可視域の偏光 vs. サブミリ波偏波−
星間塵の整列機構
Polarization in the optical and submm
regions: Grain alignment theories
松村雅文＠香川大学教育学部 & BISTRO-J TeamMasafumi Matsumura & BISTRO-J TeamMy Talk:• A brief review of grain theories• Davis and Greenstein theory (DG)
• Radiative Torque (RAT)• # Sorry, but some slides are the same as in my previous talk…

• The observed direction of pol (position angle) really shows
B-field?

• Optical vs. submm polarizations• An example: L1689 stream

Some phrases by an author…
• Cugnon P. (1987)“… What concerns coherent derivations of themagnetic field strength, my general impression
remains rather pessimistic, but not desperate.”in the proceedings of “Interstellar Magnetic Fields” ed. Beck & Graeve, pp.100-109

Grain alignment with DG mechanism• Discovery of polarization in distant stars• Alignment with paramagnetic relaxation

Hall 1949, Hiltner 1949
Davis and Greenstein 1951

• Phase lag of magnetization → torque that makes angular momentum J // B
• Alignment time scale τDG:
Draine & Weingertner 1996

• Rotational damping by gas drag:α1, δ: shape factor, α1～δ• τDG >> τdrag DG is very slow process. No alignment? A “mystery” in 1950-1980s.

Some phrases by other authors…
• Draine and Weingartner (1996)“... It therefore appears that the longstanding
mystery has been solved -- the observedalignment of interstellar grains is due to radiative
torques produced by anisotropic starlight!”
AAS 189, 1602

• Breakthrough:light scattering calculation“Dicscrete-Dipole Approximation”(DDA)
Draine 1988 ApJ 333, 848

Grain alignment by Radiative Torque (RAT)• Dolginov & Mytrophanov 1976, Draine & Weingartner 1996, 1997, Lazarianet al. many papers• Breakthrough “Discrete Dipole Approximation” (DDA) method Draine 1988

• Irregular Grains rotate rapidly by RAT → magnetized by the Barnett effect
DW 1997• → Larmor precession around interstellar magnetic field B0, i.e. directions ofJ and B0 is related, with frequency ΩB• Period of the Larmor precession τB = 1/ ΩB ～ 0.1 year ... very rapid!

Observations of Grain Alignment• Not all grains are aligned…
• Smaller grains are poorly (or not?) aligned: polarization is small in UV• Polarization is enhanced under strong radiation:• Andersson, B-G et al. 2011, AA 534 A19• HD 97300 in Chamaeleon I cloud• Matsumura et al. 2011, PASJ 63, L43• Pλ / Aλ in Pleades and other regions• Grains in in very dense regions: Starless cores Av > 20 mag.
• Jones et al. 2015, AJ 149, 31• All those observations support alignment by radiative torque (RAT)!

Ext. vs. Pol. in optical• Extinction: Aλ increases with 1/λ• Polarization Pλ takes a maximum
λmax ～ 0.4 ～ 0.8μm• → Polarization is relatively small inshorter wavelength.
• → Grains effective for shorter
wavelength, i.e. smaller grains, are lessaligned.
Fig.: Matsumura +2011 PASJ 63, L43

Tdust is high, i.e. strong radiation,→ then λmax & β are small.
（= Smaller grains are also aligned.）... This evidence supports RAT.
Tdust: Schlegel+ 1998. COBE & IRAS
Matsumura + 2011, PASJ 63, L43

Starless Cores: L183 & L43. Jones + 2015p/τ ∝ 1/τ for Av > 20 mag.
→ They suggest that grains are not alignedin Av > 20 mag.

nIRsubmm

L183

Andersson+ 2015

But, systematic difference exists
between observational methods?

Pipe-109 in Pipe nebula Alves + 2015
Opt.
submmnIR
Opt.

submmnIR

Figs are from:
Andersson+ 2015
ARAA 53, 501

Optical Polarization vs. Submm Polarization• Planck Collaboration 2015, intermediate results XXI, AA 576, 106

Optical Polarization vs. Submm Polarization (cont.)
Grains in diffuse regions:
E(B-V) < 0.8 mag.
• Beam size and los components• Dust properties in optical and submm• (Psubmm/Isubmm) / (pv/τｖ）• ～ 4.2±0.2

compatible with dust models• Ps/pv• ～ 5.4±0.2 MJy/sr

too low by factor of 2.5• → constraining dust models• Good correlations are amazing, but is it
always the case?

Planck Collaboration 2015,
intermediate results XXI, AA 576, 106

BUT,
PA really shows
the B-field?
The alignmentof grains mightchanges?
Orin KL
Rao+ 1998 ApJL 502,
L75

Fortunately,
this is not the case...

• Tang + 2010 ApJ 717, 1262

L1689 Stream• Red: Planck
353GHz submm
polarized flux
• binning:
8.5′×8.5′• The directions are90 deg. rotaed,i.e., the B-field.

• Yellow: Opticalpolarization byVrba + 1976

Polarization
around L1689(enlarged)• Red: Planck353GHz submm
polarized flux
• no binning
1.7′×1.7′• Yellow: Opticalpolarization by
Vrba + 1976

Binning 5x5,i.e.
8.5’×8.5’

Catalog ofGalactic coldclumps• Planck Coll. 2015 AA594, A28

Directions of the streams estimated cold clumps• Planck Coll. 2015 AA 594, A28
σ Sco• Binning along the line
parallel to the filament
direction.

QU plane

p: distance from theorigin:p = (q2+u2)0.5
Position Angle:
PA = 0.5*atan(u/q)
Clark, D. 2010
“Stellar Polarimetry”
p.92

↓U↑

QU planeU↑

PAgal = 45 deg.

→Q→Q

PAgal = 90 deg.

PAgal = 0 deg.
or 90 deg. for B～ PAeq = -50 deg.
or 40 deg. for B
PAgal = 135 deg.

• Direction of submm pol. No rotaion.

• 40 deg. Rotation in QU(= 20 deg. Rotaion in PA)

A

B

C

D→ NorthU↑→ NorthB A
D→QC→ North• 12COFCRAO data

12COABC

D→ NorthU↑

13COA

A

B

DC→ North

D→QB

C→ North12COU↑

13CO→Q• L1688 Stream• Only “50 deg.”
componentTemperature T2
in 2comp-model
Meisner &Finkbeiner 2015

Summary• We can now understand grain alignment qualitatively.
• Optical and Submm polarization:• Correlations are good!
• It can be understood that both are due to aligned, probably larger, grains.

• So far, at least in diffuse clouds, no evidence is reported that grains arealigned other than to B-fields.
• Comparing those properties, we can understand more about dust grains
and/or B-fields structures.

• New 2D information in submm pol. by Planck, POL2, ALMA, etc. hasbeen significantly advancing our understandings of ISM.
• E.g. ρ Oph streams

That’s it! Thank you!

• … that all
astronomers
follow the longstanding
convention …
https://www.iau.org/news/announcem
ents/detail/ann160
04/

The position angle in theHEALPix is different thanthat in IAU coordiantes,
which have been used byastronomers!

• Gorski + 2010 “The HEALPix
Primer”
https://healpix.jpl.nasa.gov/html/intronode12.htm

"Letter" from Piero Benvenuti – General Secretary• The issue: Scientists working on the polarization of the Cosmic MicrowaveBackground (CMB) use a convention for the polarization angle (PA) which is
opposite to the IAU approved standard. This may cause confusion andmisunderstandings.
• Background: The convention astronomers follow for the PA (Polarization Angle)
goes back to the 19th century and it has been in use for observations going fromradio to gamma rays: the PA increases counter-clockwise when looking at thesource. …
• Recommendation: The IAU recommends that all astronomers, including thoseworking on the CMB, follow the IAU Resolution for the Polarization Angle in all
their publications.
Paris, December 8th, 2015
https://www.iau.org/news/announcements/detail/ann16004/