William D. Watson

The Zeeman effect in astrophysical water masers and the observation of strong magnetic fields in regions of star formation

The present study solves the transfer equations for the polarized radiation of astrophysical 22-GHz water masers in the presence of a magnetic field which causes a Zeeman splitting that is much smaller than the spectral line breadth. The emphasis is placed on the relationship between the recently detected circular polarization in this maser radiation and the strength of the magnetic field. When the observed spectral line breadth is smaller than about 0.8 km/s (FWHM), it is calculated that the uncertainty is less than a factor of about 2. The accuracy is improved significantly when the angle between the line of sight and the direction of the magnetic field does not exceed about 45 deg. Uncertainty in the strength of the magnetic field due to lack of knowledge about which hyperfine transition is the source of the 22-GHz masers is removed. The 22-GHz maser feature is found to be the result of a merger of the three strongest hyperfine components.

A Non-Zeeman Interpretation for Polarized Maser Radiation and the Magnetic Field at the Atmospheres of Late-Type Giants

The linear polarization that is observed, together with likely changes in the orientation of the magnetic field along the line of sight and hence of the optical axes of the medium, can lead to the circular polarization that is observed in the radiation of the circumstellar SiO masers. A magnetic field greater than only about 30 mG is required, in contrast to 10-100 G that would be implied by the Zeeman interpretation. To assess quantitatively the likely changes in the orientation of the magnetic field, calculations are performed with representative field configurations that are created by statistical sampling using a Kolmogorov-like power spectrum.

Line Polarization of Molecular Lines at Radio Frequencies: The Case of DR 21(OH)

We present polarization observations in DR 21(OH) from thermal dust emission at 3 mm and from CO J = 1 → 0 line emission. The observations were obtained using the Berkeley-Illinois-Maryland Association (BIMA) array. Lai et al. observed this region at 1.3 mm for the polarized continuum emission and also measured the CO J = 2 → 1 polarization. Our continuum polarization results are consistent with those of Lai et al. However, the direction of the linear polarization for the J = 1 → 0 is perpendicular to that of the CO J = 2 → 1 polarization. This unexpected result was explored by obtaining numerical solutions to the multilevel radiative transfer equations for a gas with anisotropic optical depths. We find that in addition to the anisotropic optical depths, anisotropic excitation due to a source of radiation that is external to the CO is needed to understand the orthogonality in the directions of polarization. The continuum emission by dust grains at the core of DR 21(OH) is sufficient to provide this external radiation. The CO polarization must arise in relatively low density (n ~ 100 cm-3 ) envelope gas. We infer B ~ 10 μG in this gas, which implies that the envelope is subcritical.

Interacting masers and the extreme brightness of astrophysical water masers

Extreme surface brightnesses have long been reported for water masers which are well beyond what is found in self-consistent, theoretical investigations of these masers. The transport of maser radiation is calculated when two masers, having the properties inferred for the numerous weaker masers and separated by distances characteristic of the molecular cloud, are aligned to within the angular spread of the beam of maser radiation. The beam size is reduced and brightness temperatures similar to the highest observed values are readily produced. This enhanced beaming reduces the number of photons required from each maser feature to a level that can be understood within available pumping scenarios. The large number of weaker masers that have been identified in the most luminous galactic cloud of water masers (the W49 region) makes it especially plausible that alignments occur and produce the very bright masers in this region.

Astrophysical Maser Radiation from a Turbulent Medium: Application to 25 GHz Methanol Masers

Spectral and spatial distributions are calculated for astrophysical maser radiation that emerges from a turbulent medium. Turbulent-velocity fields are created by sampling from Kolmogorov-like distributions. The maps of maser emission created in a turbulent medium appear as if the emission results from a collection of isolated clumps moving with different velocities, even though the physical quantities (other than velocity) are constant within the medium. A detailed comparison is made with observational data about the 25 GHz methanol masers in OMC-1. There is evidence that key simplifications for the calculations—unsaturated masing and uniform excitation—are applicable for these masers. For the actual Kolmogorov distribution, the images are smaller and more numerous than are observed. The spectra also do not exhibit the observed irregularities. Velocity distributions that are somewhat steeper than the Kolmogorov power law lead to calculated images and spectra that reproduce characteristic features of the observations.

Formation of the HD Molecule in the Interstellar Medium

Ion-molecule, isotope exchange reactions in the interstellar gas that can form HD preferentially in comparison with H/sub 2/ are examined. Emphasis is placed upon conditions that are representative of interstellar clouds in which the unexpectedly large HD/H/sub 2/ abundance ratio has recently been observed. Enhanced abundances of the observed magnitude for HD will be produced if the D/ sup +/ + H/sub 2/ yields HD + H/sup +/ cross section has a value typical o f ion-molecule reactions. Preliminary measurements do indicate that this cross- section is large enough to produce enhancements greater than ~10/sup 2/ in the observed HD/H/sub 2/ ratio.

The Relationship between the Circular Polarization and the Magnetic Field for Astrophysical Masers with Weak Zeeman Splitting

The relationship between the magnetic field and the circular polarization of astrophysical maser radiation due to the Zeeman effect under idealized conditions is investigated when the Zeeman splitting is much smaller than the spectral line breadth and when radiative saturation is significant. The description of the circular polarization as well as inferences about the magnetic field from the observations are clearest when the rate for stimulated emission is much less than the Zeeman splitting. The calculations here are performed in this regime, which is relevant for some (if not most) observations of astrophysical masers. We demonstrate that the Stokes V parameter is proportional to the Zeeman splitting and that the fractional linear polarization is independent of the Zeeman splitting when the ratio of the Zeeman splitting to the spectral line breadth is small—less than about 0.1. In contrast to its behavior for ordinary spectral lines, the circular polarization for masers that are at least partially saturated does not decrease with increasing angle between the magnetic field and the line of sight until they are nearly perpendicular.

Dust Grains and the Luminosity of Circumnuclear Water Masers in Active Galaxies

In previous calculations for the luminosities of 22 GHz water masers, the pumping is reduced and ultimately quenched with increasing depth into the gas because of trapping of the infrared (≈ 30-150 μm), spectral line radiation of the water molecule. When the absorption (and reemission) of infrared radiation by dust grains is included, we demonstrate that the pumping is no longer quenched but remains constant with increasing optical depth. A temperature difference between the grains and the gas is required. Such conditions are expected to occur, for example, in the circumnuclear masing environments created by X-rays in active galaxies. Here, the calculated 22 GHz maser luminosities are increased by more than an order of magnitude. Application to the well-studied, circumnuclear masing disk in the galaxy NGC 4258 yields a maser luminosity near that inferred from observations if the observed X-ray flux is assumed to be incident onto only the inner surface of the disk.

Linearly polarized radiation from astrophysical masers due to magnetic fields of intermediate strength

Previous solutions for polarization of astrophysical maser radiation due to closed-shell molecules in a magnetic field have potentially serious limitations. These solutions are mostly based on the approximation that the Zeeman frequency g-Omega is much greater than the rate for stimulated emission R and the rate for decay Gamma of the molecular state. Others are asymptotic solutions obtained for an angular momentum J = 1-0 transition. It has been unclear whether the polarizations due to plausible Zeeman splittings are adequately represented by the solutions obtained for g-Omega/Gamma much greater than 1 and g-Omega/R much greater than 1. Actual masing transitions tend to involve molecular states with angular momenta that are higher than J = 1 and 0. Numerical solutions for the linear polarization are presented here which do not have the foregoing restrictions on the g-Omega and which are not limited to a J = 1-0 transition. 32 refs.

Linearly polarized radiation from astrophysical masers due to magnetic fields when the rate for stimulated emission exceeds the Zeeman frequency

The results are presented of reformulating the treatment of polarized maser radiation in the presence of magnetic fields in a way that seems somewhat more convenient for calculations with masing states having angular momenta greater than J = 1 and 0. Calculations are then performed for the case of small Zeeman splitting using idealizations which are equivalant to those made previously in calculations for a J = 1-0 transition. The results provide a complete, general description of the polarization characteristics of astrophysical maser radiation involving states of higher angular momentum of closed-shell molecules.