Formation of Fe X-Fe XIV coronal lines in the accretion shock of T Tauri stars

Abstract

Specific intensities of the strongest Fe x-Fe xiV\ coronal lines were calculated in the framework of our accretion shock model (Lamzin \cite{Lamzin1998}). These lines are formed in a region immediately behind the front of the accretion shock, therefore, the gas velocity in the line formation region is close to 1/4 of the infall velocity. It appears that iron coronal lines in the optical band (e.g. [Fe x] 6376 Å and [Fe xiV] 5304 Å) are too weak to be observed in spectra of T Tauri stars, but the UV lines (e.g. [Fe xi] 1467 Å and [Fe xiI] 1349 Å) can possibly be detected. In agreement with our calculations we could not detect the [Fe x] 6376 Å and [Fe xiV] 5304 Å lines in low noise UVES spectra of RU Lup where the accretion luminosity is ten times larger than the bolometric luminosity of the underlying star. At the same time we detected the [Fe xi] 1467 Å line in a HST/GHRS spectrum of RY Tau which suggests that the accretion rate of the star in its quiescent state is ~2 10^-9 M_sun/yr. As a byproduct of the study we found that for RY Tau the interstellar extinction coefficient A_V is closer to 0.5^{m} than to 1.0^{m}. For DF Tau, the observed upper limit for the flux of the [Fe xiI] 1349 Å line in HST/GHRS spectra is in agreement with an accretion rate of 2 10^-9 M_sun/yr as found by Lamzin et al. (\cite{Lamzin2000}). As a critical test of our calculations we predict that the [Fe xi] 1467 Å line in the spectrum of RU Lupi should be relatively strong: we expect the flux to be near 10^-15 erg/s/cm².