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Polarisation of Wolf-Rayet and Other Hot, Massive Stars

Polarisation of Wolf-Rayet and Other Hot, Massive Stars. Nicole St-Louis Université de Montréal & Centre de Recherche en Astrophysique du Québec. Astronomical Polarimetry 2008, La Malbaie, Québec 2008. Classical Wolf-Rayet Stars.

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Polarisation of Wolf-Rayet and Other Hot, Massive Stars

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  1. Polarisation of Wolf-Rayet and Other Hot, Massive Stars Nicole St-Louis Université de Montréal & Centre de Recherche en Astrophysique du Québec Astronomical Polarimetry 2008, La Malbaie, Québec 2008

  2. Classical Wolf-Rayet Stars • Wolf-Rayet stars are the descendants of the most massive O stars (>25 M) • They are characterised by their strong winds ( 10-5 M/yr, v∞ 2000 kms-1) • They represent a key evolutionary phase between the main sequence and the SN explosion • They impart to the ISM a huge amount of energy and chemically enrich it with heavy elements • They might even be the precursors to long-timescale GRBs

  3. Continuum linear polarisation curvesof Wolf-Rayet Stars WR42 WR103 WR90 WR40 St-Louis, Moffat, Drissen, Robert, Bastien et al. (>1987)

  4. Continuum linear polarisation curvesof Wolf-Rayet Stars • Massive-star winds, and in particular WR star winds, have ample free electrons to generate wavelength-independent linear polarisation. • But…

  5. Spherically symetric wind -- single star E E E E E E E Net polarisation iszero E

  6. Continuum linear polarisation curvesof Wolf-Rayet Stars • Massive-star winds, and in particular WR star winds, have ample free electrons to generate wavelength-independent linear polarisation. • But… in spherically-symmetric winds, there should be no net polarisation. • But some massive stars are binaries. WR90

  7. Spherically symetric wind --Binary Companion This angle is very small The net polarization is:

  8. Continuum linear polarisation curvesof Wolf-Rayet Stars WC7+O5-7 WR42 • The BME model works beautifully well and allows one to determine the orbital inclination which in turn can lead to the masses. Note of caution from Aspin, Simmon & Brown (1981):

  9. Mass-Loss Determination Methods • Resonnance line fits (Fullerton, Massa, Prinja 2006) • Orbital period lenghtening due to a radially symetric mass-loss (Khaliullin 1974) (B) • Photometry (atmospheric eclipses, e.g. Lamontagne et al. 1996)(B) • Polarimetry (B)  2 • Recombination line fits (H) • Radio/IR free-free emission (Wright & Barlow 1975) • H fitting based on the technique developped by Puls et al. (1996) • Improved by Markova et al. (2004) to include line blocking/blanketing

  10. Polarimetry • For WR stars, St-Louis et al. (1988) : • Apis the semi-amplitude of the ellipse in the Q-U plane. • a(R) is the semi-major axis of the orbit • v is the terminal velocity of the WR wind • i is the orbital inclination • fc is the fraction of the total flux coming from the companion • I is the numerical value of an intergral over angles. This depends on  and on the starting radius

  11. Polarimetry - WR+O Systems WR 42=HD97152 St-Louis et al. (1987)

  12. Comparison with other estimates x IR (free-free)  radio (free-free) Factor of ~3 lower than other estimates in most cases

  13. Polarimetry - the case of V444 Cygni Robert et al. (1990)

  14. Polarimetry - the case of V444 Cygni From period change, Khaliullin et al. (1984) Radio free-free emission estimates agree with polarisation. IR less so…

  15. Polarimetry of O +O systemsSt-Louis & Moffat (2008) • A litterature search yielded 6 O+O binaries with published polarimatric binary curves. Mostly III and I but two O4f systems as well. • Here there are two winds: The polarisation amplitude is the total value for both winds taken together; if the winds are equal, it is doubled. • Wind consists of H instead of He • =0.8 • In most systems, H profiles have been found to be affected by flows related to colliding winds so the can’t be used to estimate mass-loss rates

  16. Results i Luna (1988), ii Rudy & Herman (1978), iii Lupie & Nordsieck (1987), iv Niemela et al. (1992), v Morrell & Niemela (1990)

  17. The case of HD149404 • HD 14904 is the only star in our sample that we found to have other mass-loss rate determinations: • Polarimetry: 3.1 x 10-6 Myr-1 • Radio: 6.2 x 10-6 Myr-1 • PV resonnance lines: 6.2 x 10-8 Myr-1 (for qP+4=1) (two stars?) • Polarimetry seems to support a clumping factor of  3 but not 10-100!

  18. Continuum linear polarisation curvesof Wolf-Rayet Stars WR40 • Single WR stars can still show linear polarimetric variability. • The timescale is relatively short. • We now know that winds in WR stars are clumped. • Small subpeaks are superposed on top of strong emission lines and move from line center towards line edges on relatively short timescales • This can easily produce continuum linear polarimetric variability as observed.

  19. Continuum linear polarisation curvesof Wolf-Rayet Stars • One hint was found in Drissen et al. (1987). A trend for slower winds to show a smaller linear polarisation scatter. • The interpretation was that perhaps blobs formed in fast winds have more difficulty to survive; they act as an homogenezing agent. • A model of polarisation from blob ejection was devised by Davies, Vink & Oudmaijer (2008). (see also a previous model by Li et al. 2000)

  20. Clumping-induced polarimetric variability from winds (Davies et al. 08) • They first calculated P for one clump. Mass and angular sizes of clumps are conserved. • They used a  law to describe the movement of the clump: • Then they produced many randomly ejected blobs and added their Q and Us. • They examined the effect of varying many input parameters such as ejection rate, mass-loss rate, clump size, radius, etc

  21. Clumping-induced polarimetric variability from winds (Davies et al. 08) • <P> and (P) increase with mass-loss rate • The increasing (P) with decreasing v∞ is explained by the fact that blobs spend more time in the wind • The observed <P> and (P) has two possible explanations: • Because of the timescale of the variability, statistical deviations from spherical symmetry is prefered over a small number of blobs

  22. Spectropolarimetry • If there is a global asymmetry in the wind, then the so-called line effect is observed: • The global asymmetry causes the continuum polarisation from electron scattering to be polarised. • If the lines are formed by recombination, then they should not be polarised. • But the unpolarised line photons can subsequently be scattered and thus the lines can be polarised • This can be (as has been) used to detect non-spherically symmetric winds.

  23. McLean et al. (1979) Spectral resolution: 50 Å

  24. HeII 4686 must have a scattering component which increases its polarisation

  25. Schulte-Ladbeck, Nordsieck et al. (1991) IS Spectral resolution: 34 Å • Wind is not spherically symmetric • Cont pol. due to e scattering (inclined disk model) as continuum polarisation is grey • Lines are polarised due to e-scattering • Confirm ionisation stratification Schulte-Ladbeck et al. (1992) carried out a similar study and obtained similar results for WR134 except that continuum polarisation rises to the UV

  26. Harries, Hillier & Howarth (1998) 134,40 • Spectropolarimetric survey of 16 northern WR stars. • Data from the WHT. Resolution of  3 Å. • Four stars were found to show line de-polarisation (WR 134, WR 137,WR 139, WR 141 --binaries). • A statistical analysis shows that this fraction is best reproduced if only 20% of stars have an intrinsic pol. > 0.3% (equator:pole=2:3) 136 6 137 16 • Combining their data with those of Schulte Ladbeck (1994), they found that stars with the highest mass-loss rates are those found to show the line effect, i.e. to have flattened winds.

  27. Vink (2007) • Carried out a Spectropolarimetric survey of 13 WR stars in the LMC to search for a different behaviour in a low-metalicity environment (possible GRB progenitors). • Data from the VLT-FORS1. Resolution of  3 Å. • Two stars were found to show line de-polarisation (BAT22 & BAT33) • The fraction is similar to that of galactic (15%). • If metallicity is important in contributing to reduce the angular momentum loss from WR winds, the threshold is below that of the LMC (<0.5 Z)

  28. Other stars 1) Harries, Howarth & Evans (2002) -- 20 O Galactic supergiants • 5 were found to show the line depolarisation effect (25%) • Davies, Oudmaijer & Vink (2005) -- 14 Galactic and Magelanic Cloud LBV • 50% (!!) are found to show the line effect (H). • 4 were observed during multi-epochs, 3 with random polarisation angles and 1 with a constant angle. • They interpret this as due to the presence of strong clumps in the wind (consistent with their model)

  29. ESPaDOnS data of WR6=HD50896 De la Chevrotière, St-Louis, Moffat (2008)

  30. First detection of a magnetic field in a WR star: 100 Gauss To be modelled with Ignace & Gayley (2003)+ improvments to come

  31. Summary Polarisation… • Contributed to help us confirm that because winds are clumpy, mass-loss rates need to be revised • Can allow us to determine the orbital inclination in a WR+O binary system • The short timescale of random polarisation variability tells us that the winds contain many many small blobs instead of a smooth component + a few big blobs • Tells us that only 15-20% of WR stars have ‘’flattened’’ winds which is what we expect because they are not expected to have a fast rotation • Will allow us to detect magnetic fields if there are any

  32. Merci !

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