Multi-wavelength diagnostics of accretion in an X-ray selected sample of CTTSs

Abstract

The majority of CTTSs observed to date with high spectral resolution X-ray spectroscopy reveal soft X-ray emission (E<0.7 KeV) which originates from cool (1-5 MK), high density (n ˜ 10^{11}-10^{13} cm^{-3}) plasma. This is currently interpreted to be due to mass accretion. Supporting this interpretation is the fact that this plasma component is too dense to have a coronal origin, and it has never been observed in non-accreting stars. Synthesized X-ray spectra from detailed hydrodynamical modelling of the interaction between the accretion flow and the stellar chromosphere also confirm this interpretation. However, the mass accretion rates derived from X-ray data are consistently underestimated when compared to mass accretion rates derived from UV/optical data. We test the hypothesis that this soft X-ray emission originates from accretion by analysing optical, NIR and X-ray data for an X-ray selected sample of CTTSs. We derive mass accretion rates for the sample based on Hα, He I, O I and Ca II emission lines, along with the X-ray data. We draw comparisons between these mass accretion rates to understand the underestimation of the X-ray derived mass accretion rates. We discuss the possibilities of a) the X-ray emission being partially absorbed, b) the optical/NIR emission arising from different parts of the accretion stream and c) the uncertainties involved in the estimation of the mass accretion rates from different spectroscopic diagnostics.