SpectropolarimetryBag Lunch Seminar - Dec 2003

Outline1. Background

2. Applications

a) Studying the transition from AGB to post-AGB;

b) Probing the structure of Herbig Ae/Be stars;

c) Seyfert galaxies.

1 – Background• Spectropolarimetry

– Simultaneous measurement of the four stokes parameters, as a function of wavelength: I() IntensityQ() & U() linear polarization: P = (Q2+U2)1/2

V() circular polarization

• How to measure polarization:

Analiser Beam displacer

(/2 waveplate) (calcite)

Detector

E

OLight

beam

1 – Background (cont.)

• Example: HPOL @ PBO– Modified Boller and Chivens small telescope

spectrograph

DetectorWollaston

Prism

Rotating

waveplate

Grating

Camera

Glan Prism

Colimator

• 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

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)

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

2 – Applications: a) AGB to PN (cont.)

• IRAS 08005-2356 (post-AGB)

Lack of features in the linesPolarization raises to the blue. 10%!

PA change of ~ 70 deg

2 – Applications: a) AGB to PN (cont.)

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

Net pol: Net pol: Net pol:

• IRAS 08005-2356– PA flip of 70 deg indication that the symmetry axis of

the two components differ (e.g. clumpy media)

2 – Applications: a) AGB to PN (cont.)

• IRC+10420

H is less polarized

2 – Applications: a) AGB to PN (cont.)

• Line Depolarization

P

Stellar photosfere

P

Line emitting regiion

Many other line effects alter the polarization…

Continuum Line

scattering region

2 – Applications

b) Circumstellar Structure of Ae/Be Stars

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

• Most of them are intrisecally polarized in the continuum

• 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

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

2 – Applications: b) Ae/Be stars

2 – Applications: b) Ae/Be stars

Spectropolarimetric expectations No line effect

2 – Applications: b) Ae/Be stars

Spectropolarimetric expectations depolarization

2 – Applications: b) Ae/Be stars

Spectropolarimetric expectations rotating disk

2 – Applications: b) Ae/Be stars

Example: no line effect IL Cep

2 – Applications: b) Ae/Be stars

Example: depolarization BD+40 4124

2 – Applications: b) Ae/Be stars

Example: loop CQ Tau

2 – Applicationsc) 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)

• 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

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

2 – Applications: c) Seyfert Galaxies

Type I Seyfert

Type II

Seyfert

Jet of Radio Emission

Dusty

Torus

Broad Line Clouds

Narrow Line Clouds

2 – Applications: c) Seyfert Galaxies

• Antonucci & Miller 1985 spectropolarimetry of NGC 1068

Seyfert II

profile in

total flux

Seyfert I

profile in

polarized

flux!

2 – Applications: c) Seyfert Galaxies

Type I Seyfert

Type II

Seyfert

Jet of Radio Emission

Dusty

Torus

Broad Line Clouds

Narrow Line Clouds

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)