The Unusual X-ray Spectrum of FU Orionis

FU Orionis objects (FUors) have undergone strong optical outbursts and are thought to be young low-mass stars accreting at high rates of up to Ṁacc ∼ 10 −4 M⊙ yr . FUors have been extensively studied at optical and infrared wavelengths, but little is known about their X-ray properties. We have thus initiated a program aimed at searching for and characterizing their X-ray emission. First results are presented here for the prototype star FU Orionis based on observations obtained with XMM-Newton. Its CCD X-ray spectrum is unusual compared to those of accreting classical T Tauri stars (cTTS). The cool and hot plasma components typically detected in cTTS are present but are seen through different absorption column densities. The absorption of the cool component is consistent with AV ≈ 2.4 mag anticipated from optical studies but the absorption of the hot component is at least ten times larger. The origin of the excess absorption is uncertain but cold accreting gas or a strong near-neutral wind are likely candidates. The hot plasma component accounts for most of the observed X-ray flux and thermal models give very high temperatures kT ≥ 5 keV. The most prominent feature in the X-ray spectrum is an exceptionally strong Fe K emission line at 6.67 keV and weak emission from fluorescent Fe I at ≈6.4 keV may also be present. The high plasma temperature clearly demonstrates that the emission is dominated by magnetic processes. We discuss possible origins of the unusual X-ray spectrum in the context of a complex physical environment that likely includes disk accretion, a strong wind, magnetic activity, and close binarity. Subject headings: stars: individual (FU Orionis) — stars: pre-main sequence — X-rays: stars