Raman Scattering in Symbiotic Stars

Abstract

Symbiotic stars are interacting binary systems consisting of a hot star, typically a white dwarf, and a cool red giant companion. The radiation from the hot star partially ionizes the wind from the cool star, resulting in a characteristic combination of sharp nebular lines and stellar molecular absorption bands in the optical spectrum. Most of the emission lines are readily identifiable with common atoms and ions. However, roughly half of all known symbiotic stars exhibit two strong, broad emission lines at ll6825 and 7082 (D. A. Allen 1980, MNRAS, 190, 75) which defied identification for a number of years. H. M. Schmid (1989, AA W. A. Fiebelman, F. C. Bruhweiler, & S. Johanssson 1991, ApJ, 373, 649; P. S. Li & S. A. Leah 1997, ApJ, 484, 424). H. M. Schmid & H. Schild (1994, AA T. J. Harries & I. D. Howarth 1997, AA K. W. Lee & H. W. Lee 1997, MNRAS, 287, 211). The derived efficiencies for Z And, V1016 Cyg, AG Dra, and EG And imply ionization geometries –4.0, in qualiX ∼ 0.4 H tative agreement with other studies (H. Murset et al. 1991, AA H. M. Schmid et al. 1999, A&A, 348, 950). The relative mass-loss rates of Z And, V1016 Cyg, AG Dra, and EG And based on their Raman efficiencies are not generally in agreement with those derived from radio measurements (E. R. Seaquist, M. Krogulec, & A. R. Taylor 1993, ApJ, 410, 260). Since both the Raman line intensity and polarization profiles are very sensitive to the mass-loss rate (T. J. Harries & I. D. Howarth 1997, A&AS, 121, 15), detailed studies of these profiles may provide another useful method to examine the mass-loss rates of cool red giants in symbiotic systems. At present, Raman scattering is found to occur only in symbiotic stars, with the exception of the young planetary nebula NGC 7027. D. Pequignot et al. (1997, A&A, 323, 217) observed a Raman scattered He ii line in the optical spectrum of NGC 7027. This unusual planetary nebula appears to have two central stars, one of which is either a main-sequence star or subdwarf and the other a white dwarf star, and has been discussed as a possible post-symbiotic system (G. A. Gurzadyan 1997, The Physics and Dynamics of Planetary Nebulae [New York: Springer]). Spectroscopic similarities between bipolar, proto–planetary nebulae and symbiotic stars have lead many researchers to speculate on possible evolutionary links between the two types of objects (H. W. Lee & M. G. Park 1999, ApJ, 515, L89, and references therein). The binarity of symbiotic stars is well-established while that of bipolar proto–planetary nebulae remains a matter of debate. Nonetheless, it remains an exciting potential that Raman scattering may ultimately reveal evolutionary links between symbiotic stars and bipolar, proto–planetary nebulae.