Star formation

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.

Mass accretion in young stars and brown dwarfs

Different techniques can be 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. Another approach is to measure the flux and width of spectral lines (see Figure). The line emission comprises contributions from both accretion and outflows which are not easily disentangled. Mass outflows lead to a broadening of the emission lines that are produced by material moving at different (outflow) speeds. We use the broad-band spectrograph X-Shooter at the ESO/VLT on Mount Paranal in Chile to measure both the UV excess and the emission line fluxes.

Fig.: Emission lines in the optical spectrum of the young accreting star Par-Lup3-4 in the Lupus star forming regions. left - Halpha line, right - a line of the so-called Calcium infrared-triplet (IRT). Spectra taken in three subsequent years show that the two lines vary in a different way. This points at a different formation of Halpha and Calcium IRT emission within the star-disk system. (A.Klutsch)

X-ray emission of accretion shocks

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 of T Tauri stars

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.