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Spectropolarimetry

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. How to measure polarization:. Detector. Analiser. Beam displacer. E. Light beam. O.

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Spectropolarimetry

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  1. 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.

  2. How to measure polarization: Detector Analiser Beam displacer E Light beam O (calcite) (/2 waveplate) 1 – Background • Spectropolarimetry • Simultaneous measurement of the four stokes parameters, as a function of wavelength: I() Intensity Q() & U() linear polarization: P = (Q2+U2)1/2 V()  circular polarization

  3. Glan Prism Grating Camera Rotating waveplate Wollaston Prism Detector Colimator 1 – Background (cont.) • Example: HPOL @ 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

  4. 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)

  5. 2 – Applications • From the AGB to the PN phase, the circumstellar envelope goes from nearly spherical to aspherical, presenting a wide range of morphologies a) Transition from AGB to PN • 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

  6. Polarization raises to the blue. 10%! Lack of features in the lines PA change of ~ 70 deg 2 – Applications: a) AGB to PN (cont.) • IRAS 08005-2356 (post-AGB)

  7. Net pol: Net pol: Net pol: 2 – Applications: a) AGB to PN (cont.) • How to get a PA flip? Ingredients: • Bipolar geometry (two components) • Absorption ( < 1) • IRAS 08005-2356 • PA flip of 70 deg indication that the symmetry axis of the two components differ (e.g. clumpy media)

  8. H is less polarized 2 – Applications: a) AGB to PN (cont.) • IRC+10420

  9. P P Stellar photosfere Line emitting regiion 2 – Applications: a) AGB to PN (cont.) • Line Depolarization Continuum Line scattering region  Many other line effects alter the polarization…

  10. 2 – Applications • Ae/Be stars are pre-main sequence stars of intermediate mass b) Circumstellar Structure of Ae/Be Stars • 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

  11. 2 – Applications: b) Ae/Be stars

  12. 2 – Applications: b) Ae/Be stars Spectropolarimetric expectations No line effect

  13. 2 – Applications: b) Ae/Be stars Spectropolarimetric expectations depolarization

  14. 2 – Applications: b) Ae/Be stars Spectropolarimetric expectations  rotating disk

  15. 2 – Applications: b) Ae/Be stars Example: no line effect  IL Cep

  16. 2 – Applications: b) Ae/Be stars Example: depolarization  BD+40 4124

  17. 2 – Applications: b) Ae/Be stars Example: loop  CQ Tau

  18. 2 – Applications • Seyfert galaxies are named for Carl K. Seyfert who in 1943, described them as their central regions having peculiar spectra with notable emission lines. c) Seyfert Galaxies • 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

  19. 2 – Applications: c) Seyfert Galaxies Jet of Radio Emission Broad Line Clouds Type II Seyfert Dusty Torus Type I Seyfert Narrow Line Clouds

  20. Seyfert II profile in total flux Seyfert I profile in polarized flux! 2 – Applications: c) Seyfert Galaxies • Antonucci & Miller 1985 spectropolarimetry of NGC 1068

  21. 2 – Applications: c) Seyfert Galaxies Jet of Radio Emission Broad Line Clouds Type II Seyfert Dusty Torus Type I Seyfert Narrow Line Clouds

  22. 2 – Applications:c) 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)

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