X-rays from jet-driving protostars and T Tauri stars

Abstract
Aims.We study jet-driving protostars and T Tau stars to characterize their X-ray emission. We seek soft spectral components that may be due to shock emission, and search for soft, shock-induced emission displaced from the stellar position.
Methods.We study two stellar samples, the first consisting of lightly absorbed T Tau stars with strong jets, the other containing protostars with disks seen nearly edge-on. The former sample was observed in the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST), while Chandra archival data provided observations of the latter.
Results.We confirm the previously identified peculiar spectrum of DG Tau A and find similar phenomenology in GV Tau and DP Tau, suggesting a new class of X-ray spectra. These consist of a lightly absorbed, very soft component and a strongly absorbed, very hard component. The latter is flaring while little variability is detected in the former. The absorption of the harder component is about an order of magnitude higher than expected from the optical extinction assuming a standard gas-to-dust mass ratio. For the absorbed protostars, only the hard, stellar X-ray component is found.
Conclusions.The flaring hard component represents active coronal emission. Its strong absorption is attributed to mass inflow from the accretion disk. The optical extinction is small because the dust has sublimated at larger distances. The little absorbed soft component cannot originate from the same location. Because the stars drive strong jets, we propose that the X-rays are generated in shocks in the jets. We find that for the three peculiar X-ray sources, the luminosity of the soft component roughly scales with the equivalent width of the [O I]  6300 line formed in the jets, and with the mass outflow rate. In the more strongly obscured protostars, the soft component is entirely absorbed, and only the hard, coronal component penetrates the envelope or the near-edge-on disk.