  (Abridged) H2O masers in 14 SFRs have been monitored once every 2-3 months
for up to 13 years. We investigate the dependence of the overall spectral
morphology of the maser emission and its variability on the luminosity of the
YSO, and look for bursts and gradients in individual components. We find that
higher-luminosity sources tend to be associated with stronger and more stable
masers.Higher-luminosity YSOs can excite more emission components over a larger
range in velocity, yet the emission that dominates the spectra is at a velocity
very near that of the molecular cloud in which the objects are embedded. For
Lfir > 3E+04 Lo the maser emission becomes increasingly structured and more
extended in velocity with increasing Lfir. Below this limit the maser emission
shows the same variety of morphologies, but without a clear dependence on Lfir
and with a smaller velocity extent. Also, for sources with Lfir above this
limit, the water maser is always present above the 5sigma-level; below it, the
typical 5sigma detection rate is 75-80%. We conclude that there must be a lower
limit to Lfir, below which the associated maser is below the detection level
most of the time. These results can be understood in terms of scaled versions
of similar SFRs with different YSO luminosities, each with many potential sites
of maser amplification, which can be excited provided there is sufficient
energy to pump them, i.e. the basic pumping process is identical regardless of
the YSO luminosity. In SFRs with lower input energies, the conditions of maser
amplification are much closer to the threshold conditions, and consequently
more unstable. We find indications that the properties of the maser emission
may be determined also by the geometry of the SFR, specifically by the beaming
and collimation properties of the outflow driven by the YSO.

Astrophysics