**Table 1:** Model parameters
Maser and continuum ring properties:
Maser ring inner radius $R_{\rm in}$ ^a	22 pc
Maser ring radial thickness $\Delta R$ ^a	3 pc
Maser ring height Ð^a	6 pc
Maser ring Rotation velocity $v_{\rm rot}^{b}$	57 km s^-1
Continuum peak emissivity radius $R_{\rm c}$ ^b	37 pc
Inclination angle i^b	60 $^\circ$
Free-free absorption bicone properties:
Opacity $\tau_{\rm ff}$	0.7
Opening angle ( FWHM)	$60^{\circ}$
Cloud properties:
Cloud size a^c	<0.7 pc
Internal velocity width $\delta v$ ^c,d	20 km s^-1
Random velocity dispersion $\Delta v$ ^c,d	60 km s^-1
Outflow velocity v_z	60 km s^-1
Outflow velocity dispersion $\Delta v_{z}^{d}$	30 km s^-1
Number of clouds along radius N₀	1.8
Cloud volume filling factor f	<0.08
Cloud maser optical depth $\tau _{\rm c}$	1.5

Refs: ^aT97; ^bP01; ^cD99;
^d The dispersions represent the FWHM of a Gaussian
probability distribution.

	1 Introduction
	2 Source properties and observations
	3 Clumpy ring model
	4 Modelling and results
	5 Discussion of maser properties
	6 Physical considerations
	7 Conclusions and future work
	Appendix A: Maser gain in clumpy media
	References