Maser Polarization and Magnetic fields during Massive Star Formation

W. H. T. VlemmingsASP Conference Series, Vol. 387, 2008

Do-Young Byun

Outline• Theoretical Background– Zeeman Effect– Para- and non-paramagnetic Species

• Observational results on HMSF– OH, H2O, Methanol

• Summary• KVN Single Dish Polarization

Background : Zeeman Effect• Occurs when the degeneracy of magnetic substates is

broken under the influence of B field

• Zeeman splitting– ~ 103 larger in paramagnetic molecules – OH : paramagnetic– SiO, H2O, CH3OH : non-paramagnetic– Z > D for paramagnetic case

• Z : Zeeman splitting, D : Doppler Line Width– Z < D for non-paramagnetic case

Large Zeeman Splitting : paramagnetic species

• Z > D : no theoretical ambiguity– Zeeman components are well separated and resolved. No

significant influence on each other.

– B field Strength• Observed splitting directly gives Bcos.• : angle between B and maser propagation direction

– B field direction• Magnetic transitions mF = 1 give rise to Elliptically polarized

components (V ~ Bcos, Q ~ -Bsin2). Polarization vector B• Magnetic transitions mF = 0 give rise to Linearly polarized com-

ponents (Q ~ Bsin2). Polarization vector || B

• Z < D : Complex – B field direction

• < crit ~ 55, Polarization vector || B• > crit ~ 55, Polarization vector B• 90 flip of EVPA• Cannot directly determine B field direction

– B field strength • B field strength depends on circular pol. fraction and

maser saturation level.• need to know saturation level

Small Zeeman Splitting : non-paramagnetic species

Observational Results on High Mass Star Formation

• Most information on small scale magnetic field information has come from OH maser observa-tion

• H2O and methanol maser observations are in-creasing.

• SiO maser polarization observations are rare.– Orion IRc2 : magnetic field along the disk (Plam-

beck+ 2003)

HMSF : OH masers• 1.6 GHz, Paramagnetic• Trace ~105 – 108 cm-3 density region• direct measurement of |B|

– |B| ~ 1mG , up to ~40mG (Fish&Reid 2007)

• Observation toward Cep A (Bartkiewicz+ 2005) – The B direction derived from circular pol. seems to follow

ambient B-field, not outflows

• B field measurements using 6 and 13GHz OH maser– Consistent with 1.6GHz measurements– Suffer less from Faraday Rotation than 1.6GHz

HMSF: H2O masers• First discovery of H2O Zeeman

splitting with single dish (Fiebig & Gusten 1989)

• High resolution circular pol.– Typically, |B| ~ 15-150mG at 108 –

1011 cm-3

– |B| ~ 650mG in Cep A HW2 (Vlemmings+ 2006b)

• Linear Pol– Typically 2%– Rapid change in direction over

small scale

HMSF: 6.7GHz Methanol masers• Pol. observations of the 6.7G

and 12G masers are rare.– Polarization fraction ~ 1.5% – First 6.7GHz polarization map

of W3(OH) (Vlemmings+ 2006c)

– Better probes of B direction than OH because of less Fara-day rotation

– Difficult to determine |B| due to Small Zeeman Splitting

Pol. Map of W3(OH)- Methanol : Contours and Black lines- OH : Triangles and Red lines

HMSF: mm Methanol masers• Lin. Pol. fraction of up to 40% using IRAM 30m.

(Wiesemeyer+ 2004)– Lin. Pol detection : 10/14 (Class I), 3/7 (Class II)– Cir. Pol detection towards 2 Class I sources

HMSF: Summary • |B| n0.49 over wide density

scale

• B remains partly coupled to the gas up to very dense re-gion.

• Dynamical importance of B during HMSF process in shap-ing outflows and jets.

Receiver CabinObservation

Building

43 GHz FE

22 GHz FE

Down Con-

verter

Down Con-

verter

Receiver system DAS Clock Field system

GUI

Control PC

1 pps Distributor

100 MHz Distributor

10 MHz Distributor

10 MHz Distributor

1 pps Distributor

5 MHz

10 MHz

100 MHz

1 pps

1.4 GHz

ClockSystem

H-maser

GPS

Digital O/E

WDMOptical

MUX

DigitalE/O

Sampler

Sampler

WDMOpticalDMUX

DigitalSpectro-meter

Mark5B

DIR-1000

DigitalFiltersDown

Con-verter

Down Con-

verter

Sampler

Sampler

LCP

RCP

StokesI,Q,U,V

KVN Data Acquisition Sys-tem

• 4 Samplers– 4 Receiver Outputs with 512MHz bandwidth– 2bit quantization

• Digital Filter Bank (DFB)– 16x16MHz Streams :– Total Band Width : 256MHz bandwidth

• E.g. 1x256MHz, 2x128MHz, 4x64MHz, 8x32MHx, 16x16MHz

• Digital Spectrometer (DSM)– FX Type Correlator– Auto and Cross Power Spectrum– Wide : 4x512MHz Streams– Narrow : Max 8 streams w/ 4K channels

Continuum Observation• Polarization Observation at K and Q

– 3C454.3 Monitoring– MFPOL (Multi-Freq Polarization Observation)– MOGABA (Monitoring of Gamma-ray Bright AGN)

• Sensitivity of Linearly Polarization Flux– |Q+jU| ~10mJy (~15mJy) at K(Q) for 15 min obs.

• Phase Stability– Random Error < 0.2 (0.3) deg at K (Q)– Systematic Error < 2 deg at K/Q

• Instrumental Cross Pol– |dLR – d*RL| < 5% (10%) at K (Q)

• Simultaneous Dual Frequency Polarimetry Lee,S.-S.+ in prep

I/Io V/I

Amp(Q+jU)/I Phs(Q+jU)

To Do• Spectroscopy Polarimetry using DFB+DSM

– DFB Quantization Correction, Post Doppler Correc-tion

– H2O, CH3OH Single Dish observations• 86, 129GHz Test • VLBI Polarization

– Multi-epoch H2O• Calibration Procedure

– Noise source– P-Cal System