AAS 195th Meeting, January 2000
Session 50. Stellar Atmospheres
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

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[50.12] EG Andromedae's UV Light Curve and the Hydrogen Absorption Observed by HUT

B.R. Espey (ESA/STScI), S.R. McCandliss (JHU)

We present an analysis of 37 archival IUE spectra, spanning 16 years, of the symbiotic binary system EG And. The UV continuum is found to vary with a 482~±~0.5 day period, as the white dwarf component periodically passes behind the red giant primary. This period agrees well with that found by a host of other observers using a variety of optical and infrared, photometric and spectroscopic techniques. Analysis of the UV emission lines shows that many share the same or a similar period. Although the UV continuum generally returns to the same level outside eclipse, we note that there are episodes where it is roughly twice as strong, and we discuss possible physical mechanisms for this phenomenon. Despite these continuum excursions, we note that our period analysis is relatively immune to these effects because the eclipse is so complete and regular.

We use the total Rayleigh scattering cross-section for neutral hydrogen tabulated by Schmid (1995) to derive the column density as a function of phase. The \ion{H}{1} column is asymmetric with respect to ingress and egress with the column density rising more rapidly on ingress than it falls on egress. We also find that the \ion{O}{1} \lambda~1304 line goes from emission to absorption before the surrounding continuum is significantly depressed through Rayleigh scattering by the neutral hydrogen region. We find that the start of the \ion{O}{1} absorption coincides with N(HI) \ge 1021 {\rm cm}2.

Hopkins Ultraviolet Telescope (HUT) spectra covering the region 912 -- 1840Å\ were obtained at two phases during egress. Fits to the HUT data using molecular hydrogen models provide evidence for absorption by excited ground state vibrational bands. These data show the need for high resolution FUSE and STIS observations to fully resolve the atomic and molecular lines, as well as determine the molecular excitation states in intervening regions during ingress and egress. Such data would provide probes of the complex boundaries between ionized and neutral regions about the two stars, and enable the cool gas temperature to be assessed.

Work supported by NASA grants NAS 5-27000 & NAG5-5122 to JHU.

The author(s) of this abstract have provided an email address for comments about the abstract: espey@stsci.edu

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