All Figures

$\begin{figure} \par\mbox{\includegraphics[width=8.2cm,clip]{figures/2971fi1a.ps}... ...ce*{5mm} \includegraphics[width=8.2cm,clip]{figures/2971fi1b.ps} } \end{figure}$ Figure 1: Images reproduced from P01 showing observations of OH maser and continuum emission at EVN+MERLIN resolution a) Greyscale shows the velocity integrated OH emission at a resolution of $34\times 29$ mas. The contours show continuum emission at the same resolution. Contour levels are at -1, 1, 2, and 4 times the 3 $\sigma$ noise of 0.42 mJy/beam. b) Corresponding OH maser velocity centroid field. The greyscale is between 8224 (lightest grey) and 8300 km s^-1 (black). Contours are from 8230 km s^-1 increasing at 5 km s^-1 intervals up to 8290 km s^-1.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=8cm,clip]{figures/2971fi2a.ps}\hspace*{1.5cm} \includegraphics[width=7cm,clip]{figures/2971fi2b.ps} } \end{figure}$ Figure 2: a) Sketch of the proposed source geometry. The inner dark grey ring represents the region where OH masing clouds are confined. The outer light grey ring depicts an isosurface of the smoothly distributed continuum emissivity. Note that although the radius of peak continuum emissivity lies outside the OH maser ring, some continuum emission interpenetrates and even lies within the OH ring (not shown in this figure). This geometry explains the large line to continuum ratio on the eastern side of the source because here the majority of the continuum is background to the OH. In contrast on the western side only the smaller fraction of the continuum which interpenetrates and lies inside the OH maser zone is available as a source of seed photons. In order to explain the relative weakness of the absolute brightness of both line and continuum on the eastern side of the source (see Fig. 1) the model also includes a region of free-free absorption within a bicone which covers the eastern side of the source. b) Detailed representation of the OH maser ring indicating positions and dimensions referred to in the main text. Arrows indicate components of cloud rotation around the ring ( $v_{\rm rot}$ ) and outflowing from the ring midplane (v_z). See Sect. 3.6 for more details.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=8.2cm,clip]{figures/2971fi3a.ps}... ...ce*{5mm} \includegraphics[width=8.2cm,clip]{figures/2971fi3b.ps} } \end{figure}$ Figure 3: a) Model continuum emission superimposed on the velocity integrated OH maser emission. The contours, greyscale and the resolution are equivalent to those in Fig. 1a. b) Corresponding modelled OH maser velocity centroid field. Greyscale is between -60 and 60 km s^-1 around the systemic velocity. Contours are from -30 km s^-1 and increasing by 5 km s^-1 up to 30 km s^-1. Compare with Fig. 1b.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=17.8cm,clip]{figures/2971fi4.ps}\end{figure}$ Figure 4: The first three columns show sample realisations of the Monte-Carlo simulation. The rightmost column shows the average over 100 realisations. Intensity and flux are in arbitrary units, normalized to their peaks. Images: top row: greyscale and contours represent the velocity integrated OH maser emission convolved with a simulated EVN+MERLIN beam ( ${\it FWHM} \approx 35$ mas). Contours are logarithmically spaced from 2^-1/2 to 2^-7/2 of the peak. Middle row: Integrated emission at global VLBI resolution ( FWHM = 0.92 mas). Spectra: the solid line is the spectrum integrated over the entire source. Dashed and dotted lines represent spectra taken at the southern and northern tangent regions respectively.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{figures/2971fi5.ps}\end{figure}$ Figure 5: Top: detail of the northern region of the best matching model realisation at global VLBI resolution. The diameter of the circles plotted are proportional to the maser spot velocity integrated emission. For comparison with the results of D99, only spots brighter than 5% of the peak are shown. The inset is the spectrum of the brightest feature Bottom: position-Velocity diagram of the brightest maser spots. The data suggests the presence of a linear gradient $\sim$ $30\pm 12$ km s^-1 pc^-1 for spots on the eastern side of the plot.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{figures/2971fiA1.ps}\end{figure}$ Figure A.1: Variation of the mean gain G with the mean number $\bar{N}$ of overlapping maser clouds when the individual cloud optical depth is $\tau _{\rm c}$ = 1.5. The crosses show the results of a numerical Monte-Carlo calculation that averages over M = 50 lines of sight. The solid line is the result of analytic calculation using Eq. (A.3). The dashed line plots the result of an infinite-series summation (Eq. (A.2)). The top axis shows the distribution mean optical depth $\tau_{\rm mean} = \hbox{$\bar{N}$ }\tau_{\rm c}$ , the right axis its effective optical depth $\tau_{\rm eff} = \ln G_{\rm mean}$ .

In the text