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Observations of Circularly Polarized OH Emission and Narrow Spectral Features
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.
... 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 . ...
Experimental realization of mirrorless lasers in the last decade have resulted in hectic activity in this field, due to their
novelty, simplicity and ruggedness and their great potential for application. In this article, I will review the various developments
in this field in roughly chronological order, and discuss some possible applications of this exciting phenomenon, also termed
as ‘random lasing’.
... 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. ...
The hydroxyl radical (OH) is found in various environments within the
interstellar medium (ISM) of the Milky Way and external galaxies, mostly either
in diffuse interstellar clouds or in the warm, dense environments of newly
formed low-mass and high-mass stars, i.e, in the dense shells of compact and
ultracompact HII regions (UCHIIRs). Until today, most studies of interstellar
OH involved the molecule's radio wavelength hyperfine structure (hfs)
transitions. These lines are generally not in LTE and either masing or
over-cooling complicates their interpretation. In the past, observations of
transitions between different rotational levels of OH, which are at
far-infrared wavelengths, have suffered from limited spectral and angular
resolution. Since these lines have critical densities many orders of magnitude
higher than the radio wavelength ground state hfs lines and are emitted from
levels with more than 100 K above the ground state, when observed in emission,
they probe very dense and warm material. We probe the warm and dense molecular
material surrounding the UCHIIR/OH maser sources W3(OH), G10.62-0.39 and NGC
7538 IRS1 by studying the rotational
transition of OH in emission and, toward the last source also the molecule's
ground-state transition in absorption. We used the
Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe these OH
lines, which are near 1.84 THz (m) and 2.51 THz (m). We
clearly detect the OH lines, some of which are blended with each other.
Employing non-LTE radiative transfer calculations we predict line intensities
using models of a low OH abundance envelope versus a compact, high-abundance
source corresponding to the origin of the radio OH lines.
Until Karl Jansky's 1933 discovery of radio noise from the Milky Way, astronomy was limited to observation by visible light. Radio astronomy opened a new window on the Universe, leading to the discovery of quasars, pulsars, the cosmic microwave background, electrical storms on Jupiter, the first extrasolar planets, and many other unexpected and unanticipated phenomena. Theory generally played little or no role – or even pointed in the wrong direction. Some discoveries came as a result of military or industrial activities, some from academic research intended for other purposes, some from simply looking with a new technique. Often it was the right person, in the right place, at the right time, doing the right thing – or sometimes the wrong thing. Star Noise tells the story of these discoveries, the men and women who made them, the circumstances which enabled them, and the surprising ways in which real-life scientific research works.
A survey of H2CO absorption in dark clouds in IC 1795 (W3) shows that the material in the direction of W3 can be distinctly separated into three groups: one, immediately surrounding W3, has a uniform velocity of about —41 km−1 s; the second, associated with the W3 (OH) source has a velocity of about —49 km−1 s; the third, as a velocity of about —20 km−1 s and may be associated with an interarm spur in the foreground.
A survey of H 2 CO absorption in dark clouds in IC 1795 (W3) shows that the material in the direction of W3 can be distinctly separated into three groups: one, immediately surrounding W3, has a uniform velocity of about −41 km –1 s; the second, associated with the W3 (OH) source has a velocity of about −49 km –1 s; the third, as a velocity of about −20 km –1 s and may be associated with an interarm spur in the foreground.
In 1954 Dicke [1] discussed the possibility of a coherent cooperative process between an ensemble of two-level atoms. His paper has become a classic, and discusses the process now known as super-radiance. At the time the paper attracted relatively little attention except that of a few who worked in r.f. spectroscopy.
Progress in modern radio astronomy led to the discovery of space masers in the microwave range, and it became a powerful tool for studies of interstellar star-forming molecular clouds. Progress in observational astronomy, particularly with ground-based huge telescopes and the space-based Hubble Space Telescope, has led to recent discoveries of space lasers in the optical range. These operate in gas condensations in the vicinity of the mysterious star Eta Carinae (one of the most luminous and massive stars of our Galaxy). Both maser and laser effects, first demonstrated under laboratory conditions, have now been discovered to occur under natural conditions in space too. This book describes consistently the elements of laser science, astrophysical plasmas, modern astronomical observation techniques, and the fundamentals and properties of astrophysical lasers. © Vladilen Letokhov and Sveneric Johansson 2009. All rights reserved.
The following topics were dealt with: from quantum physics to quantum electronics, noncoherent and nonresonant scattering feedback, space masers with scattering feedback, stellar lasers with a resonance scattering feedback.
The high intensity anomalous OH emission which has been detected in the neighbourhood of HII regions exhibits remarkable polarization characteristics, narrow spectral features, and unusual ratios of line strengths. All of these unusual properties are generally believed to be attributable to maser action of some form. A knowledge of the structure of the emitting regions and their brightness temperature is necessary for the development of a satisfactory theory of the emission mechanism.
DOI:https://doi.org/10.1103/PhysRevLett.17.821
DOI:https://doi.org/10.1103/PhysRevLett.17.450
RECENT radio astronomical investigations of the 18-cm ground-state xwedge-doublet transition of the OH radical have suggested that maser amplification of the radiation is taking place in the interstellar medium. So far, no entirely satisfactory theoretical explanation of the observations has been published. A number of workers (for example, Cook1 and Litvak et al.2) have suggested that optical pumping of the OH molecule is responsible for the apparent population inversion. Another possibility is that the OH molecule is formed in an excited state in the vicinity of HII regions. In either case, an appreciable number of OH molecules may be found in excited levels. If the populations of the excited rotational levels of the ground electronic and vibrational state were sufficiently large, it would be expected that it would be possible to observe xwedge-doublet transitions originating in these levels.
With the aid of nanosecond laser flash photolysis OH is chemically produced and observed “collision free”. The rotational states show inverted population in the A doublets J = , , , and of 2Π and anti-i higher rotational states. The intensities of the main and satellite absorption lines further indicate an inversion in special hyperfine states. Application to interstellar OH maser emission is considered.
In this paper we consider a general problem of astrophysical lasers operating with optical pumping by accidental resonance (PAR) with strong spontaneous emission lines of other elements. The origin of population inversion and the coefficient of amplification due to the optical excitation of atoms (ions) by a four-level scheme with PAR are analyzed. We elucidate conclusively the question of the manifestation of astrophysical lasers in the optical region of the spectrum, which in principle differs from that of astrophysical masers in the microwave range.
The observation of the OH radicals, formed in the reaction of O(1D) atoms with water, was carried out by their uv absorption. The four branches Q1, Q21, R1 and R21 were registrated for the rotational states K'' = 1 to K'' = 6. The measured intensity ratios of corresponding main and satellite lines deviate strongly from those calculated from the transition probabilities. A non thermal population of the hyperfine levels, resulting from the reaction is supposed and the consequence for maser amplification will be discussed.
The radio line emission of interstellar molecules routinely shows deviations from thermal equilibrium which culminate with strong maser radiation in some sources. Like its laboratory counterpart, the maser radiation is amplified through the effect of induced processes in a region where a population inversion exists and displays many of the basic features of laboratory lasers. This review is intended to explain the basic theory of astronomical masers and to survey the theoretical models which were developed for specific sources.
DOI:https://doi.org/10.1103/PhysRevLett.17.452
The available data on the microwave spectrum of the hydroxyl radical are critically reviewed for information applicable to radio astronomy. Molecular properties such as the rotational constants, spin-orbit, spin-spin and hyperfine coupling constants and centrifugal distortion parameters employed in or derived from the analysis are tabulated. All the observed and predicted transitions of 16OH, 16OD, and 18OH below 300 GHz and lower state energy levels less than 4000 cm−1 are presented for the ground vibrational state. The laboratory data on 17OH is included, but no predicted transitions are presented due to the limited data available. In addition to the transition frequencies the table contains the calculated line strengths and energies of the levels involved in the transition. An extensive bibliography of laboratory and astronomical studies of the hydroxyl radical is presented as an aid to workers in both fields.
DOI:https://doi.org/10.1103/PhysRevLett.17.774
The theory of Heer and Graft is used to develop expressions for the first- and third-order contributions to the electric polarization of a maser medium undergoing simultaneous maser action on a number of hyperfine transitions. Detailed expressions for the electric polarization of the 2Pi3/2 state of OH are given for simultaneous maser amplification of the (22)-1667, (11)-1665, (21)-1720, and (12)-1612 megahertz transitions between the hyperfine levels (F+F-) of the Lambda doubled states. If the level occupation and widths are comparable in magnitude for all levels and maser action occurs, the analysis of the resulting nonlinear problem indicates that all four transitions saturate toward circular polarization and is in accord with the suggestion of Heer. The state of circular polarization of the (11), (21), and (12) transitions is strongly dependent on the (22) transition, and each line saturates toward a state of circular polarization opposite to the state of circular polarization of the (22) line. These conclusions require that the saturation line width exceed the Zeeman splitting. A formulation of the exact problem is given and saturation line widths are discussed.
An improved version of the theory of amplified spontaneous emission (ASE) is presented and shown to be satisfied by experimental data obtained using the 3.39-μm transition of He-Ne. The new approach allows the effect of an external signal injected at one end of the system to be investigated and its influence on both the positive-going and negative-going waves to be calculated for He-Ne. A special case occurs when the injected signal is replaced by a mirror which reflects the negative-going wave back into the medium and the theory of this commonly occurring laboratory system is experimentally verified at 3.39 μm. The treatment allows the importance of the interaction between spontaneous and stimulated emission to be considered. The beam divergence predicted by the theory is evaluated and compared with experiment. The spectral distribution for a pure ASE system is considered, as is the distribution of inversion which results from the positive- and negative-going waves and which is shown to have minima at the medium extremities. Finally the implications of this work for the problem of interstellar OH emission is considered.
A number of 1720-MHz OH maser sources have relatively simple spectra consisting of doublets with opposite senses of circular polarization. Such doublets could be due to the Zeeman splitting of the ²Pi(3/2)F = 2 to 1 transition. A 3-station VLBI experiment was conducted to measure the spatial separation of the emitting regions of the components of opposite-sense polarization. The doublets in the 1720-MHz OH spectra of W3(OH), W51, NGC6334(N) and NGC7538(N) are found to be spatially coincident to within a linear dimension of about 100 AU, suggesting that they originate from the same region in space. This is taken to be strong evidence for identifying the doublets as Zeeman components. The magnetic field within the OH maser sources responsible for the observed splitting is typically a few times .001 gauss. (GRA)
The existence is reported of a spike and a plateau-like emission feature
in the Orion nebula located in or near the Kleinmann-Low nebula and the
Becklin-Neugebauer source. Values are presented for the antenna
temperatures in the directions of the spike and the plateau, and it is
concluded that the gas-phase chemical composition of the Orion molecular
cloud is not homogeneous and that a change from the usual dominance by
carbon-containing molecules to dominance by inorganic molecules is
confined to a small region distinguished by large internal motions. Two
papers estimating the magnetic field strengths of the Orion
nebula/molecular cloud are reviewed. It is argued that the strengths of
the magnetic fields are uncertain by several orders of magnitude and
that the large motions in the region dominated by inorganic molecules
are responsible for the broad millimeter-wavelength lines of SO observed
in the molecular cloud, rather than the large magnetic fields as
suggested in one of the previous papers.
In order to define the spatial relationship of maser emissions from
different transitions of the hydroxyl radical and to provide constraints
on how OH masers are pumped, attempts were made to determine accurate
relative positions of the emission from all ground-state transitions in
two sources. Four ground-state transitions were observed from the
sources W3(OH) and Sgr B2. The positions of the strongest maser features
at 1612, 1667, and 1720-MHz were obtained relative to the 1665 MHz
feature by an analysis of the fringe-rate residuals. All four
ground-state masers come from the same general region, within 2 arcsec
of the center of a very compact H II region, and thus all appear to
share a common source of excitation. A comparison of total power spectra
suggests that most OH maser features have lifetimes greater than several
decades.
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The authors have conducted a six-station, VLBI observation of the ground
state 1665 MHz OH emission source in W3(OH) with full polarization
information. They have been able (1) to produce spectral line VLBI
synthesis maps in all Stokes parameters; (2) to identify several Zeeman
pairs; (3) to determine the percent of linear, circular, and total
polarization of the emission of each feature; and (4) to infer some
information about the three-dimensional structure of the magnetic field.
Eighty-one maser components stronger than 1.5 Jy were detected.
Sixty-five exhibited pure circular polarization. These observations
suggest that the OH cloud is permeated with a magentic field having an
average strength of 6 mG.
: The two-element variable baseline interferometer of the Owens Valley Radio Observatory has been used to determine positions of the OH sources near the galactic radio sources W3, Ori A, NGC6334, Sgr B2, W49, W51 and W75. The accuracy of the absolute positions is 4 sec. to 6 sec., that of relative positions of different features belonging to one source is approximately 3 sec. The relative distances between different components of one source seem to be either between 0.0 and 0.1 pc or between 1 and 10 pc. The sample of sources may be too small to conclude whether this is a significant property of OH emission centers. There is a strong indication that four of the OH sources (W3, Ori A, W51 and W75) are located near regions of infrared activity. For three sources spectra in terms of Stokes parameters are given for the 1665.401 MHz line and for the 1667.358 MHz line. The remaining four sources have been observed at 1665.401 MHz only. Strong circular polarization characterizes the spectra of most sources, but many unpolarized and weakly polarized features have been found in the spectra of the Ori A and W51 sources. (Author)
A detailed classification of the available data on W3 is presented.
Radio, optical and infrared maps of the object are shown in order of
increasing angular resolution. The physical parameters and energy
spectra of the various components of W3 and their sources are presented.
A summary of the current theories on W3 is also included.
MANY experimental data have been collected recently concerning emission at radiowavelengths from molecules in the interstellar medium, particularly from OH. It has been increasingly argued that stimulated emission occurs in the medium and that this accounts for the anomalously high intensity and the nature of the polarization of the OH radiation. Turner1 has reviewed this field and has given an expression for the stimulated emission intensity from an ensemble of molecules possessing a population inversion, in terms of the amplification of both the background radiation and the spontaneous emission from the medium.
A review of lasers operating under natural astrophysical conditions is presented. A principal difference between the conditions for obtaining inversion and amplification in an astrophysical laser and in well-known astrophysical masers is considered. The data for a recently discovered Fe II ion astrophysical laser are presented. The parameters of the astrophysical laser amplifier are discussed and the possibility of its conversion into the astrophysical laser with the incoherent feedback is considered, as well as the methods for studying the width of the emission spectrum of the astrophysical laser.
The present state of the art concerning astrophysical lasers (APL) and nonlinear optical effects under astrophysical conditions is reviewed. The operational conditions of an APL (amplification under non-LTE conditions) and astrophysical predecessors of the lab-oratory lasers are considered in the introduction of the review. The rareness of observed APL action in the visible range in comparison with astrophysical masers (APM) in the microwave range is explained. Early proposals of APLs with collisional and optical pumping are discussed. APL/M in the mid-IR and submillimeter ranges linking APL and APM are also discussed. APLs in the Weigelt blobs of Eta Carinae operating in FeII and OI with a Bowen type optical pumping are considered in detail. General questions (narrowing of APL spectral lines, the possibility of scattering feedback and ways of measuring the true spectral width of an APL) are considered. Nonlinear optical effects in astrophysical conditions and resonance-enhanced two-photon conditions, in particular, are discussed in the conclusion.
The paper describes the early endeavours by radioastronomers to detect the weak signature of the Zeeman effect in interstellar neutral hydrogen clouds in an effort to measure the Galactic magnetic field strength. The search is set in the context of the neutral hydrogen programme at Jodrell Bank. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
A brief historical account is given of the discovery of OH emission and the experiments that led to the identification of the radiation mechanism as maser amplification. An outline is presented of the progress by the MIT, CfA and Jodrell Bank groups in mapping the prototypical maser, W3(OH). The measurements of the structure and proper motions of masers are described and their limitations explained. Studies of the structure of extragalactic water vapor masers are summarized.
An upper limit of 0.02 second of arc has been determined for a hydroxyl radical (OH) emission region associated with the radio source W3, with the use of a Michelson interferometer consisting of two radio telescopes 845 kilometers apart. Timing was provided at the stations by independent atomic frequency standards. The 1665-megahertz radiation was translated to video frequency and recorded digitally on magnetic tapes which were later processed by computer, yielding fringe phase and amplitude as a function of frequency over the received bandwidth.
The Stokes parameters were measured as a function of frequency for the anonmalous 1665-megacycles-per-second OH emission originating near the thermal radio source W3. The emission is highly polarized, and the polarization parameters vary rapidly with frequency. The observed polarization can be described in terms of narrow, roughly Gaussian, emission features, all with uniform polarization but with several features overlapping without coherence near the center of the spectrum. Most of the individual features may be 100-percent polarized. Detailed examination of the brightest features suggest that they are not exactly Gaussian in shape.
Anomalous OH emission sources, emphasizing differences involving radio polarization properties, linewidths and angular sizes
A group of stars embedded in an emission nebula near the cluster NGC 7822 has been discovered to have a unique interstellar spectrum, characterized by exceedingly strong CN lines. Evidence is presented favoring the hypothesis that the CN molecules arise in the immediate neighborhood of the H 11 region boundary. It is further suggested that the CN molecules are formed in the solid particles of the Hr
region when flashed by the UV radiation in the advancing ionization front.
WE have previously found1 intensity ratios of the four microwave lines of OH in disagreement with the theoretical and laboratory ratios2 of 1 : 5 : 9 : 1 for the lines at 1,612, 1,665, 1,667 and 1,720 Mc/s. For the strong OH absorption of the radio source Sagittarius A it appeared that the ratios might be attributed to high opacity3. We now report a more detailed series of measurements which shows that high opacity alone cannot account for the observations and that disturbed populations of the levels have to be invoked.
