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Masers observations of Magnetic fields
during Massive Star Formation
Masers observations of Magnetic fields
during Massive Star Formation
Wouter Vlemmings (Argelander-Institut für Astronomie, Bonn)
with Gabriele Surcis, Rosy Torres, Ramiro Franco (Bonn) Kalle Torstensson, Huib Jan van Langevelde (Leiden/JIVE, Netherlands), Richard Dodson (ICRAR, Australia)
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IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
MSF Magnetic Fields•Role relative to turbulence and gravity?
•B launches outflows, stabilize accretion disks, facilitate enhanced accretion rate?
•B suppresses fragmentation?
•Need B-field probes at high densities and small scales
• complement larger scale dust
• Zeeman splitting of masers and other spectral lines!
Pudritz & Banerjee 2005
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IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Zeeman Splitting•“splitting of a spectral line into several components in the presence of a static magnetic field”
B=0 B≠0
• Circular polarization, V∝dI/dv • Not not for all masers!
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
SF Masers
•Maser polarization observed from:
• OH (1.6 and 6 GHz)
• Faraday rotation
• H2O (22 GHz)
• Shocks
• SiO (43 and 86 GHz)
• rare, polarization interpretation
• CH3OH (methanol, 6.7, 12.2, 36 GHz)
• Common MSF maser, strong
Poster I.24, Green et al.& II.32 Torres et al.
Poster II.26, Surcis et al.
Poster II.26, Surcis et al.
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Methanol Masers• Non-paramagnetic:
• Zeeman splitting << doppler line-width
• Using g-Landé factor from 25 GHz laboratory measurements (Jen 1951):
• Vz = 49 m/s G-1 for 6.7 GHz methanol
• Circular polarization ~0.1-0.3% for mG B-fields
• Linear polarization weak (Ellingsen 2002; Vlemmings et al. 2006; Dodson 2008)
• Typical 2-3% for 6.7 and 12.2 GHz masers
• Occasionally as high as 8-10%
• Depends on e.g. maser saturation
• Analysis requires maser radiative transfer
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Effelsberg survey• Detection of Zeeman splitting requires
high SNR
• SNR~3000 needed to detect 1 mG
• Strong masers (>50Jy)
• Effelsberg 100m telescope:
• 51 hr; ~50 strong Northern methanol maser regions
• Southern sources done with Parkes MB system
• Stability problem, scan-to-scan Right-Left CP variability
• Still simultaneous 6 GHz OH (poster II.32, Torres et al.)
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Results (I)Cepheus A
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Survey Results (II)• Significant Zeeman splitting detected
in 35/47 6.7 GHz methanol maser sources associated with high-mass star formation
• Corresponds to |B|=23 ± 5 mG in the methanol region (nH2~107-109 cm-3)
• Larger than Bcrit~12 mG ⇒ dynamically important
• Average field ~6 times higher than average 1.6 GHz OH maser field
• Line-of-sight direction consistent with OH maser measurements
• Probes overall Galactic field direction?
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Further observations
•High resolution follow-up (EVN; Surcis poster II.26)
• confirmed field
• indicate connection to larger scale structure
•6 GHz OH Effelsberg survey
• B vs. density relation
W75N(Surcis et al. 2010)
1000 AU
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
The case of Cepheus A• Cepheus A HW2 ~20 M☉ @ 700 pc
(Jiménez-Serra et al. 2007; Moscadelli et al. 2009)
• Thermal radio jet, ionized gas at ~500 km/s (Curiel et al. 2006)
• Rotating dust (R~330 AU) and molecular gas (R~580 AU) disk structure ⊥ to outflow (Patel et al. 2005, Jiménez-Serra et al. 2007, Torrelles et al. 2007)
• made up of at least 3 YSOs (e.g. Comito et al. 2007)
• Flattened 6.7 GHz methanol maser structure near disk plane (R~650 AU, h~300 AU)
• infall at ~1.7 km/s (Torstensson et al. 2007, 2010)
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IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Cepheus A results
MERLIN image of the polarization of methanol masers around the outflow Cepheus A HW2
750
AU
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Cepheus A B-field
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Vlemmings et al. 2010
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
e-MERLIN MSF legacy project
•Upgrade to MERLIN array
• 4 GHz bandwidth, L,C and K-band, ~1 μJy sensitivity, 10-150 mas resolution
•‘Feedback during Massive Star Formation’
• 450 hrs allocated at C-band (5-7 GHz)
• simultaneous map radio continuum emission and the 3D magnetic field from methanol/OH masers
NGC 7538 IRS1 and associated masers
(VLA, Galván-Madrid et al. 2010)
e-MERLIN beam
IntroductionMaser PolarizationEffelsberg survey(e-)MERLINConclusions
Conclusions• Significant Zeeman splitting detected
in 35/47 6.7 GHz methanol maser sources associated with high-mass star formation
• Corresponds to |B|=23 ± 5 mG in the methanol region (nH2~107-109 cm-3)
• Larger than Bcrit~12 mG ⇒ dynamically important
• Average field ~6 times higher than average 1.6 GHz OH maser field
• High-resolution studies show relation with large scale fields
• Cepheus A: magnetic field regulated infall
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