Evidence for Accretion in the High - resolution X - ray Spectrum of the T Tauri Star System Hen 3 - 600

We present high-resolution X-ray spectra of the nearby, multiple T Tauri star (TTS) system Hen 3-600, obtained with the High Energy Transmission Grating Spectrograph (HETGS) aboard the Chandra X-ray Observatory (CXO). Both principle binary components of Hen 3-600 (A and B, separation 1.4) were detected in the zeroth-order CXO/HETGS X-ray image. Hen 3-600-A — the component with a large mid-infrared excess — is a factor ∼ 2− 3 fainter in X-rays than Hen 3-600-B, due to a large flare at Hen 3-600-B during our observation. The dispersed X-ray spectra of the two primary components overlap significantly, so spectral analysis was performed primarily on the first-order spectrum of the combined (A+B) system, with analysis of the individual dispersed spectra limited to regions where the contributions of A and B can be disentangled via cross-dispersion profile fitting. This analysis results in two lines of evidence indicating that the X-ray emission from Hen 3-600 A+B is derived, in part, from accretion processes: (1) The line ratios of He-like O vii in the spectrum of Hen 3600 A+B indicate that the characteristic density of its X-ray-emitting plasma is significantly larger than those of coronally active main sequence and pre-main sequence stars. (2) A significant component of low-temperature (2–3 MK) plasma is present in the Hen 3-600 A+B spectrum; this “soft excess” appears somewhat stronger in component A. These results for Hen 3-600 A+B are consistent with, though less pronounced than, results obtained from X-ray gratings spectroscopy of more rapidly accreting TTS systems. Indeed, all of the emission signatures of Hen 3-600 A + B that are potential diagnostics of accretion activity — from its high-resolution X-ray spectrum, through its UV excess and Hα emission line strengths, to its weak near-infrared excess — suggest that its components (and component A in particular) represent a transition phase between rapidly accreting, classical T Tauri stars and non-accreting, weak-lined T Tauri stars. Subject headings: Xrays: stars — stars: individual (Hen 3-600, TW Hya, HD 98800) — stars: pre-main sequence — stars: coronae — accretion, accretion disks 1