We present high resolution X-ray spectra of the X-ray bright classical T Tauri star, TW Hydrae, covering the wavelength range of 1.5-25Å. The differential emission measure derived from fluxes of temperature-sensitive emission lines shows a plasma with a sharply peaked temperature distribution, peaking at log T = 6.5. Abundance anomalies are apparent, with iron very deficient relative to oxygen, while neon is enhanced relative to oxygen. Density-sensitive line ratios of Ne ix and O vii indicate densities near log n e = 13. A flare with rapid (∼ 1 ks) rise time was detected during our 48 ksec observation; however, based on analysis of the emission-line spectrum during quiescent and flaring states, the derived plasma parameters do not appear strongly time-dependent. The inferred plasma temperature distribution and densities are consistent with a model in which the bulk of the X-ray emission from TW Hya is generated via mass accretion from its circumstellar disk. Assuming accretion powers the X-ray emission, our results for log n e suggest an accretion rate of ∼ 10 −8 M ⊙ yr −1 .

The isolated, young, sunlike star TW Hya and four other young stars in its vicinity are strong x-ray sources. Their similar x-ray and optical properties indicate that the stars make up a physical association that is on the order of 20 million years old and that lies between about 40 and 60 parsecs (between about 130 and 200 light years) from Earth. TW Hya itself displays circumstellar CO, HCN, CN, and HCO+ emission. These molecules probably orbit the star in a solar-system-sized disk viewed more or less face-on, whereas the star is likely viewed pole-on. Being at least three times closer to Earth than any well-studied region of star formation, the TW Hya Association serves as a test-bed for the study of x-ray emission from young stars and the formation of planetary systems around sunlike stars.

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We report the detection of quiescent H 2 emission in the v ¼ 1 ! 0 S(1) line at 2.12183 lm in the circumstellar environment of two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution (R ' 60; 000) spectra, bringing to four, including TW Hya and the weak-lined T Tauri star DoAr 21, the number of T Tauri stars showing such emission. The equivalent widths of the H 2 emission line lie in the range 0.02-0.10 Å , and in each case the central velocity of the emission line is centered at the star's systemic velocity. The line widths range from 9 to 14 km s À1 , in agreement with those expected from gas in Keplerian orbits in circumstellar disks surrounding K-type stars at distances !10 AU from the sources. UV fluorescence and X-ray heating are likely candidate mechanisms responsible for producing the observed emission. We present mass estimates from the measured line fluxes and show that the estimated masses are consistent with those expected from the possible mechanisms responsible for stimulating the observed emission. The high temperatures and low densities required for significant emission in the v ¼ 1 ! 0 S(1) line suggests that we have detected reservoirs of hot H 2 gas located in the low-density upper atmospheres of circumstellar disks of these stars.

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