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V. Swisher, E. L. N. Jensen, D. H. Cohen (Swarthmore College), M. Gagné (West Chester University)
High-resolution grating spectroscopy of young stars can reveal physical conditions associated with these stars' x-ray emission, providing information about stellar activity, dynamo mechanisms, and the effects of x-rays on disks during the planet formation epoch. Unfortunately, most very young stars are too faint in x-rays to yield high-quality grating spectra. We present a high-resolution Chandra grating spectrum of the low-mass pre-main-sequence star DoAr 21, one of the brightest x-ray sources in the \rho Oph region and, at an age of 1--3 Myr, one of the youngest stars for which a high-resolution x-ray spectrum has been obtained.
The x-ray spectrum of DoAr 21 shows substantial flaring and is very hard, with strong continuum and relatively weak lines. The entire observation is well-fit with a two-temperature thermal model with components at 1.5 keV (T \approx 17 MK) and 5.5 keV (T \approx 64 MK). During a large flare, the hot component increased in temperature from 2.8 keV to 5.8 keV in less than two hours.
The strongest line in the spectrum (Si XIV 6.18 Å) is marginally resolved, with a FWHM of 450 ± 120 km/s. The other strong lines in the spectrum are consistent with having similarly broad widths, but given the noise level, zero broadening cannot be ruled out.
New high-resolution optical spectra of DoAr 21 show filled-in H\alpha with broad emission wings, and a transient, weak H\alpha emission component with a FWHM of 300 km/s, indicating continued low-level accretion onto DoAr 21. Combined with its weak mid-infrared excess, this suggests that the star is in the process of losing its circumstellar disk. Interestingly, the density-sensitive forbidden-to-intercombination line ratio in the x-ray spectrum shows a value intermediate between the high-density limit seen in the classical T Tauri star TW Hya and the low-density limit seen in the diskless T Tauri star HD 98800 A.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.