(abridged:) We study X-rays from jet-driving protostars and T Tau stars. We
seek soft spectral components that may be due to shock emission, and
shock-induced emission displaced from the stellar position. Two stellar samples
are used, 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. 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. 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 weakly 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 [OI] 6300A 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.Comment: 17 pages, 27 figures, A&A style. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST). Version with higher resolution figures
available at http://www.issibern.ch/teams/Taurus/papers.html or
http://www.astro.phys.ethz.ch/papers/guedel/guedel_p_nf.htm