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Astron. Astrophys. 363, 1106-1114 (2000)

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1. Introduction

Circumstellar disks in one form or another appear to be a prevalent and important feature of many stars, such as protostars, Be stars, the evolved B[e] stars, Luminous Blue Variables, Asymptotic Giant Branch stars, cataclysmic variables, and on galactic scales the Active Galactic Nuclei. Understanding these disks in terms of their accretion properties and mass loss is an important area of current research (the subject is far too broad to list the many important studies regarding disks, but see for example reviews by Papaloizou & Lin 1995 and Lin & Papaloizou 1996 that touch on topics relevant to many disk systems). Consequently, new and improved diagnostic methods are of great benefit for interpreting the observations of disk spectra, which can then be used to constrain and guide theoretical models.

This paper is the third in a series to investigate anisotropic resonance line scattering as a probe of the structure of circumstellar media. As discussed more thoroughly in Paper I (Ignace 1998a), the anisotropic and polarizing effects of scattering by resonance lines was considered by Hamilton (1947), who described how resonance scattering may be treated as a mix of isotropic and dipolar scattering, with the parameter [FORMULA] characterizing the fraction of the total scattering that is dipolar (see also Chandrasekhar 1960). Hence, [FORMULA] is pure isotropic scattering, whereas [FORMULA] is pure dipole scattering, like that of free electrons.

In Paper I, anisotropic line scattering in spherically expanding winds was discussed, especially of the redshifted emission where absorption can largely be ignored. It was shown that the emission profile shape can be altered by 10% at most for realistic wind models. Of course, there is no net polarization in the line profile for an unresolved spherical source. In Paper II (Ignace 1998b), circumstellar disks were considered. Unlike the spherical case where viewing inclination is arbitrary, it was shown that interesting profile effects can be expected of equatorial disks. However, only the total emission profile was modelled. This third paper extends the work on disks to report on polarized emission profiles  1.

The continuum polarization that arises from electron scattering in equatorial disks has been considered in numerous papers. Especially popular have been studies of Be star disks (Brown & McLean 1977; Brown & Fox 1989; Fox & Brown 1991; Fox 1991; Bjorkman & Bjorkman 1994; Wood et al. 1997 to name a few). Relatively few papers have focussed on the polarization of line profiles in stellar winds, some exceptions being Caroff et al. (1972), McLean (1979), Poeckert & Marlborough (1978), McKenna (1984, 1985), Jeffery (1989, 1990), Wood & Brown (1994a,b), Brown & Wood (1994), and Harries (2000). And of these, only a couple were concerned with polarization arising from resonance line scattering (although it is worth noting that quite sophisticated advances in line scattering polarization have been considered in static and plane-parallel geometries, such as Trujillo Bueno & Manso Sainz 1999 and Rangarajan 1999). However, polarized line profiles potentially contain a wealth of important information about the disk structure. The scattering geometry relates directly to (a) the Doppler shifted frequency at which resonance scattered light will appear in the profile and (b) its polarization.

In Sect. 2, emission profile shapes are derived for optically thin planar disks in pure expansion or rotation. Initially the expansion and rotation are assumed constant leading to analytic expressions for the profile shape. Numerical profile calculations are also given for disks in linear expansion or Keplerian rotation. A discussion of the results for observations and directions for further study are presented in Sect. 3.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000