Charge Exchange X-Rays Beyond the Solar System

Solar-wind charge exchange X-rays are emitted throughout the solar system, and this heliospheric emission contributes a significant fraction of the apparent cosmic soft X-ray background. The same phenomenon must occur around other stars with highly ionized stellar winds, which offers the possibility of measuring properties of other stars' astrospheres such as stellar mass-loss rate, local neutral gas density, and stellar-wind composition, velocity, and geometry. Such measurements are just beyond the capabilities of current X-ray telescopes, although useful mass-loss limits have been obtained from a few Chandra observations. The prospects for astrospheric studies using future X-ray missions such as Constellation-X and XEUS are bright, although not immediate. Moving beyond the local stellar neighborhood, charge exchange emission may be an important contributor to the diffuse line emission seen in the Galactic Ridge and Galactic Center. Observations with current X-ray telescopes should be able to address this more controversial hypothesis.