PHOTOSPHERIC HEATING IN COLLIDING-WIND BINARIES

The spectra of many massive binaries show secondary line depths that a re deeper when the secondary is approaching, a phenomenon we refer to as the Struve-Sahade effect. Such systems are expected to contain coll iding stellar winds, and we show how the X-ray flux from the bow shock that wraps around the secondary will preferentially heat one hemisphe re of the secondary. If the bow shock suffers any significant Coriolis detection due to orbital motion, then the heated surface of the secon dary will be best seen during orbital phases of secondary approach. We present model calculations for the system AO Cassiopeiae that illustr ate how the secondary's light curve appears brighter during these orbi tal phases (as observed). We iind that the model profiles of spectral lines that are insensitive to or that strengthen with heating will app ear deeper when the secondary is approaching, but the same healing eff ects may be nulled or even reversed in lines that weaken with increase d temperature. This differing response of lines to heating may be at o dds with reports of systematic deepening of UV and optical lines, and thus the connection between such heating and the Struve-Sahade effect needs further observational and theoretical investigation.