Spectropolarimetry

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Spectropolarimetry
Bag Lunch Seminar - Dec 2003

• Outline
• Background
• Applications
• Studying the transition from AGB to post-AGB
• Probing the structure of Herbig Ae/Be stars
• Seyfert galaxies.

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1 Background

• Spectropolarimetry
• Simultaneous measurement of the four stokes
  parameters, as a function of wavelength
• I(?) ? Intensity
• Q(?) U(?) ? linear polarization P
  (Q2U2)1/2
• V(?) ? circular polarization

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1 Background (cont.)

• Example HPOL _at_ PBO
• Modified Boller and Chivens small telescope
  spectrograph

• Difficulties

• Polarization optics have to be achromatic

• Measurement require much more photons
• observation must be made for at least 4
  positions of the waveplate
• polarization do not follow Poisson statistics ?
  statistical errors for Q, U and V larger than for
  I

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1 Background (cont.)

• Typical resolution
• In general, largely confined to low spectral
  resolution (R 500-2000)
• Most studies concentrate on the continuum
• Studies of polarimetric line profiles are scarce
  (or used to be) expect for greatly broadened
  lines (QSOs, Seyfert, SNe)

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2 Applications

• a) Transition from AGB to PN

• From the AGB to the PN phase, the circumstellar
  envelope goes from nearly spherical to
  aspherical, presenting a wide range of
  morphologies

• When and how the transition starts?

• Trammell et al. 1994 ? investigated 31 stars
  believed to be undergoing the early phases of the
  transition

• 24 were intrinsically polarized ? evidence that
  aspherical structure appears very early in the
  transition

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2 Applications a) AGB to PN (cont.)

• IRAS 08005-2356 (post-AGB)

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2 Applications a) AGB to PN (cont.)

• How to get a PA flip? Ingredients
• Bipolar geometry (two components)
• Absorption (? lt 1)

• IRAS 08005-2356
• PA flip of 70 deg ? indication that the symmetry
  axis of the two components differ (e.g. clumpy
  media)

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2 Applications a) AGB to PN (cont.)

• IRC10420

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2 Applications a) AGB to PN (cont.)

• Line Depolarization

Continuum
Line
scattering region
? Many other line effects alter the polarization
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2 Applications

• b) Circumstellar Structure of Ae/Be Stars

• Ae/Be stars are pre-main sequence stars of
  intermediate mass

• Spectropolarimetry of emission lines gives the
  possibility of probing the circumstellar
  structure near the star

• Vink et al. 2002 ? Studied H? in 22 Herbig Ae/Bes
  

• Most of them are intrisecally polarized in the
  continuum

• 15 out of 22 objects show clear line effects ?
  indication that the geometry near the star is not
  spherical

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2 Applications b) Ae/Be stars
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2 Applications b) Ae/Be stars
Spectropolarimetric expectations ? No line effect
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2 Applications b) Ae/Be stars
Spectropolarimetric expectations ? depolarization
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2 Applications b) Ae/Be stars
Spectropolarimetric expectations ? rotating disk
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2 Applications b) Ae/Be stars
Example no line effect ? IL Cep
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2 Applications b) Ae/Be stars
Example depolarization ? BD40 4124
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2 Applications b) Ae/Be stars
Example loop ? CQ Tau
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2 Applications

• c) Seyfert Galaxies

• Seyfert galaxies are named for Carl K. Seyfert
  who in 1943, described them as their central
  regions having peculiar spectra with notable
  emission lines.

• They are spiral galaxies and account for about
  10 of all galaxies (Ho et al. 1997)

• They are one class of AGN galaxies ? broad
  emission lines which is evidence of highly
  excited gas

• Two types, depending on the line profiles
• Type I very broad lines ? gas velocities over
  1000 km/s
• Type II much narrower lines

• SU theory ? same type of object ? observational
  differences arise from differing viewing angles

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2 Applications c) Seyfert Galaxies
Jet of Radio Emission
Broad Line Clouds
Type II Seyfert
Dusty Torus
Type I Seyfert
Narrow Line Clouds
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2 Applications c) Seyfert Galaxies

• Antonucci Miller 1985 ? spectropolarimetry of
  NGC 1068

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2 Applications c) Seyfert Galaxies
Jet of Radio Emission
Broad Line Clouds
Type II Seyfert
Dusty Torus
Type I Seyfert
Narrow Line Clouds
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2 Applicationsc) Seyfert Galaxies
Watanabe et al. 2003

• 3 polarizing mechanisms
• Electron scattering
• Dust scattering
• Dichroic absorption

• 2 polarizing sources
• Electron scattering in the jets (optical)
• Dust scattering in the torus (optical)
• Dichroic absorption (NIR)