Accretion in pre-main-sequence binaries

Mass accretion at stellar surfaces occurs in pre-main-sequence binaries of all separations. The accretion rates are comparable to those found for single stars. This paper describes several recent observations which shed light on the accretion process in the binary environment. 1) Most classical T Tauri binaries include two classical T Tauri stars, suggesting that the lifetimes of their circumstellar disks are correlated. 2) The classical T Tauri binary DQ Tau shows evidence for enhanced accretion rates at periastron passage, as predicted for accretion streams from circumbinary disks. 3) Two infrared companions show evidence for shocked molecular hydrogen emission, indicative of continued infall onto these stars or associated disks. Timescale arguments indicate that for many binaries existing circumstellar material cannot supply the observed accretion rates for as long as the ages of the binaries. Presuming steady accretion rates, this requires replenishment. Infall from circumbinary envelopes or accretion streams from circumbinary disks have been suggested, although such circumbinary envelopes or disks have not yet been directly detected around most actively accreting binaries.