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Session 12 - Binary Stars.
Display session, Monday, June 08
Atlas Ballroom,
W Serpentis (P = 14 day) is the prototype of a family of hyper-active Algol-type interacting binaries. Its light curve is well known not to repeat from cycle to cycle as a result of large-scale mass-transfer activity. The spectrum is generally classified as an F-type supergiant. Strong double-peaked emission components are evident at H-alpha, which is attributed to an extensive accretion disk and mass-transfer stream. Strong absorption lines that give rise to the supergiant classification are not photospheric, but rather "shell lines." Moderate-dispersion Cassegrain spectroscopy was obtained at SDSU's Mount Laguna Observatory in July and August 1994 of W Ser over one full orbit, and parts of another, at H-alpha, O I 7774, and Mg II 4481. Mild V/R variations in H-alpha were seen near primary eclipse. Such variations are normally interpreted as disk-eclipse effects, but there also may be complications caused by winds. Radial velocities derived from Mg II 4481 and various nearby metal lines give incoherent velocity curves, which are completely inconsistent with orbital motion. However, the equivalent-width variation with orbital phase of the anomalously strong O I 7774 line is very similar to that seen in long-period Algols with accretion disks as presented by Etzel, Olson amp; Senay (1995, AJ 109, 1269). Surprisingly, the variation in equivalent width repeats fairly well from cycle to cycle, although the velocities do not repeat well. This work was supported by NSF grants AST91-15104 and AST94-17035.
