Narrow polarized components in the OH 1612-MHz maser emission from supergiant OH–IR sources

Cohen, R. J.; Downs, G. S.; Emerson, R. F.; Grimm, Michael; Gulkis, S.; Stevens, Gary L.; Tarter, Jill

doi:10.1093/mnras/225.3.491

Cited by 65 publications

(41 citation statements)

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“…Indeed, since it is very unlikely that the magnetic field energy density in the wind is larger than its kinetic energy density, i.e., B 2 /8π ≤ ρv 2 w /2, we should expect a magnetic field not in excess of B ≤ 3.4 mG at r ∼ 3 × 10 16 cm (if it arises from a compression of the magnetic field in the wind). This value of the magnetic field is in agreement with the values obtained from polarization observations of OH masers in supergiants (e.g., Cohen et al 1987;Nedoluha & Bowers 1992), which give magnetic field estimates at distances of ∼3 × 10 16 cm of ∼1−10 mG. However, we find that the magnetic field is actually about 0.58 G, i.e., 170 times larger.…”

Section: Resultssupporting

confidence: 91%

The role of synchrotron self-absorption in the late radio emission from SN 1993J

Pérez-Torres

Alberdi

Marcaide

2001

A&A

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Abstract. The standard model for radio supernovae considers that the observed synchrotron radio emission arises from the high-energy shell that results from the strong interaction between the expanding supernova ejecta and the circumstellar medium. This emission is considered to be only partially absorbed by ionized thermal electrons in the circumstellar wind of the progenitor star. Based on a study of the radio light curves of the type II supernova SN1993J, we present evidence of synchrotron self-absorption. Our modeling of the radio light curves requires a large initial magnetic field, of about 30 Gauss, and the existence of an (initially) highly-relativistic population of electrons. We also show that while at early epochs the dominant absorption mechanism is external absorption by thermal electrons, at late epochs and long wavelengths the dominant absorption mechanism is synchrotron self-absorption. Consequently, the spectral turnover takes place at much shorter wavelengths than expected in the standard model, and at long wavelengths (≥90 cm at current epochs) the flux predictions depart substantially from those of the standard model.

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Section: Resultssupporting

confidence: 91%

The role of synchrotron self-absorption in the late radio emission from SN 1993J

Pérez-Torres

Alberdi

Marcaide

2001

A&A

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“…The polarization information is not available or neglected in the interferometric surveys of the Galactic plane (Sevenster et al 1997a(Sevenster et al ,b, 2001) in numerous single-dish studies of IRAS colour-selected objects (e.g., Eder et al 1988;Le Squeren et al 1991;te Lintel Hekkert et al 1991) and in earlier blind surveys (e.g., Bowers 1978;Baud et al 1979a,b). Earlier studies (e.g., Wilson et al 1970;Wilson & Barrett 1972) suggested that the 1612 MHz circumstellar emission is essentially unpolarized but the observations with a spectral resolution better than 0.1 km s −1 revealed several circularly polarized features (Cohen et al 1987;Zell & Fix 1991). The OH emission in the 1665 and 1667 MHz lines shows a substantial degree of circular polarization up to 50−80% Claussen & Fix 1982) but linear polarization occurs in only ∼10% of sources (Olnon et al 1980).…”

Section: Introductionmentioning

confidence: 92%

“…This finding is at odds with early reports, which demonstrated a paucity of polarized features in the 1612 MHz spectra and only occasional detections of linear polarization at 1667 MHz (Cohen 1989, for review). Thus, spectral resolution appears to be a crucial factor (Cohen et al 1987;Zell & Fix 1991).…”

Section: Occurence Of Polarizationmentioning

confidence: 99%

Polarization properties of OH masers in AGB and post-AGB stars

Wolak

Szymczak

Gérard

2011

A&A

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Context. Ground-state OH maser emission from late-type stars is usually polarized and remains a powerful probe of the magnetic field structure in the outer regions of circumstellar envelopes if observed with high angular and spectral resolutions. Observations in all four Stokes parameters are quite sparse and this is the most thorough, systematic study published to date. Aims. We aim to determine polarization properties of OH masers in an extensive sample of stars that show copious mass loss and search for candidate objects that are well-suited for high angular resolution studies. Methods. Full-polarization observations of the OH 1612 and 1667 MHz maser transitions were carried out for a sample of 117 AGB and post-AGB stars. Several targets were also observed in the 1665 MHz line. Results. A complete set of full-polarization spectra together with the basic polarization parameters are presented. Polarized features occur in more than 75% of the sources in the complete sample and there is no intrinsic difference in the occurrence of polarized emission between the three classes of objects of different infrared characteristics. The highest fractional polarization occurs for the post-AGB+PN and the Mira+SR classes at 1612 and 1667 MHz, respectively. Differences in the fractional polarization between the sources at different evolutionary stages appear to be related to depolarization caused by blending. The alignment of the polarization angles at the extreme sides of the shell implies a regular structure of the magnetic field of a strength of 0.3-2.3 mG. Conclusions. Polarized OH maser features are widespread in AGB and post-AGB stars. The relationship between the circular and linear fractional polarizations for a representative sample are consistent with the standard models of polarization for the Zeeman splitting higher than the Doppler line width, whereas the polarized features are the σ components.

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“…Such effects are not seen in normal AGB stars (e.g., Cohen et al 1987). The variability of upto a factor of 2 suggests the effect may be related to line overlap between different Zeeman components along the line of sight (Deguchi & Watson 1986).…”

Section: Summary: the Two Componentsmentioning

confidence: 94%

Bipolar Outflows from OH/IR Stars

Zijlstra

2001

Post-Agb Objects as a Phase of Stellar Evolution

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We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of ten OH/IR stars with irregular OH spectra and unusually large expansion velocities is observed at high angular resolution. The sample includes bipolar nebulae (e.g. OH231.8+4.2), bright post-AGB stars (HD 101584) and reflection nebulae (e.g. Roberts 22). The IRAS colour-colour diagram separates the sample into different types of objects. One group may contain the immediate progenitors to the (few) extreme bipolar planetary nebulae. Two objects show colours and chemistry very similar to the planetary nebulae with late IR-[WC] stars. One object is a confirmed close binary.A model is presented consisting of an outer AGB wind which is swept up by a faster post-AGB wind, with either the AGB or Post-AGB wind being non-spherically symetric. The interface of the two winds is shown to exhibit a linear relation between velocity and distance from the star, giving the impression of an accelerating outflow. The OH data confirms the predicted linear velocity gradients, and also reveals toruslike, uniformly expanding components.All sources are discussed in detail using optical/HST images where available. ISO data for Roberts 22 reveal a chemical dichotomy, with both crystalline silicates and PAHs features being present. IRAS 16342−3814 shows a dense torus with mass 0.1 M ⊙ and density of 10 8 cm −3 ; HST data shows four point-like sources located symmetrically around the nebula, near the outer edge of the dense torus.Lifetimes for the bipolar OH/IR stars are shown to be in excess of 10 4 yr, longer than normal post-AGB timescales. This suggests that the toruses or disks are nearstationary. We suggest that accretion from such a disk slows down the post-AGB evolution. Such a process could explain the link between the long-lived bipolar nebular geometry and the retarded star.

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Narrow polarized components in the OH 1612-MHz maser emission from supergiant OH–IR sources

Cited by 65 publications

References 0 publications

The role of synchrotron self-absorption in the late radio emission from SN 1993J

The role of synchrotron self-absorption in the late radio emission from SN 1993J

Polarization properties of OH masers in AGB and post-AGB stars

Bipolar Outflows from OH/IR Stars

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