O STARS are characterized by their dense, supersonic stellar winds. These winds are the site of x-RAY emission from shock-heated plasma. By analyzing high-resolution x-RAY spectra of these o STARS, we…

O STARS are characterized by their dense, supersonic stellar winds. These winds are the site of x-RAY emission from shock-heated plasma. By analyzing high-resolution x-RAY spectra of these o STARS, we can learn about the wind-shock heating and x-RAY production mechanism. But in addition, the x-rays can also be used to measure the mass-loss rate of the stellar wind, which is a key observational quantity whose value affects stellar evolution and energy, momentum, and mass input to the galactic interstellar medium. We make this x-RAY based mass-loss measurement by analyzing the profile shapes of the x-RAY emission lines observed at high resolution with the chandra and XMM-newton grating spectrometers. One advantage of our method is that IT is insensitive to small-scale clumping that affects density-squared diagnostics. We are applying this analysis technique to o STARS in the chandra archive, and are finding mass-loss rates lower than those traditionally assumed for these o STARS, and in line with more recent independent determinations that do account for clumping. By extending this analysis to the XMM RGS data archive, we will make significant contributions to the understanding of both x-RAY production in o STARS and to addressing the issue of the actual mass-loss rates of o STARS. The XMM RGS data archive provides several extensions and advantages over the smaller chandra hetgs archive: (1) there are roughly twice as many o and early b STARS in the XMM archive; (2) the longer wavelength response of the RGS provides access to diagnostically important lines of nitrogen and carbon; (3) the very long, multiple exposures of zeta PUP provide the opportunity to study this canonical o supergiant's x-RAY spectrum in unprecedented detail, including looking at the time variability of x-RAY line profiles. Our research team has developed a sophisticated empirical line profile model as well as a computational infrastructure for fitting the model to high-resolution x-RAY spectra in order to determine the values of physically meaningful model parameters, and to place confidence limits on them. We have incorporated second-order effects into our models, including resonance scattering. We have also developed tools for modeling the x-RAY opacity of the cold, x-RAY absorbing wind component, which is a crucial ingredient of the technique we have developed for determining wind mass-loss rates from analyzing the ensemble of emission lines from a given star's x-RAY spectrum. In addition to testing state-of-the-art wind shock models and measuring o star mass-loss rates, an important component of our proposed research program is the education of talented undergraduates. Swarthmore undergraduates have made significant contributions to the development of our line profile modeling, the wind opacity modeling, and related research topics such as laboratory astrophysics before going on to PHD programs. Two have been named as finalists for the APS's apker prize. The research we propose here will involve two undergraduates and will likely lead to honors theses, refereed papers, and the opportunity to present their research results at national and international meetings. By measuring mass-loss rates for all the o STARS for which high-resolution x-RAY spectra exist and by constraining x-RAY production mechanisms, we will address issues important to our understanding of stellar and galactic evolution: including the frequency of core collapse supernovae, the energetics of the galactic interstellar medium, and the radiation conditions in star formation regions where not only new, solar-type STARS form, but also where their planetary systems form and are subject to effects of high-energy emission from nearby STARS. In this way, the work we are proposing in this project will make a contribution to NASA's mission to understand cosmic evolution and the conditions for generating and sustaining life in the universe.