High-Resolution X-ray Spectroscopy of the Accreting Weak-Line T Tauri Star DoAr 21

1. High-Resolution X-ray Spectroscopy of the Accreting Weak-Line T Tauri Star DoAr 21 Victoria Swisher, Eric L. N. Jensen, David H. Cohen (Swarthmore College), and Marc Gagné (West Chester University) X-rays from Pre-Main-Sequence (PMS) stars PMS stars have stronger and harder x-ray emission than cool main-sequence stars. They also exhibit significant x-ray time variability. Is PMS x-ray activity similar in origin to solar-type x-ray activity (alpha-omega dynamo driven)? If so, why is it stronger? Is it connected to accretion? New Chandra grating (HETGS) spectroscopy 94 ks observation in May 2003. The spectrum (at right) is very hard – dominated by bremsstrahlung continuum, but emission lines from some highly ionized species are also seen (up to helium-like iron, Fe XXV). MEG Global spectral modeling produces a good fit with a two-temperature thermal plasma (APEC): kT = 1.5 keV, 5.6 keV; abundances ~ 0.3 solar. ISM absorption consistent with extinction. DoAr 21 (V2246 Oph) in the ρ Oph star forming cloud, is a Weak-lined T Tauri Star (WTTS), having shown H-alpha in emission in the 1950s but without H-alpha emission reported subsequently and without a strong IR excess. It is only about 1 Myr old (left) and has already been identified, via low-resolution x-ray spectroscopy, to have strong, hard, and variable x-ray emission (Imanishi et al. 2002). HEG High-resolution x-ray diagnostics Chandra grating spectra provide several diagnostics of the physical properties of hot plasma that might be useful for discriminating among theories of PMS x-ray production. A large x-ray flare (below) was seen in our Chandra observation. The x-ray flux increased by a factor of three and the emission hardened during the flare. Spectral modeling showed that the hard component nearly doubled in temperature (from kT = 2.8 keV to kT = 5.8 keV). The strongest line in the spectrum – Si Lyman-alpha (left) – shows modest, but statistically significant broadening (FWHM = 460 +/- 120 km/s). Widths for the strongest lines are summarized at left. One of the few PMS stars that is x-ray bright enough to produce a high-quality grating spectrum, DoAr 21 is perhaps the youngest PMS star observed with the Chandra gratings. In terms of its disk properties, it is an intermediate case between the CTTS TW Hya, which has x-ray properties that have been interpreted in terms of accretion (Kastner et al. 2001) and HD 98800, which is a naked T Tauri star that has x-ray properties similar to main-sequence stars (Kastner et al. 2003). Forbidden-to-intercombination line intensity ratios in He-like ions are diagnostics of density. Kastner et al. (2002) find small f/i ratios in the CTTS TW Hya for O and Ne, indicating high densities (~ 1013 cm-3). The naked T Tauri star HD 98800 shows large f/i ratios, indicating low densities. Our Chandra observations of DoAr 21 show intermediate f/i ratios for Si (right) and S. The implied densities are more in-line with the high values seen in TW Hya, but our results have large uncertainties. F R I New high-resolution spectroscopy: We obtained H-alpha data showing broad emission wings and night-to-night variability. We also measured v sin i ~ 80 km/s. There seem to be several indicators of circumstellar material including at least modest accretion. The SED (above) shows a modest IR excess with a PAH feature (inset) near 11 microns. Recently, evidence has been presented for circumstellar molecular hydrogen emission (Bary et al. 2003). Copies of this poster are available at astro.swarthmore.edu/~cohen/projects/doar21/