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

¹ Sternberg Astronomical Institute, Moscow V-234, 119899 Russia e-mail: lamzin@sai.msu.ru
² Uppsala Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden e-mail: stempels@astro.uu.se

Corresponding author: N. Piskunov, piskunov@astro.uu.se

Received: 26 October 2000
Accepted: 24 January 2001

Abstract

Specific intensities of the strongest Fe x-Fe xiv coronal lines were calculated in the framework of our accretion shock model (Lamzin [CITE]). 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 Åand [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 Åand [Fe xii] 1349 Å) can possibly be detected. In agreement with our calculations we could not detect the [Fe x] 6376 Åand [Fe xiv] 5304 Ålines 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 Åline in a HST/GHRS spectrum of RY Tau which suggests that the accretion rate of the star in its quiescent state is ≈ /yr. As a byproduct of the study we found that for RY Tau the interstellar extinction coefficient A_V is closer to than to . For DF Tau, the observed upper limit for the flux of the [Fe xii] 1349 Åline in HST/GHRS spectra is in agreement with an accretion rate of /yr as found by Lamzin et al. ([CITE]). As a critical test of our calculations we predict that the [Fe xi] 1467 Åline in the spectrum of RU Lupi should be relatively strong: we expect the flux to be near 10^-15 erg/s/cm².