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Polarisation of WolfRayet and Other Hot, Massive Stars

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in spherically-symmetric winds, there should be no net polarisation. ... Here there are two winds: The polarisation amplitude is the total value for both ... – PowerPoint PPT presentation

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Title: Polarisation of WolfRayet and Other Hot, Massive Stars


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 (gt25 M?)
  • They are characterised by their strong winds
  • (? 10-5 M?/yr, v8 ? 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.
(gt1987)
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 is zero
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
WC7O5-7
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
??
??2
  • 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)
  • 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)
  • Ap is 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 - WRO Systems
WR 42HD97152
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 systems
St-Louis Moffat (2008)
  • A litterature search yielded 6 OO 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 cant 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
    qP41) (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)
  • ltPgt and ?(P) increase with mass-loss rate
  • The increasing ?(P) with decreasing v8 is
    explained by the fact that blobs spend more time
    in the wind
  • The observed ltPgt 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
  • 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

Spectral resolution 34 Å
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. gt 0.3 (equatorpole23)

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 (lt0.5 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 WR6HD50896 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 WRO 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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