Long-term study of water masers associated with Young Stellar Objects. II: Analysis

Abstract

(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.