Tim's Research Interests

Research Interests

Raman scattering in Symbiotic Stars

Raman scattering is simply the inelastic version of Rayleigh scattering. In the Rayleigh scattering case the incoming photon and outgoing photon have the same frequency and the eigenstate of the scatterer is unchanged. In Raman scattering the outgoing photon is of a different frequency (usually lower) than the incoming photon and the scatterer is left in a different eigenstate (see schematic below).

[No Imaging]

The cross-section for Raman scattering is usually rather small, so in order to observe Raman scattered lines we need an object with

A high flux of "parent" photons, and
A large mass-column of scatterers

These two criterea are met in

Symbiotic Stars

Symbiotic stars are systems that display both the spectrum of a hot, ionized plasma and a cool, low-gravity star. It is now generally accepted that these objects are binary systems containing a compact hot star (such as a white dwarf) which ionizes material shed by a companion red giant star (normally in a cool, slow stellar wind). Two of the optical emission lines (6830 and 7088 angstroms) are the result of Raman scattering of OVI photons (1032 and 1038 angstroms) by neutral hydrogen in the cool stellar wind. Since the scattering geometry is highly asymmetric (with UV photons produced in the ionized nebula and scattering in the locale of the red giant) the lines should be highly linearly polarized. A spectropolarimetric survey of 28 symbiotic stars made using the 4-m Anglo-Australian Telescope (AAT, see below) revealed that the vast majority of the Raman lines do indeed show significant linear polarization and that the polarization spectra are highly structured.

The Anglo-Australian Telescope

The following plot is the polarization spectrum of the 6830 line observed in symbiotic star M1-21.

Spectropolarimetric analysis of WR star winds

1. EZ Canis Majoris

Although the WR star EZ CMa (HD 50896) has become the prototype WN5 star, notably following the non-LTE modelling work of Hillier, it is well known that the star exhibits abnormal properties, particularly in its photometric and polarimetric behaviour.
The light-curve of EZ CMa has a 3.77 day period, with a pattern of one high and two low maxima per period although the shape of the curve changes from year to year (e.g. Robert el al. 1991). A careful study of the optical emission-line and RV variations also revealed a 3.77 day period.
McLean (1980) found that the polarization of EZ CMa is also modulated on the 3.77 day period with a peak to peak amplitude of more than 0.3% in both Stokes parameters. It was discovered that the dominant variation is the second harmonic (that is a period of 1.89 days), a phenomenon that is most commonly found in close binary systems. Aperiodic variations in the polarimetry rendered the binary interpretation uncertain, but it was suggested an orbital inclination of 71 degrees was consistent with the data. When combined with the mass-function this implies a 1.3 solar mass companion assuming a WR mass of 10 solar masses.
Polarization structure across the emission lines of EZ CMa was first observed by McLean, who presented a polarization spectrum with 50 angstrom resolution covering the range 3400 to 6000 ansgtroms. It was suggested that ionization stratification could explain the polarization spectrum, with the lines of more highly ionized species being produced deeper in the wind, and hence undergoing electron scattering. In addition, it was noted that the observed rapid variations in the polarization implied non-steady mass-loss and that the envelope is not axisymmetric.
Observations were obtained using the RGO spectrograph and polarimetry module on the 3.9m AAT telescope, NSW. Time series data were obtained on the nights of the 28--29 Jan 1991 and 12--13 March 1992 by R. Schulte-Ladbeck. Further observations were obtained on 17 Mar 1992 and 6--9 May 1994. The reduced polarization spectra are displayed below.

1. Zeta Puppis

The O4 I(n)f star (Walborn 1972) Zeta Puppis (HD 66811) is the archetypal O-supergiant star. It is the second brightest O-star in the night sky, with an apparent V-magnitude of 2.25. Its massive stellar-wind has been extensively observed over virtually all wavelengths, from radio to X-rays, and the system has become the proving ground for stellar wind theorists.
Spectropolarimetric observations of Zeta Puppis were made with the 3.9m Anglo-Australian telescope in NSW using the RGO spectrograph and polarimetry module from the 14-17th March 1992. The 25 cm camera and 1200V grating were employed while the Thompson CCD was used as the detector. The system gave a mean reciprocal dispersion of 0.69 Angstroms per pixel and a resolution of 1.3 Angstroms.
The following plot shows a rectifed H-alpha polarization spectrum of Zeta Puppis. Note the change in polarization through the H-alpha emission line. This is the first unambiguous evidence for a wind asymmetry in an O-star.

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