Magnetic Field Structure of Orion Source I

We observed polarization of the SiO rotational transitions from Orion Source I (SrcI) to probe the magnetic field in bipolar outflows from this high-mass protostar. Both 43 GHz J = 1-0 and 86 GHz J = 2-1 lines were mapped with ∼20 au resolution, using the Very Large Array (VLA) and Atacama Large Mil...

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Bibliographic Details
Published in:The Astrophysical journal 2020-06, Vol.896 (2), p.157
Main Authors: Hirota, Tomoya, Plambeck, Richard L., Wright, Melvyn C. H., Machida, Masahiro N., Matsushita, Yuko, Motogi, Kazuhito, Kim, Mi Kyoung, Burns, Ross A., Honma, Mareki
Format: Article
Language:English
Subjects:
Arrays
Astrophysical masers
Astrophysics
Circular polarization
Faraday effect
Field strength
Interferometers
Magnetic fields
Masers
Massive stars
Outflow
Polarimetry
Polarization
Radio telescopes
Silicon monoxide masers
Star formation
Stellar jets
Vibrational states
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Summary:We observed polarization of the SiO rotational transitions from Orion Source I (SrcI) to probe the magnetic field in bipolar outflows from this high-mass protostar. Both 43 GHz J = 1-0 and 86 GHz J = 2-1 lines were mapped with ∼20 au resolution, using the Very Large Array (VLA) and Atacama Large Millimeter/Submillimeter Array (ALMA), respectively. The 28SiO transitions in the ground vibrational state are a mixture of thermal and maser emission. Comparison of the polarization position angles in the J = 1-0 and J = 2-1 transitions allows us to set an upper limit on possible Faraday rotation of 104 rad m−2, which would twist the J = 2-1 position angles typically by less than 10°. The smooth, systematic polarization structure in the outflow lobes suggests a well-ordered magnetic field on scales of a few hundred au. The uniformity of the polarization suggests a field strength of ∼30 mG. It is strong enough to shape the bipolar outflow and possibly lead to sub-Keplerian rotation of gas at the base of the outflow. The strikingly high fractional linear polarizations of 80%-90% in the 28SiO v = 0 masers require anisotropic pumping. We measured circular polarizations of 60% toward the strongest maser feature in the v = 0 J = 1-0 peak. Anisotropic resonant scattering is likely to be responsible for this circular polarization. We also present maps of the 29SiO v = 0 J = 2-1 maser and several other SiO transitions at higher vibrational levels and isotopologues.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ab959e