Derck P. Smits

Spectra of 4.7 and 6.0 GHz excited OH masers from 21 pointings towards known star-forming regions are reported. The C -band observations, using the Green Bank Telescope in full Stokes mode, have measured how polarization properties vary across the maser profiles in each spectrum and vary between different epochs of observation. Seven sources had no detections of masers at any of the OH transitions. Nine sources were observed to have 4.766 GHz masers, which include two new detections in G188.946+0.886 and G196.454–1.677. No new linear or circular polarization was found in any of these masers. Seven and two sources had 6.035 and 6.031 GHz masers, respectively. All 6.035 GHz masers had some level of circular polarization, and three also had linear polarization. Both linear and circular polarization were found in the 6.031 GHz masers in G133.947+1.064 and weak, 100% left circular polarization in G141.918+1.902. Magnetic fields have been calculated from measured Zeeman splittings. The strength and orientation of the fields agree with most previous measurements, although there are some indications that the magnetic fields could be changing. Thermal emission or absorption is seen in several of the 4.7 and 6.0 GHz transitions, many of which have not been reported before.

Spectra of 6.7 GHz methanol masers from 21 pointings of known star-forming regions are reported. C -band observations, using the Green Bank Telescope in full Stokes mode, have measured how polarization properties vary across the maser profiles in each spectrum and between different epochs of observation. Two-thirds of the sources are observed to have 6.7 GHz methanol masers, including one new detection (G240.316+0.071). Linear polarization is in the range of 0%–15%, and circular polarization ranges from 0 to ±9%, in line with previously reported values. The only instances where polarization is not observed is when these polarization ranges are below the 3 σ detection limits. Zeeman splitting is observed in several sources, with splitting values derived from the velocity separation between right circularly polarized and left circularly polarized components via Gaussian fitting. These values are seen to change with time and appear to correspond to changes in the linear and circular polarizations. The polarization properties varying across the spectra and changing with time are most likely due to variations in the magnetic fields.

We report the first detection of circular polarization in 4.7 GHz excited OH masers in star-forming regions made using full Stokes measurements with the Green Bank 100 m telescope. The Zeeman shift between the two circular components provides a measure of the magnetic field pervading these maser spots. Three different methods are used to determine the shift in velocity between the RCP and LCP components. We find fields with B ∼ 100 mG using archival molecular parameters that have limited precision and uncertain values. Reservations of using 1.7 and 6.0 GHz OH masers to estimate magnetic fields in star-forming regions are discussed.

C-band Mueller matrices for the Green Bank Telescope are presented here which enable on-sky Stokes parameters for point sources at the beam center to be determined. Standard calibrators, 3C 138 and 3C 286, were observed using the Spider program to steer the telescope across a broad range of R.A. on both sides of the zenith transit. For this analysis, only the observations at the peak of the Spider pattern were used rather than the full sweep of the runs. Therefore, the results presented here only apply to point sources at the beam center. The Mueller matrices are shown to vary with frequency and with use of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain between the X and Y components of the feed. However, the relative calibration gain can be determined from observations of a source with known polarization. Correcting the data for the relative calibration gain prior to data analysis allows for use of a frequency-independent Mueller matrix. This generic Mueller matrix is shown to provide reliable C-band polarization measurements.

C-Band Mueller matrices for the Green Bank Telescope are presented here which enable on-sky Stokes parameters for point sources at the beam center to be determined. Standard calibrators, 3C138 and 3C286, were observed using the Spider program to steer the telescope across a broad range of Right Ascensions on both sides of the zenith transit. For this analysis, only the observations at the peak of the Spider pattern were used rather than the full sweep of the runs. Therefore, the results presented here only apply to point sources at the beam center. The Mueller matrices are shown to vary with frequency and with use of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain between the X and Y components of the feed. However, the relative calibration gain can be determined from observations of a source with known polarization. Correcting the data for the relative calibration gain prior to data analysis allows for use of a frequency independent Mueller matrix. This generic Mueller matrix is shown to provide reliable C-Band polarization measurements.

A new tool to identify possible Zeeman pairs is presented here: the right circularly polarized (RCP) and left circularly polarized (LCP) components of a pair with velocity drift at the same rate. Long-term monitoring of hydroxyl transitions towards NGC 6334I-MM3 has resulted in the identification of three possible Zeeman pairs, one each at the 1.667, 6.030, and 6.035 GHz transitions, where the rate of drift of the RCP feature equals that of the LCP component. Historical observations suggest that their drift has been occurring over 40 yr. A fourth pair in the 1.720 GHz OH transition may indicate magnetic field variation over 50 yr.

The source G9.62+0.20E surprises yet again! Analysis of the associated methanol maser feature at v_lsr = +8.8 km s⁻¹ revealed a second period of P = 52.1 ± 0.3 d, along with the previously reported period P = 243 d for this source. Various explanations, harmonic flaring, Dicke’s superradiance, a secondary pulsation, or two different periodically varying radio sources in the field of view with associated methanol masers are presented. The latter two hypotheses appear more plausible but it is not clear which, if any, best describe these observations.

The first confirmed periodically varying 6.031 and 6.035 GHz hydroxyl masers are reported here. They vary contemporaneously with the 6.7 GHz methanol masers in G323.459–0.079. The 1.665 GHz hydroxyl and 12.2 GHz methanol masers associated with G323.459–0.079 are also periodic. Evidence for periodicity is seen in all features in all transitions save a single 1.665 GHz hydroxyl maser feature. Historical excited-state hydroxyl maser observations set a stricter upper limit on the epoch in which a significant accretion event occurred. The associated burst in 6.7 GHz methanol maser activity has subsided significantly while the hydroxyl transitions are brightening possibly the result of changing physical conditions in the masing cloudlets. Time lags in methanol are confirmed and may be the result of the periodic flaring propagating outward from the central region of maser activity. A possible magnetic field reversal occurred during the accretion event.

Atomic hydrogen masers occur in recombination plasmas in sufficiently dense H ii regions. These hydrogen recombination line (HRL) masers have been observed in a handful of objects to date and the analysis of the atomic physics involved has been rudimentary. In this work a new model of HRL masers is presented which uses an nl-model to describe the atomic populations interacting with free-free radiation from the plasma, and an escape probability framework to deal with radiative transfer effects. The importance of including the collisions between angular momentum quantum states and the free-free emission in models of HRL masers is demonstrated. The model is used to describe the general behaviour of radiative transfer of HRLs and to investigate the conditions under which HRL masers form. The model results show good agreement with observations collected over a broad range of frequencies. Theoretical predictions are made regarding the ratio of recombination lines from the same upper quantum level for these objects.