X-ray Jets

Bi-polar jets from young stellar objects drive internal shocks and bow shocks against the interstellar medium. If the shock speed is high enough (several 100 km/s) then the shocked gas may reach temperatures of T > 1 MK. Why is this important? Extended X-ray sources are interesting irradiation sources for the stellar environment in particular in protostellar phases (where the emission remains undetected owing to strong photoelectric absorption); such radiation fields may be important drivers of chemical reactions far from the star, but they also efficiently ionize their immediate environment, making the gas accessible to magnetic fields.

A few Herbig-Haro objects have shown faint X-ray emission (e.g., Pravdo et al. 2001). In contrast, we have for the first time mapped an entire bi-polar jet from a flat-spectrum (transition Class I-II) source, DG Tau. We detect both the forward and the counter jet (Fig. below), each showing a temperature of a few million degrees. Estimating the cooling time (from the lack of X-rays outside 5″) leads to hot-gas volumes (volume filling factor of 1E-6 to 1E-5) and electron densities (~1E4 cm-3) that suggest a strong pressure non-equilibrium, making the hot gas expand the jet laterally. The DG Tau jet indeed shows an unusually large opening angle (10-20 deg).

X-ray spectral phenomenology first discovered in our XEST project (for DG Tau and other jet-driving TTS) suggest that X-ray jet is present even closer to the star (unresolved by current instruments), with a total X-ray luminosity rivaling that of modest T Tauri stars (of order 1E29 erg/s). The spectra show two components with entirely different absorption column densities, the strongly absorbed hot component originating from the stellar magnetosphere and the weakly absorbed component from the jet base (Güdel et al. 2007bc).