Stellar Coronae
Time series analyses of EUV and X-ray observations of active stars have provided evidence that plasma at temperatures >= 4 × 106 K arises purely from flares, analogous to the idea of "nanoflare" theories of solar coronal heating. Constellation-X will provide a sensitive test of flare heating through both Doppler shifts and photon arrival times. A Constellation-X XMS effective area of 6,000 cm2 at 6 keV and resolving power of E/ΔE > 1,000 brings within reach Doppler diagnostics in H-like and He-like S (λ4.73, 5.04), Ar (λ3.95, 3.73) and Fe (λ1.85).
Another major Constellation-X breakthrough in the study of stellar flares will be the enormous improvement in photometric precision of flare light curves and spectra, allowing direct measurement of coronal loop resonant frequencies themselves. Loop "wobble" velocities on the Sun have reached up to 200 km s-1. Constellation-X detections of loop oscillations, both spectroscopically and photometrically, could provide unique measurements of these quantities in a wide range of stars, from accreting T Tauri stars to evolved giants.
Detection of hard X-rays in stellar flares would define a major breakthrough for stellar physics. This emission is unequivocally related to impulsively accelerated electrons and ions that do not suffer from magnetic trapping (as radio-emitting electrons do). In the case of the Sun, hard X-rays and gamma rays have been the prime source for the study of energy release physics, particle acceleration in magnetic fields, and coronal heating. The different, and probably more extreme, magnetic configurations in magnetically active stars could lead to quite different acceleration histories and heating efficiencies in large flare events. Detection of hard X-ray components would thus open an entirely new avenue in the study of the energetics of hot, magnetized coronal plasma. For the Constellation-X HXT area of 150 cm2, bright flares on nearby stars can be detected in only 100 sec.
