Stellar Astrophysics

Participants: Prof JC Brown, Dr D Clarke, Dr MA Hendry, Dr L Oskinova, Ms D Telfer, Mr D MacDonald, Mr C Alexander
 
Structure of Stellar Mass Loss, Atmospheres and the Interstellar Medium

Our work is aimed at advancing understanding of fine structure in stellar atmospheres,  powerful stellar mass loss (up to a billion times the solar wind rate), and other forms of distributed stellar/interstellar matter. This is achieved by drawing on (1) spectropolarimetry as a unique diagnostic for flow geometry (the Group is a world leader in this field), (2) source plasma and radiation physics theory and (3) data on stellar X-ray spectra and variability from Chandra and Newton. We are also active in stellar astrophysics applications of gravitational microlensing. Among the many topics studied are the following:

• Discrete Absorption Components in Hot Star Spectra   (Brown,Oskinova,MacDonald)
  Collaborations: Amsterdam; Bartol; Potsdam
  Intense study of the optical and UV spectra of the winds of hot giant stars has revealed that their spectrum lines exhibit discrete (narrow) absorption components (DACs) moving across the line, sometimes as occasional transients and sometimes with a pattern recurring on longer timescales. The origin of these DACs is not known, and could be locally enhanced wind density in sectors of the stellar surface and/or density enhancements formed in the wind itself. For comparison with complex radiative hydrodynamic simulations we are developing a simplified analytic method of modelling DACs from time dependent spectra, using Genetic Algorithm codes to infer wind structure from recurrent DAC data. 
  

   

• Be Star Disks (Brown, Telfer)
  Collaborations: Madison; Toledo; Iowa; ESO
  From broad band polarimetry, Be stars have long been known to possess flattened disks which are variable on both short and long timescales. Detailed models explaining the formation of Be star disks, such as the Bjorkman-Cassinelli Wind Compressed Disk model, are difficult to reconcile with the near-Keplerian nature of the disk. We are working on a new magnetic rotator model for Be star disks based on the idea that: there must  be a domain of field strength large enough to lift material off the star but not so large as to centrifuge the material away entirely nor to make the  equatorial disk predominantly expanding rather than Keplerian. 

Numerical Simulation of a Wind Compressed Disk for hot-stars with equatorial rotations speeds ranging from 200 km/s (left) to 450 km/s (right).

• Stellar and Galactic Jet Outflows (Brown, Hendry, Alexander)
  Collaborations:  Iowa
  We are exploring the extension of our studies of stellar winds and disks to other astrophysical situations such stellar and  galactic jets. Inclusion of time-of-flight effects in the latter case essentially extends reverberation mapping into the (spectro) polarimetric regime.  We are also investigating the diagnostic potential of emission spectra from supernova ejecta,  where the ejected material in a geometrically thin shell is undergoing homologous expansion (i.e. with radial velocity proportional to distance from the source). Within the framework of Sobolev theory, we have devised a method for inverting the profiles of emission lines of arbitrary optical depth, yielding the line intensity as a function of radius.
   
• Wolf-Rayet Star Variability (Oskinova,  Brown)
  Collaborations: Montreal; Madison;  Iowa;  Beijing;  Potsdam
  We are carrying out simulations of the effect of stochastic emission of dense blobs into otherwise spherical winds of WR stars - a model which we have found can reproduce the observed levels of photometric and polarimetric optical variability, for sufficiently large wind acceleration. We are extending this work to evaluate more fully the X-ray variability of WR winds. Also more consistent Radiative Transfer treatments (with Potsdam) will allow us to understand whether X-ray emitting material is related to those dense cold blobs which cause optical variability. We have been awarded XMM-Newton time with Ignace for this study. 
   

  Simulations of line profile variability of CIII for inhomogeneous atmosphere Wolf-Rayet star. 

• Eta Carinae Outbursts and Luminous Blue Variables (Brown)
  Collaborations: Madison
  The detailed physics of Luminous Blue Variables remains far from clear but is central to understanding the evolution of very massive stars, such as Eta Carinae. With Cassinelli (Madison) we are developing models of Eta Car outbursts in terms of long term evolutionary variability plus the effects of a highly eccentric binary companion. Super-enhancements of mass loss rate at long intervals may occur by resonant tidal effects when periastron happens to coincide with maximal pulsational expansion. We are investigating how large a mass loss rate this model might produce, predicting  its polarimetric signature and addressing whether it can explain the position of Eta Car in the HR diagram.

• Structures in the Interstellar Medium (Brown)
  Collaborations:  Montreal; Amsterdam; Madison
  Short time scale variability has recently been reported in stars previously regarded as constant polarimetric standards. One suggested explanation is the movement of these stars across lines of sight containing irregularities in the polarising interstellar medium. If this interpretation is correct it offers a potential probe of density fluctuations in ISM dust structure on the scale of typical stellar radii, complimentary to the study of ISM HI density via narrow absorption lines and of ISM electron density via pulse dispersion and angular sizes of pulsars. We are quantifying this idea by developing the theory of polarisation changes expected in terms of the ISM dust density distribution.