Multiwavelength diagnostics of accretion in an X-ray selected sample of CTTSs

Context. High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in classical T Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies. Aims. We aim to test the hypothesis that the high density soft X-ray emission originates from accretion by analysing, in a homogeneous manner, optical accretion indicators for an X-ray selected sample of CTTSs. Methods. We analyse optical spectra of the X-ray selected sample of CTTSs and calculate the accretion rates based on measuring the H alpha, H beta, H gamma, HeII 4686 angstrom, He I 5016 angstrom, He I 5876 angstrom, OI 6300 angstrom, and He I 6678 angstrom equivalent widths. In addition, we also calculate the accretion rates based on the full width at 10% maximum of the H alpha line. The different optical tracers of accretion are compared and discussed. The derived accretion rates are then compared to the accretion rates derived from the X-ray spectroscopy. Results. We find that, for each CTTS in our sample, the different optical tracers predict mass-accretion rates that agree within the errors, albeit with a spread of approximate to 1 order of magnitude. Typically, mass-accretion rates derived from H alpha and He I 5876 angstrom are larger than those derived from H beta, H gamma, and OI. In addition, the H alpha full width at 10%, whilst a good indicator of accretion, may not accurately measure the mass-accretion rate. When the optical mass-accretion rates are compared to the X-ray derived mass-accretion rates, we find that: a) the latter are always lower (but by varying amounts); b) the latter range within a factor of approximate to 2 around 2 x 10(-10) M-circle dot yr(-1), despite the former spanning a range of approximate to 3 orders of magnitude. We suggest that the systematic underestimate of the X-ray derived mass-accretion rates could depend on the density distribution inside the accretion streams, where the densest part of the stream is not visible in the X-ray band because of the absorption by the stellar atmosphere. We also suggest that a non-negligible optical depth of X-ray emission lines produced by post-shock accreting plasma may explain the almost constant mass-accretion rates derived in X-rays if the effect is larger in stars with higher optical mass-accretion rates.