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Session 92 - Very Young Stars, T-Tauri Stars, H-H Objects.
Oral session, Friday, January 09
At a distance of \sim60 pc and age of \sim20 Myr, TW Hya is the nearest and perhaps oldest known classical T Tauri star (Kastner et al.\ 1997, Science, 277, 67). Due to their proximity, TW Hya and the four other known members of the TW Hya Association constitute the brightest association of T Tauri stars in terms of X-ray flux received at the Earth. Their large fluxes make the TW Hya Association stars excellent subjects to investigate the origin of X-ray emission from recently formed, Sun-like stars, and to study the effect of pre-main sequence X-ray irradiation on the physics and chemistry of protoplanetary disks. TW Hya itself is of particular interest, since it displays numerous molecular emission lines that likely originate in a circumstellar disk of roughly solar system dimensions (Kastner et al.\ 1997).
We have observed TW Hya with ASCA, and we have analyzed archival ROSAT observations of TW Hya. These data likely represent the highest signal-to-noise ratio X-ray spectra yet obtained for any T Tauri star, classical or weak-lined. Fits of Raymond-Smith (R-S) plasma models to the ROSAT PSPC spectrum indicate a characteristic temperature T_x \sim 6\times10^6 K and absorbing column N_H \sim 10^20.5 cm^-2. Adopting these values, the pointed PSPC count rate of TW Hya (0.31 s^-1) suggests an intrinsic X-ray luminosity L_x = 1.1\times10^30 ergs s^-1. Preliminary analysis of ASCA data yields a SIS (CCD camera) count rate of 0.12 s^-1, consistent with the R-S model and source flux that best fits the PSPC spectrum. We will present results of detailed modeling of the ASCA data which, unlike the ROSAT data, specify the hard (2--10 keV) component of the spectrum of TW Hya and thereby should constrain the X-ray flux that is incident on molecular gas at large optical depths within the TW Hya circumstellar disk.
Program listing for Friday