Polarization in Astronomy 天文学中的偏振

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POLARIZATION
IN
ASTRONOMY
天文学中的偏振
Cheng Zhao
2014/05/09
CONTENTS
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Introduction
Measurement
Mechanisms
Applications
References
INTRODUCTION
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Theory
Transverse electromagnetic waves.
Electric field  is the main component
interacting with matter, hence it is
used to describe polarization.
Polarization:
Asymmetry of the oscillation direction
INTRODUCTION
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Classification
Basis: oscillation feature of 
Facing the propagation direction of light, there are only two
polarization states (basis): linear polarization in x, y direction.
INTRODUCTION
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QM Description
=1
Photon
=0
Eigenvalue
of spin
gauge symmetry
Symbol
Helicity = ±ℏ
Helicity
Polarization state
m = -1
-ℏ
Left-handed
circular polarization
m = +1
+ℏ
Right-handed
circular polarization
Transformation with linear polarization basis
| = 1/ 2 (| +| )
| = −/ 2 (| − | )
INTRODUCTION
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Formalism
Stokes vector:  = (, , , )
Degree of polarization 
I:intensity
Q, U:linear component
V:circular component
Natural light: P=0
Linear polarization: P=1
(For linear polarization V=0)
MEASUREMENT
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Polarizer – Sheet or Plate Polarizers
 Sheet polarizer
Aligned anisotropic crystals or polymers. Extinction ratio
(transmitted intensity for the polarizing direction over that for
the perpendicular direction) is typically about 100.
 Wire-grid polarizer
Conducting wires with spacing smaller than the
wavelength (From sub-millimeter to optical regime). Fairly
large extinction ratios, and applicable over large wavelength
ranges.
 Brewster window
A simple piece of glass. The only polarizer in far UV
regime. Extinction ratio mostly depends on the incidence
angle.
MEASUREMENT
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Polarizer – Polarizing Beam-Splitters
Keep all the incident light as output. The polarizer acts as a
beam-splitter.
 Cube beam-splitter with multilayer coating
Fairly large extinction ratios are obtained when there are
internal reflections at the Brewster angle.
 Birefringent crystals
The refractive index in the crystal is an anisotropic tensor.
Incident light that is not in the direction of the crystal axes will
be split into two beams with different linear polarization states
and speeds.
MEASUREMENT
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起偏器
MEASUREMENT
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Retarder
Retard the phase of electric field. Manipulate polarization by
rotating the angle of linear polarization and thus converting
circular into linear polarization.
Quarter-wave and half-wave retarders
MEASUREMENT
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Retarder
MEASUREMENT
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Novel Components
Mostly owing to developments in the field of nanophotonics
and by the liquid crystal display and telecommunications
industries.
 Circular polarizer
Based on chiral nanostructures. [Gansel et al. 2009]
 Passive devices (theta cell)
Based on twisted nematic liquid crystals that locally
rotate the direction of linear polarization.
Many astronomical objects exhibit a centrosymmetric
linear polarization pattern around a central source. Such a
passive liquid crstal device aligns this pattern in one direction
so that only one Stokes parameter has to be measured. [Snik,
Frans, 2009]
MEASUREMENT
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Novel Components
 Polarization grating
Such a grating not only acts as a dispersion element, but
as a polarizing beam-splitter as well. This polarization grating
may replace the combination of a spectrograph and a
polarizer. [Packham et al. 2010]
 Polarization-maintaining (birefringment) fiber
Polarimetric versions of fiber-based integral field units.
[Lin and Versteegh 2006]
MEASUREMENT
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Detector
Similar as other astronomical instruments: low dark current,
high linearity, high dynamic range, small gain variations, etc.
 Read-out noise
Polarimetric noise levels should be smaller than photon
noise. Nonideal detector properties can create unwanted
polarization signals [Keller 1996]. Electron-multiplying CCD
(EMCCD) can reduce the influence of read-out noise.
 Read-out speed
Polarimetry often requires fast read-out rates, sometimes
>1 kHz. Single-pixel detectors like photomultiplying tubes
(PMT) and avalanche photodiodes (APD) can cope with this
speed. CCD and other 2D detectors are usually too slow.
MEASUREMENT
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Detector
 ZIMPOL (Zurich Imaging Polarimeter)
Shifting charges back and forth in synchrony with the
polarization modulation, without reading out. Developed for VLT
for the direct detection of extra solar-planets.
Advantage: images in two polarization sates are registered
practically simultaneously, no flat field problems. Atmosphric and
instrumental aberrations affecting both images are identical.
Disadvantage: The many charge transfers and asymmetries
of charge transfer limit the polarimetric performance.
MEASUREMENT
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Detector
 CMOS
Each pixel has a individual read-out capacitor [Keller, 2004].
However it may have high dark current.
MECHANISMS
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Scattering and Reflection – Theory
Fresnel equations describe the phase shift (electric
field) of the reflected light when light moves from a
medium into a second medium.
If the light is incident at the Brewster angle, the
reflection light is perfectly linearly polarized.
For complex particles having
a preferred handedness (like
biology molecules), the
reflection light can be circularly
polarized.
MECHANISMS
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Scattering and Reflection – Sources
 Solar system bodies
Microphysical properties of small particles like dust
grains and atmospheric aerosols can only be unambiguously
determined using spectropolarimetry [Hansen and Travis,
1974], such as the small sulphuric acid cloud droplets in
Venus’atmosphere.
 Stellar ejecta and
circumstellar material
Disks, jets, (AGN) halo,
exoplanets, etc.
MECHANISMS
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Multiple Scatering – Theory
When aligned dust grains (by magnetic field) are
illuminated anisotropically and scatter this light, they
can also produce circular polarization.
MECHANISMS
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Multiple Scattering – Sources
 Interstellar dust aligned with the galactic magnetic field
Smallest dimension of nonspherical dust grains are
always aligned with the direction of the local magnetic field
[Hough, 2007]. Perfect for mapping and understanding the
galactic magnetic field structure.
 Dense molecule clouds
Star formation area, disks around young stars (protostar
disk and protoplanetary disk).
MECHANISMS
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Cyclotron and Synchrotron Radiation – Theory
Non-relativistic electron gyrates around a strong magnetic
field: cyclotron radiation. Linear polarization in the direction
perpendicular to the magnetic field; circular polarization
along the direction of the magnetic field.
For relativistic electron, the radiation is
synchrotron radiation. Strongly linearly
polarized. Circular polarization is also
generated when the magnetic field has
a net component along the line of sight.
In extreme environments, synchrotron
radiation dominates.
MECHANISMS
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Cyclotron and Synchrotron Radiation – Sources
 AGN jets
 Neutron star
Study the structure of magnetic fields.
Jet of M87
Crab nebula
MECHANISMS
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Zeeman Effect – Theory
Magnetic field modifies the energy of the magnetic
sublevels of an atom (-μ·B), and splits the spectral lines.
According to the selection law of transition, Δm=0,±1.
For Δm=±1, angular momentum of the emergent photon
is ±ℏ (projection along the direction of the magnetic
field), corresponding to left-hand and right-hand
polarization respectively (σ polarization).
For Δm=0, the photon has no angular momentum
component along the magnetic field direction, and is
linearly polarized (π polarization).
MECHANISMS
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Zeeman Effect – Theory
Polarization state depends on the orientation of the
magnetic field with respect to the line of sight.
The emergent light can be red/blue-shifted by the strong
magnetic field.
Polarization due to Zeeman effect
MECHANISMS
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Zeeman Effect – Sources
 Sunspot
The magnetic field is fairly weak in this case.
 Large-scale and strong magnetic fields
Particularly applies to unresolved stars.
Surface magnetic field
of SU Aur obtained
through Zeeman effect
MECHANISMS
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Hanle Effect – Theory
When there is magnetic field along the line of sight, the
scattering linear polarization will precess around it, and
is mostly depolarized.
It is highly complementary to Zeeman effect: the
sensitive magnetic field strength is much smaller,
typically mG – 100G. Furthermore, Hanle effect can be
applied for mixed-polarity magnetic field [Trujillo Bueno,
2006 & 2009].
MECHANISMS
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汉勒效应(Hanle Effect)-原理
Hanle Effect
Polarization by different directions of magnetic field
MECHANISMS
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Hanle Effect – Sources
 Sun (corona)
A standard tool in solar physics.
 Stellar atmosphere and stellar wind
Brightness and magnetic field
of a sunspot group
APPLICATIONS
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Application of Polarization in Astronomy
Bastien, Manset, et al., 2011,
Astronomical Polarimetry 2008: Science from Small to Large
Telescopes
APPLICATIONS
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Solar Physics & Space Physics
目前,太阳表面和内部的许多物理过程和活动现象都还未被完
全了解清楚,主要原因在于获取一些物理参量(如磁场、密度
等)较为困难。
偏振被广泛地应用到了太阳物理中,例如探测日冕的电子密度、
磁场、冕流等。
太阳的许多剧烈活动现象,例如耀斑等,都与太阳上的磁暴有
关,因此偏振的观测还有助于了解太阳活动的过程、预测空间
“天气”。
APPLICATIONS
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Star Formation and Circumstellar Disks
过去的20年中,星周环境(circumstellar environment)的
模型取得了重大的进展。现在已经有了年轻星体(YSO)周围
尘埃密度分布的详尽模型,包括图像、色图与偏振图。
偏振观测能够给出尘埃的性质与排列,从而检验理论模型。
在更高的分辨率上,偏振可以用于原行星盘的研究。在光学和
近红外波段,偏振可以给出尘埃微粒非常详尽的性质,包括它
们的大小、形状与孔隙等。利用这些性质可以推断尘埃(与行
星)的演化过程和原行星盘动力学模型。
APPLICATIONS
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Stellar Evolution and Magnetic Stars
2005年后,通过光谱偏振测量,在一些低温度、低质量、充
分对流的主序星上发现偶极主导的磁场,与现有的动力学理论
相矛盾。
偏振观测有助于了解恒星内的对流、对流层与辐射层的相互作
用,从而改进恒星演化和恒星结构的理论模型。
由于对磁场和尘埃散射敏感,偏振还可用来研究演化晚期的星
体,例如中子星、超新星遗迹等。对超新星偏振辐射的研究,
可以推测超新星爆发的一些细节。
此外,由于偏振能够确定一些几何性质,它也被广泛地用来研
究双星,限制其理论模型。
APPLICATIONS
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Extrasolar Planet Detection
目前已知的地外行星中,大部分都依赖于间接观测,因为它们
相对于宿主恒星而言太昏暗了。考虑到行星反射的光是高度偏
振的,而恒星发出的光在很大程度上不具有偏振特性,现在偏
振正被用来探测地外行星。
理论上讲,对地外行星的偏振观测可以获取大量信息,例如轨
道倾角、质量、大气微粒性质、反射率、行星半径、温度等。
偏振测量因而被当作将来地外行星探测的重要手段,并能对行
星大气模型给出更多的约束。
偏振可以用于对行星云层或海洋组成的研究,而一些复杂的生
物分子能散射出圆偏振光,因此偏振又是极少的能够探测地外
生命的方法之一。
APPLICATIONS
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Interstellar Medium
从上世纪50年代起,就提出了尘埃排列理论,用以解释银河系
中的线偏振。近几年该理论取得了显著进展。
偏振光谱能够确定尘埃的组成、大小、增长和消失,从而确定
分子云与星际介质的相互作用。
在远红外和亚毫米波段观测到的去偏振效应需要用磁场来解释,
而偏振观测可以很好地确定磁场的性质,但仍需要更多高空间
分辨率的观测。
对恒星形成星云的圆偏振观测,还可能使我们了解地球生物分
子同手型的来源。
APPLICATIONS
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Galaxies and AGN
偏振在AGN统一模型的建立和发展上起了重要作用。
最近的高空间分辨率偏振观测显示出核附近的信息,有助于发
展AGN的统一理论,和研究AGN与宿主星系的相互作用。对
更高红移处AGN的偏振研究可能使我们了解AGN的演化和与
环境的相互作用。
在光学和近红外波段的光谱偏振测定,可以确定无法直接分辨
出的几何结构和散射颗粒的运动速度。
APPLICATIONS
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Cosmology
宇宙微波背景(CMB)的偏振在微开尔文的水平,有两种偏
振模式,称为E模式和B模式。
CMB的偏振证实了早期宇宙密度的不均匀性(各向同性的入
射光通过汤姆孙散射无法产生偏振),更进一步,CMB偏振
的研究,可以确定早期宇宙密度(温度)起伏的性质和成分。
通过CMB偏振的研究,可以获知宇宙早期的信息,以及宇宙
大尺度结构的形成。
REFERENCES
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Snik, Keller, 2013, Astronomical Polarimetry: Polarized Views of Stars
and Planets, Springer Science+Business Media Dordrecht, p.175
Bastien, Manset, et al., 2011, Astronomical Polarimetry 2008: Science
from Small to Large Telescopes
赵凯华, 2004, 新概念物理教程·光学, 高等教育出版社
Schmid, Gisler et al., 2005, ZIMPOL/CHEOPS: a Polarimetric Imager for
the Direct Detection of Extra-solar Planets
Trujillo Bueno, 2009, Diagnostic methods based on scattering
polarization and the joint action of the Hanle and Zeeman effects
Raouafi, 2011, Coronal Polarization
Hough, 2007, New opportunities for astronomical polarimetry, J.
Quantit. Spectr. Rad. Transfer, 106, 122–132

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