Article

Observations of Circularly Polarized OH Emission and Narrow Spectral Features

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Abstract

IN a recent publication we reported the detection of linearly polarized OH emission from a localized region near the radio source W3 (ref. 1). Although the mechanism of production of the polarized emission was not understood, we pointed out that, among several possible mechanisms, the Zeeman effect was unique because it alone would give rise to circularly polarized radiation. This communication reports observations made with a circularly polarized feed horn on the 140 ft. radio telescope of the National Radio Astronomy Observatory, Green Bank, West Virginia, between November 11 and 12, 1965. The-receiver used was an autocorrelation-type radiometer with 100 channels2. Bandwidths of 2.5 Mc/s, 625, 250, 62.5 and 15.6 kc/s could be studied with a frequency resolution of 0.012 of the bandwidth. Therefore, on some occasions, our maximum frequency resolution was 190 c/s. The system temperature, including losses in the antenna feed line, was 250° K; the antenna beamwidth was 19 min of are; coupling between the two senses of circular polarization in the antenna feed horn was approximately − 15 db.

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... The brightness temperature of some of the spectral lines of these sources is as high as 10 13 -10 17 K, whereas the Doppler width of these lines corresponds to kinetic temperatures of 10-100 K that are usual for neutral clouds of interstellar gas. It was then proposed that these anomalies were the result of coherent amplification of the radio emission of the star by the surrounding stellar medium, that was in a population inverted state [5][6][7]. This mechanism could yield amplifications as large as 10 7 . ...
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... This and the detection of weaker emission at the frequencies of the F = 2 − 2 and 1 − 2 OH hfs lines (at 1667 and 1720 MHz) allowed Weinreb et al. (1965) to identify of OH as the carrier of this peculiar emission. It was readily identified as due to maser action by the latter authors because of the observed peculiar profiles with multiple, narrow velocity components of polarized emission with highly anomalous relative hfs component intensity ratios, with the 1665 MHz F = 2 − 1 line usually being the strongest (Weinreb et al. 1965, Barrett & Rogers 1966. The maser nature was clinched by interferometry, which showed that the OH emission near the prominent HII region W3 had brightness temperatures exceeding 10 9 K and arose from compact (< 0. ′′ 05) spots spread over a few arc sec (thousands of AU) and significantly offset (by 14 ′ or ≈ 8 pc) from the radio continuum emission maximum of the HII region , Moran et al. 1967, Davies et al. 1967. ...
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DOI:https://doi.org/10.1103/PhysRevLett.17.774
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