Magnetospheric accretion in substellar objects

The shock resulting from magnetospheric accretion in low mass T Tauri stars produces a characteristic veiling continuum in the ultraviolet and blue spectral regions. Variability across the optical bands is expected due to rotational modulation of the hot spots formed at the base of the accretion column and from the non-periodic variability due to the inherently non- equilibrium nature of the accretion process. In addition, characteristic spectral features including signs of low surface gravity and strong permitted emission lines that form in the ballistic gas flows are key identifiers of accreting Classical T Tauris. Finally, a contribution to the near-IR flux is seen from thermal emission arising from the inner regions of the dusty circumstellar disk.

We use these observational signatures obtained from multi-epoch imaging of the Orion and South Taurus equatorial regions by the Sloan Digital Sky Survey to study the variation of these accretion signatures across the substellar boundary. We find that, with the exception of the near-IR excess, these signatures are present in both low mass stars and young brown dwarfs. The absence of detectable circumstellar disk emission at the lowest masses is attributed to a decrease in the contrast between the disk and the stellar photosphere. The persistence of these signatures across a range of inferred masses suggests a common underlying physics in the magnetospheric accretion process, and thus a common formation mechanism and environment.

We find that the variability as measured by the rms variation in magnitudes decreases for lower mass objects. Examination of the variability across the optical spectrum finds trends that are consistent with earlier studies of low mass T Tauris but indicates that other factors besides changes in the veiling continuum may contribute to the brightness fluctuations. A proposed candidate is the deutrium-burning pulsational instability that may occur in very low mass stars and brown dwarfs at very young ages.