Polarization of the broad Hα wing in symbiotic stars

Abstract

In many symbiotic stars there appear broad wings around Hα, of which the formation mechanisms proposed thus far include a fast outflow, motion of the inner accretion disc, electron scattering and Raman scattering of Lyβ. We adopt a Monte Carlo technique to simulate the Raman scattering of ultraviolet photons that are converted into optical photons around Hα, forming broad wings, and compute its polarization. Noting that many symbiotic stars exhibit a bipolar nebular morphology and polarization flip in the red‐wing part of the Raman‐scattered O vi features, we assume that the neutral scattering region is composed of two components. The first component is a static cylindrical shell with finite thickness; and the second component is a finite slab that is moving away with velocity vp= 100 km s−1 along the symmetry axis of the first component. The cylindrical shell component yields polarization in the direction parallel to the cylinder axis. The strongest polarization is obtained in the limit where the height of the cylinder approaches zero and the scattering region effectively becomes a circular ring. As the height of the cylinder increases, the resultant polarization decreases and becomes negligible in the limit of the infinite cylinder. The polarization near the line centre is weaker than in the far‐wing regions because of the large Rayleigh scattering numbers arising from the large scattering cross‐sections near the line centre. The receding polar scattering component produces strong polarization in the direction perpendicular to the cylinder axis. In the presence of a Lyβ emission‐line component with an equivalent width ∼ 0.5 Å, the polarized flux exhibits a local maximum at λ= 6578 Å that corresponds to a recession velocity 6.4vp relative to Hα. When both of the scattering components co‐exist, the polarization is characterized by weak parallel polarization near the line centre and strong perpendicular polarization in the red part. We discuss the observational implications of our computation.

The polarization of scattered Lyα radiation around high-redshift galaxies

“…Raman scattering of Lyβ wing photons may produce highly polarized Hα emission in a radiative cascade of the form (e.g. Lee & Yun 1998; Yoo, Bak & Lee 2002). This mechanism requires the presence of large H i column densities N H i ≳ 10 20 cm −2 , and has been observed to operate around symbiotic stars (Ikeda et al 2004), and potentially in close proximity to quasars (Lee & Yun 1998).…”

“…Nevertheless, polarized Hα lines produced by this mechanism are typically broad (FWHM ∼20–40 Å) and symmetric, with wings extending into both the longer and the shorter wavelengths (see e.g. Yoo et al 2002, their figs 4–8); these features clearly distinguish them from Lyα line emitters.…”

A Monte Carlo study of polarization structures in the Thomson-scattered line radiation

“…As is shown by Yoo, Bak & Lee (2002), in the case of Raman scattered Hα wings, the linear degree of polarization is lower in the vicinity of the centre than in the far wing part. This is because the Raman scattering optical depth is smaller in the far wing part and therefore the far wing photons suffer fewer scatterings than those photons near the line centre.…”

Polarized Hα Wings in the Symbiotic Stars AG Draconis and Z Andromedae

“…The redshifting of the polarization profiles from the peaks of the intensity profiles may be due to a dilution effect from a component of the H emission nebula around the binary system, and/or a depolarization effect by multiple-scattering in an optically thick H i nebula. The depolarization in the center of the line was suggested by Yoo, Bak, & Lee (2002), who concluded that the degree of polarization tends to be higher through the wing than around the center. This is because the cross section of Raman scattering steeply increases with proximity to the center of Ly in wavelength and the opacity becomes thick at the line center.…”

“…The ratio ðLyÞ=ðO viÞ is $3000 at the velocity of maximum polarization in AG Dra (Yoo et al 2002). From far-UV spectra obtained with the Hopkins Ultraviolet Telescope (HUT; see Kruk et al 1995), 6 we obtained the intensity ratio IðLyÞ=IðO viÞ 1=1000.…”