Near-simultaneous Observations of Direct and Raman-scattered Linesin the Symbiotic Star Z Andromedae

Abstract

Symbiotic binary stars typically consist of a hot compact star orbiting a cool giant. Overlaid on the continuum from these stars is a nebular spectrum produced by the photoionization of the cool star wind by the hot star. Most of the observed emission lines are readily identifiable with common ions; however, a pair of lines at λλ6825 and 7082 remained unidentified until relatively recently. A case has been made that these long-unidentified emission lines result from the Raman scattering of O VI λλ1032, 1038 resonance photons. We present near-simultaneous far-UV and optical observations of the symbiotic star Z Andromedae, obtained with the space-borne Hopkins Ultraviolet Telescope and the Mount Hopkins 1.5 m telescope. Our data show the presence of both the O VI λλ1032, 1038 resonance doublet and the λ6825 and λ7082 emission lines and provide strong evidence for the Raman effect in Z And. We show that the unusual line ratio of 7:1 observed for the O VI lines is due to the effects of interstellar H2 absorption. Correction for this effect results in a line ratio of close to2:1, consistent with optically thin emission. We derive the efficiency for the Raman-scattered O VI lines and compare them with previously reported measurements for RR Telescopii.