Magnetic Structure of Milky Way Galaxy
Derived from Observation of Variable Stars
Kyoto University
Master’s program 2nd year

Tetsuya ZenkoCollaborator
Shun Nagatomo1 ,Mikio Kurita1 ,Kino Masaru1 ,Tetsuya Nagata1
Shogo Nishiyama2 ,Noriyuki Matsunaga3
1 Department of Astronomy, Kyoto University
2 Miyagi University of Education

3 Department of Astronomy, The University of Tokyo

Nov. 24th 2016 DTA symposium @ NAOJ

Outline
473 Miras in 20’×30’ (50pc ×75pc @ 8kpc)
Galactic Center (d ~ 8kpc) objects: approximately 300
・Position Angle(PA) peaks ~ 16 degree<=> Galactic Plane 31.04 degree
Closer (d ~ 6kpc) objects: 28
・PA : distributed widely
・Average ~ 22.52degree (wide 14.11degree)First discussion about kpc-scale magnetic structureNew!!

21 Classical Cepheids in 4×20deg

・16 Cepheids show the polarization (P/σ > 2.5)
・One area objects(-6.7°≤ l ≤ -9.3°): Position Angle shows large
inclination to the Galactic PlaneIntroduction
Mathewson & Ford (1970), Heiles(2000)

Extinction

Latitude

NIR or visual wavelengthobject

Magnetic Field

dust
Observe

Longitude

Polarization parallel to the Galactic Plane@visual wavelength
Davis-Greenstein or Radiation Torque, or …

Interstellar Polarization results as INTEGRALof the line of sightObservedust clouddust cloud~1kpc

dust cloudvisual wavelength Polarization

More than 3kpc

Where do magnetic field change ?
We need distance information

How can we measure the distance?
Diﬃcult near Galactic Center(GC:d~8kpc)

Changing Magnetic Field with distance?
A few attempts in literature

Polarimetry of Red Clump Stars (Pavel 2014)
・Catalog:2MASS & GPIPS(Clemens+2012)

GPIPS:Galactic Plane Infrared Polarization Survey“Position Angle change with Distance??”

Color Excess as proxy of distance(l, b)=(19.49°,0.56°)(Hatano+2013, Nishiyama+2009, Kobayashi+1983)
・H-K color toward the Galactic center
・blue H-K color = foreground , red H-K color = backgroundCompare NIR and visual wavelengths
Blue square:3°×3°
Mathewson & Ford (1970), Heiles(2000)

Latitude

9286 objects

LongitudeHeiles(2000)
visual wavelength polarization: 29 objects @ 3×3deg GC

8Number

6d ~ 1kpc

420

155~160160~165165~170170~175175~180155
160 165170
175
0 0~5 5 5~10 1010~151515~2020 20~2525

PA(degree)However…
NIR polarization ~ 16degree (average)(Kobayashi+ 1983, Nishiyama+ 2009, Hatano+ 2013)

The number of stars

Nishiyama+(2009)

Where does the magnetic field change?

Change with distance?

Observedust clouddust cloud~1kpc

dust cloudMore than 3kpc

visual wavelength polarization

This work

NIR Polarization of variable stars
Pulsating Variable Stars : Period-Luminosity Relation=> distance determination

Data analysis

+2°

+1°

l=0°

Hatano+2013+1°

IRSF:SIRPOL,SIRIUS

N

1024pixel×1024pixel HAWAⅡ array
J,H,Ks simultaneous observationPolarimetry Hatano+ 20132006-2009
limiting magnitude(δp < 1%)
J〜 14.0mag
H〜 13.4mag
Ks 〜 12.5mag

Variability Observation

b=0°

31.04°-1°lMatsunaga+ 2009
2001-2008
20’×30’(corresponds to 50×75pc@8kpc)
549 Miras => 545 Polarimetry observation
473 Miras (Hband)Matsunaga+2009bGalactic Coordinates
0.3

25HPol 5.5%

Extinction
Ak5A(Ks)=50.2

20

Ak3
A(Ks)=3b[degree]b[degree]

0.1

15010-0.1

GalacticCenter(~8kpc)

North Celestial Pole-0.2
Galactic Plane

531.04°

Blue:less than 6kpc
(28objects)

-0.3
0.3

distance[kpc]Ak1A(Ks)=1

0.2

0.1

0l[degree]

-0.1

-0.2

-0.3

0 sun

bin = 2[degree]mode= 16.55[degree]=16.36[degree]
PositionAngle all473
Miras &average
28 Miras(d < 6kpc)
60
Miras 473
Red:473 Miras
distribution
28

Blue:28 distribution

50

Average =16.36 degree (width 11.56degree)

NumberNumber4030

20100Diﬀerent Trend
Closer(d<6kpc) objects
Average = 22.52degree(width 14.11degree)
-80

-60

-40

-20

0PA(H)[degree]

PA(H)[degree]

2040

6080

Point・Most of Mira : PA ~ 16degree
・Closer (d < 6kpc) Objects have diﬀerent trend
We need the polarimetric observation with distanceHow does the magnetic field change with distance?
・Area (20’×30’)
・Closer ( d < 6kpc) objects are a few=>Other area show the same trend?

But…
・No large Polarization survey(IR) in the Galactic Center・Few variable star survey (IR)=>Other area show the same trend?
The dependence of Polarization with longitude

Observation
Classical Cepheids(Dékány+ 2015)

VISTA Variables in the Vía Láctea (VVV)2010~2014
Range: -10.5°≤l≤10°, -1.7°≤b≤2°
Ks-band multi epoch Z,Y,J,H-band single epoch

Polarimetry observation by IRSF
2016/6/25~7/5(only 5 nights observation)seeing : 1.2 ~ 2.9 arc second @ Ks (strong wind & bad weather)
21 Classical Cepheids (blue circle)

Dékány+ 2015

VVVCepheid28 P_T=18.239day NCP=34.02degree

16 Cepheids show polarization (P/σ > 2.5)
Distance:0.2
11~13kpc(±1kpc)
7’.7×7’.7
Most of Cepheid’s PA is approximately parallel to the Galactic Plane
Newlarge inclination to the Galactic Plane !!
( -6.7°≤ l ≤ -9.3° 555pc @ 12kpc)

0.15

2
1.5
1
0.5
0-0.5
-1
-1.5
-2

b[degree]b[degree]b[degree]

VVVCepheid

0.1

0.05
50
l[degree]l[degree]-50Galactic Center: PA 17° @ ~8kpc (Polarization 2%)
-7.45

-7.5

-7.55
l[degree]

-7.6

-7.65

-10

Conclusion
473 Miras (Hband) 50×75pc@~8kpc
・28 Miras (d < 6kpc )・Most of Mira : PA ~ 16degree @ ~8kpc<=>Galactic Plane 31.04 degree
less than 6kpc…
・Closer ( d < 6kpc) objects have diﬀerent trendFirst discussion about kpc-scale magnetic structure
21 Classical Cepheids (Ks band)・16 Cepheids show the polarization (P/σ > 2.5)
・-6.7°≤ l ≤ -9.3° : PA shows large inclination to the Galactic Plane
・Other objects: PA is approximately parallel to the Galactic PlaneDiﬀerent Magnetic Structure (-6.7°≤ l ≤ -9.3°)