Observability of stellar winds from late-type dwarfs via charge exchange x-ray emission

Despite the fact that the overwhelming majority of stars are of late spectral type (F-M) and lie on the main sequence, we know nothing about their stellar winds. Existing measurements of winds only apply to high-mass O and B stars, red giants, and supergiants and only extend down to a few times 10(-10) M. yr(-1), as compared to the solar rate of similar to2 x 10(-14) M. y(r-1). Attempts to detect winds from late-type dwarf stars have to date resulted only in loose upper limits of order 10(-12) to 10(-11) M. yr(-1). We propose a novel method of studying stellar winds through observation of charge exchange-induced X-ray emission. Recent X-ray detections of comets suggest that charge transfer between highly charged ions in the solar wind and neutral gases in cometary atmospheres is responsible for much or all of the observed emission, a hypothesis that has been strengthened by Chandra observations of comet C/1999 S4 (LINEAR). It has also been proposed that charge transfer between the solar wind and the local interstellar medium (ISM) produces a substantial fraction of the soft X-ray background observed by ROSAT and various rocket experiments. We show that the same process may be observable in nearby dwarf star systems using Chandra and future large-area high-resolution observatories, which would provide hitherto unobtainable information on wind geometry, ion composition, mass-loss rates, and the distribution of neutral gas in the ISM.