Context. Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, which indicates the existence of two emitting regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The broad emission lines of H I, He II, Ca II, Fe II, and other species are thought to form in a larger volume of gas.
Aims. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe I and other metals represent a standard chromosphere of a late-type star, or are induced by mass accretion.
Methods. We carried out high-resolution spectroscopy of selected cTTS with a special focus on DR Tauri and followed variations of chromospheric features, such as narrow Fe I emission lines, and accretion signatures such as the veiling continuum and the He II line emission.
Results. We found that the amount of veiling in DR Tau varies from practically nothing to factors more than 10 times the stellar continuum intensity, and that the veiling is caused by both a non-photospheric continuum and chromospheric line emission filling in the photospheric absorption lines. The latter causes differential veiling because stronger lines are more veiled. We developed methods to separate the two sources of veiling. Several veiled T Tauri stars show a common effect: the radial velocities of photospheric and chromospheric lines vary in anti-phase. This is caused by an area with enhanced chromospheric emission, which is offset from the pole of rotation and is associated with the hot spot formed at the footprint of the magnetic funnel of mass accretion.
Conclusions. The enhanced chromospheric emission in cTTS is linked not only to solar-like magnetic activity, but is powered to a greater extent by the accreting gas. We suggest that the area of enhanced chromospheric emission is induced by mass accretion, which modifies the local structure of stellar atmosphere in an area that is more extended than the hot accretion spot. The narrow emission lines from this extended area are responsible for the extra component in the veiling through line-filling of photospheric absorption lines.
2011. Vol. 535, A6- p.
stars: variables: T Tauri, Herbig Ae/Be, stars: individual: DR Tau, stars: individual: S CrA, stars: individual: RW Aur, stars: individual: RU Lup, stars: individual: DI Cep