The formation of stars proceeds in various phases which are dominated by the interaction of the object forming in the center (protostar, T Tauri star) with the material surrounding it (envelope, accretion disk). We examine these interactions through observations at different wavelengths: IR radiation from envelope and disk, optical emission from accreting material and central star, and X-rays from star and accretion shocks.
Different techniques are used to measure the mass accretion rates of T Tauri stars and young brown dwarfs. The most direct method is the determination of the excess emission at UV wavelengths with respect to a non-accreting star.
The bulk of the X-ray emission of cool stars is produced in the outermost part of the stellar atmosphere, the corona (see topic "Magnetic activity"). The latest generation of X-ray Observatories (Chandra and XMM-Newton) enabled measuring plasma densities leading to the discovery that accreting T Tauri stars show an additional "soft" X-ray component. This emission was attributed to accretion shocks. We examine the densities and temperatures of the hot plasma in T Tauri stars through X-ray spectroscopy with the aim of determining the origin of the X-ray emission (corona vs accretion shock).
Coronal X-ray emission is particularly strong in young stars. This allows X-ray measurements to be used for the identification of complete samples of T Tauri stars (see topic "Magnetic activity"). We perform combined X-ray, optical and IR surveys with the aim of assessing the complete population of pre-main sequence population in star forming regions.