The structure of interstellar hydroxyl masers - VLBI synthesis observations of W3/OH/

Reid, M. J.; Haschick, A. D.; Burke, B. F.; Moran, J. M.; Johnston, K. J.; Swenson, George W.

doi:10.1086/158092

Cited by 126 publications

(98 citation statements)

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“…To preserve the low stokes V signature, we performed amplitude calibration of OH 44.8-2.3 using the AIPS task "ACFIT". In this method, each circular polarization autocorrelation spectrum was calibrated independently using the method described by Reid et al (1980). A template auto-correlation spectrum was selected for ∼20 min interval from the Los Alamos antenna at a time range with sufficiently high elevation.…”

Section: Vlba Observations and Reductionmentioning

confidence: 99%

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

Amiri

Vlemmings

Kemball

et al. 2012

A&A

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Context. SiO maser emission occurs in the extended atmosphere of evolved stars and can be studied at high angular resolution. As compact, high-brightness components they can be used as important tracers of the dynamics at distances close to the central star. The masers also serve as probes of the evolutionary path from spherically symmetric AGB stars to aspherical PNe. Very long baseline interferometry (VLBI) observations of Mira variables indicate that SiO masers are significantly linearly polarized with linear polarization fractions up to 100%. However, no information is available at high angular resolution for SiO masers in higher mass loss OH/IR stars. Theory indicates a different SiO pumping mechanism in higher mass loss evolved stars. Aims. We extend the VLBI SiO maser studies to OH/IR stars. The observations enable us to understand the SiO pumping mechanisms in higher mass-loss evolved objects and to compare them with Mira variables. Additionally, polarimetric observations of SiO masers help us to understand the magnetic field strength and morphology and to distinguish between conflicting polarization theories. Methods. The 43 GHz SiO maser observations of the OH/IR star OH 44.8-2.3 were performed with the VLBA in full polarization spectral line mode. Auxiliary EVLA observations were performed to allow for the absolute calibration of the polarization angle. The Zeeman splitting was measured by cross correlating the right and left circular polarization spectra, as well as the S-curve fitting. Additionally, we analyzed the 1612 MHz OH maser observations of OH 44.8-2.3 from the VLA archive. Results. The SiO masers of OH 44.8-2.2 form a ring located at ∼5.4 AU around the star. The masers appear to be highly linearly polarized with fractional linear polarization up to 100%. The linear polarization vectors are consistent with a dipole field morphology in this star. We report a tentative detection of circular polarization of ∼0.7% for the brightest maser feature. The magnetic field measured for this feature corresponds to 1.5 ± 0.3 G. Additionally, the distribution of the 1612 MHz OH maser emission could indicate an elongated morphology. Conclusions. The SiO masers in OH 44.8-2.3 exhibit a ring morphology. Even though the central AGB star of OH 44.8-2.3 is expected to be larger than typical Mira variables, the SiO masers occur at a similar distance from the stellar photosphere as Mira variables. The SiO masers and the 1612 MHz OH maser emission suggest a mildly preferred direction of the outflow in the CSE of this star. Significant linear polarization is measured for the SiO region of this star, which could develop from either collisional or radiative pumping. In either case, the observed polarization is also consistent with magnetic field structures along the preferred outflow direction. This could reflect the possible role of the magnetic field in shaping the circumstellar environment of this object. Although we cannot firmly distinguish between the different polarization theories, the derived magnetic...

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Section: Vlba Observations and Reductionmentioning

confidence: 99%

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

Amiri

Vlemmings

Kemball

et al. 2012

A&A

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“…Using a larger window (generally twice the clustering radius) can be justified for two reasons. First, the window radius of 3 Â 10 15 cm is based on the clustering scale analysis of Reid et al (1980), which is itself based on only one source: W3(OH), assuming the photometric distance of 2.2 kpc (Humphreys 1978). The kinematic distance of 4.3 kpc, assuming the NH 3 local standard of rest (LSR) velocity of À45 km s À1 (Reid, Myers, & Bieging 1987) and a flat R 0 ¼ 8:5 kpc, ÂðRÞ ¼ 220 km s À1 rotation curve, would correspond to a clustering scale of 6 Â 10 15 cm, or twice that which is derived from the photometric distance.…”

Section: Zeeman Pair Selection Criteriamentioning

confidence: 99%

Interstellar Hydroxyl Masers in the Galaxy. II. Zeeman Pairs and the Galactic Magnetic Field

Fish

Reid

Argon

et al. 2003

ApJ

115

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We have identified and classified Zeeman pairs in the survey by Argon, Reid, & Menten of massive starforming regions with 18 cm ( 2 Å 3=2 ; J ¼ 3=2) OH maser emission. We have found a total of more than 100 Zeeman pairs in more than 50 massive star-forming regions. The magnetic field deduced from the Zeeman splitting has allowed us to assign an overall line-of-sight magnetic field direction to many of the massive starforming regions. Combining these data with other data sets obtained from OH Zeeman splitting, we have looked for correlations of magnetic field directions between star-forming regions scattered throughout the Galaxy. Our data do not support a uniform, Galactic-scale field direction, nor do we find any strong evidence of magnetic field correlations within spiral arms. However, our data suggest that in the solar neighborhood the magnetic field outside the solar circle is oriented clockwise as viewed from the north Galactic pole, while inside the solar circle it is oriented counterclockwise. This pattern, including the magnetic field reversal near the Sun, is in agreement with results obtained from pulsar rotation measures.

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“…Once interferometric maps became available, one usually finds that the proposed Zeeman components came from widely separated regions in a same maser source. Verification of OH maser sources came from VLBI observations (Reid et al 1980;Zheng et al 2000) which showed some pairs to come from the same position within the size of a maser spot. OH masers have now been detected in the prestellar cores of clouds, L1544 (Crutcher & Troland 2000).…”

Section: The Zeeman Effectmentioning

confidence: 99%

Milestones in the Observations of Cosmic Magnetic Fields

Han

Wielebinski

2002

Chin. J. Astron. Astrophys.

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Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from analysis of rotation measures of extragalactic radio sources and pulsars, which can be used to construct the 3-D magnetic field structure in the Galactic halo and Galactic disk. Radio synchrotron spurs in the Galactic center show a poloidal field, and the polarization mapping of dust emission and Zeeman observation in the central molecular zone reveal a toroidal magnetic field parallel to the Galactic plane. For nearby galaxies, both optical polarization and multifrequency radio polarization data clearly show the large-scale magnetic field following the spiral arms or dust lanes. For more distant objects, radio polarization is the only approach available to show the magnetic fields in the jets or lobes of radio galaxies or quasars. Clusters of galaxies also contain widely distributed magnetic fields, which are reflected by radio halos or the RM distribution of background objects. The intergalactic space could have been magnetized by outflows or galactic superwinds even in the early universe. The Zeeman effect and polarization of sub-mm and mm emission can be used for the study of magnetic fields in some Galactic molecular clouds but it is observed only at high intensity. Both approaches together can clearly show the role that magnetic fields play in star formation and cloud structure, which in principle would be analogous to galaxy formation from protogalactic clouds. The origin of the cosmic magnetic fields is an active field of research. A primordial magnetic field has not been as yet directly detected, but its ⋆

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The structure of interstellar hydroxyl masers - VLBI synthesis observations of W3/OH/

Cited by 126 publications

References 0 publications

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

Interstellar Hydroxyl Masers in the Galaxy. II. Zeeman Pairs and the Galactic Magnetic Field

Milestones in the Observations of Cosmic Magnetic Fields

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