9417035 Etzel Post main-sequence stars, having exhausted the hydrogen fuel in their core, undergo core contraction, thermonuclear ignition of hydrogen in a thin shell, and expansion of their outer layers. When such a star is in a close binary system, evolutionary expansion causes it to fill its so-called Roche surface, a tear-dropped shaped lobe with a cusp pointed at the other star. Matter flows along the surface of this star (the "loser"), through the cusp (where the gravitational forces are equal), and is collimated in a stream that sends mass to the other star (the "gainer"). This evolution by mass exchange occurs on a time scale of one million to ten million years, some 100 to 1000 times faster than the rate of evolution when the star is on the main sequence path of stellar evolution. Nature has conspired to make the details of mass flows and accretion processes especially observable in Algol-type binary star systems. Research on these systems will improve our physical understanding of these processes. The research effort will be concentrated on studying long-period Algol systems in which the transferred mass supplies a flattened accretion disk rotating around the "gainer". The viscous dissipation in the disk allows matter to spiral inward to be accreted by the "gainer", supplying mass and angular momentum to that star. The mass, structure, composition, viscous heating, and instabilities of accretion disks will be studied using the strong hydrogen-alpha wavelength emission lines and infrared wavelength absorption lines of oxygen produced in these disks. A diagnostic model to predict line profiles will be developed. The orbits of binary star systems are viewed nearly edge-on, so the stars eclipse each other. The "losers" also partially eclipse the disks, and the disks partially eclipse losers, adding greatly to the observed spatial resolution of the images. Multi-color photometry of the stellar light variations and spectroscopic observations of stellar radial velocities will provide data to calculate stellar masses and radii, which will set the gravitation stratification in the accretion disks. Equatorial accretion of matter from the disk onto the "gainer" leads to rapid rotation of that star. Unless the angular momentum transport to the star is very efficient, the surface rotation of the "gainer" should be differential. The detection of the differential rotation will be sought by observing spectral line profiles as eclipses of the gainer proceed and various portions of the stellar surface are eclipsed or exposed. Results from using other techniques that gauge the average rotation of the stellar envelope, will be combined with the differential rotation data to shed light on the ultimate mass and angular momentum assimilation by "gainers". Estimates of the rate of mass transfer in some systems will be made. It will be possible to judge critically from photometric solutions whether "losers" truly fill their Roche surfaces. In a significant number of cases, the Roche volumes are not filled. Some mechanism other than simple lobe overflow must therefore drive mass transfer and evolution in these systems. Magnetic activity on "loser" surfaces, and cool stars in related systems, will be studied.
