Raman-Scattering Wings of H alpha in Symbiotic Stars

Abstract

Nussbaumer et al. (1989) proposed that broad H$\alpha$ wings can be formed through Raman scattering of Ly$\beta$ photons, and in this Letter we argue that the H$\alpha$ wings prevalently seen in symbiotic stars may be indeed formed in this way. Assuming a flat incident UV radiation around Ly$\beta$, we generate template wing profiles around H$\alpha$ that are formed through Raman scattering in a plane-parallel H~I region. We perform profile fitting analyses to show that the template wing profiles are in excellent agreement with the observed ones that are provided by van Winckel et al. (1993) and Ivison et al. (1994). The wing flux is determined by the scattering H~I column density and the incident Ly$\beta$ flux strength and profile. From our profile analysis it is proposed that the Raman scattering component may be identified with the neutral envelope with a column density ranging $10^{18-20} {\rm cm^{-2}}$ that surrounds the binary system. We briefly discuss alternative candidates for the wing formation mechanism and observational implications of Raman scattering in symbiotic stars and in other astronomical objects including planetary nebulae, post AGB stars and active galactic nuclei.