviour is mostly consistent with the presence of spots with temperature of $\sim300$ K above the photosphere and filling factors between 0.2-0.4, in contrast to typical hot spots observed in more massive stars. The remaining targets near $\epsilon$ Ori, likely to be older, show eclipse-like lightcurves, no significant H$\alpha$ activity and are better represented by variable extinction due to circumstellar material. Interestingly, two of them show no evidence of infrared excess emission. Our study shows that high-amplitude variability in young very low mass stars can be caused by different phenomena than in more massive T Tauri stars and can persist when the disk has disappeared and accretion has ceased.