X-ray diagnostics of pre-main sequence accretion and outflow activity
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I summarize the mounting evidence for a direct link between pre-main sequence (pre-MS) X-ray emission and pre-MS accretion and outflow processes. Such a link is supported by three lines of observational inquiry rendered feasible by the extraordinary capabilities of the Chandra and XMM-Newton X-ray observatories,working in concert with space- and ground-based imaging and spectroscopy in the optical and infrared regimes. These threads are:(1) the measurement of anomalous line ratios of He-like ions in high-resolution X-ray spectra of certain young, actively accreting, low-mass (classical T Tauri) stars; (2) the detection of an X-ray eruption from V1647 Ori (``McNeil's Star'') that coincided with the accretion-driven optical/infrared outburst from this dust-enshrouded protostar; and (3) the discovery of soft X-ray emission from the immediate vicinities of pre-MS stars that drive well-collimated outflows. Taken together, these results indicate that, in addition to solar-like coronal activity, X-rays may be produced by pre-MS stars via a variety of mechanisms, including accretion shocks, disk wind shocks, and star-disk magnetic reconnection events. Further progress will require intensive modeling efforts as well as sensitive, high-resolution X-ray spectroscopic surveys involving much larger samples of pre-MS stars.
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