Full-Polarization Observations of OH Masers in Massive Star-Forming Regions: II. Maser Properties and the Interpretation of Polarization

Abstract

We analyze full-polarization VLBA data of ground-state, main-line OH masers in 18 massive star-forming regions previously presented in a companion paper. The OH masers often arise in the shocked neutral gas surrounding ultracompact Hii regions. Magnetic fields as deduced from OH maser Zeeman splitting are highly ordered, both on the scale of a source as well as the maser clustering scale of ~10^15 cm. Results from our large sample show that this clustering scale appears to be universal to these masers. OH masers around ultracompact Hii regions live ~10^4 years and then turn off abruptly, rather than weakening gradually with time. These masers have a wide range of polarization properties. At one extreme (e.g., W75 N), pi-components are detected and the polarization position angles of maser spots show some organization. At the other extreme (e.g., W51 e1/e2), almost no linear polarization is detected and total polarization fractions can be substantially less than unity. A typical source has properties intermediate to these two extremes. In contrast to the well ordered magnetic field inferred from Zeeman splitting, there is generally no clear pattern in the distribution of polarization position angles. This can be explained if Faraday rotation in a typical OH maser source is large on a maser amplification length but small on a single (e-folding) gain length. Increasing or decreasing Faraday rotation by a factor of ~5 among different sources can explain the observed variation in polarization properties. We suggest that almost all pi-components acquire a signficant amount of circular polarization from low-gain stimulated emission of a sigma-component from OH appropriately shifted in velocity and lying along the propagation path.

AASTeX, 57 pages including 2 tables and 21 figures (1 color), accepted for publication in ApJS

Cited by in corpus (24)

• Interferometric observations of magnetic fields in forming stars
• Super-resolution Full Polarimetric Imaging for Radio Interferometry with Sparse Modeling
• Evolution of Magnetic Fields in High Mass Star Formation: Linking field geometry and collapse for the W51 e2/e8 cores
• A new probe of magnetic fields during high-mass star formation: Zeeman splitting of 6.7 GHz methanol masers
• Methanol Maser Polarization in W3(OH)
• The Galactic distribution of magnetic fields in molecular clouds and HII regions
• Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
• ALMA reveals the magnetic field evolution in the high-mass star forming complex G9.62+0.19
• Proper Motions of OH Masers and Magnetic Fields in Massive Star-Forming Regions
• Enhanced density and magnetic fields in interstellar OH masers
• A Very High Spectral Resolution Study of Ground-State OH Masers in W3(OH)
• Large Magnetic Fields and Motions of OH Masers in W75 N
• Linear Polarization of Class I Methanol Masers in Massive Star-Forming Regions
• First images of 6.7-GHz methanol masers in DR21(OH) and DR21(OH)N
• A Review of Maser Polarization and Magnetic Fields
• VLBA Observations of G5.89-0.39: OH masers and magnetic field structure
• The magnetic field in the dense photodissociation region of DR 21
• Masers and star formation
• OH maser toward IRAS 06056+2131: polarization parameters and evolution status
• Intrinsic short time scale variability of W3(OH) maser
• Amplification of OAM Radiation by Astrophysical Masers
• The magnetic field of a magnetohydrodynamic disk wind: Water maser observations and simulations
• OH Masers in G11.904-0.141
• Physical conditions around high-mass young star-forming objects via simultaneous observations of excited OH and methanol masers