Long-term study of water masers associated with Young Stellar Objects

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Abstract
We present the analysis of the properties of water maser emission in 14 star forming regions (SFRs), which have been monitored for up to 13 years with a sampling rate of about once every 2–3 months. The 14 regions were chosen to span a range in luminosity Lfir of the associated Young Stellar Object (YSO) between 20 and . The general scope of the analysis is to investigate the dependence of the overall spectral morphology of the maser emission and its variability on the luminosity of the YSO. 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 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 5σ-level; below it, the typical 5σ detection rate is 75–80%. Although the present sample contains few objects with low YSO luminosity, we can 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. For individual emission components the presence of velocity gradients seems to be quite common; we find both acceleration and deceleration, with values between 0.02 and 1.8 km s-1 yr-1. From the 14 “bursts” that we looked at in some detail we derive durations of between 60 and 900 days and flux density increases of between 40% and 1840% (with an absolute maximum of ∼820 Jy over 63 days). The ranges found in burst- intensity and -duration are biased by our minimum sampling interval, while the lifetime of the burst is furthermore affected by the fact that bursts of very long duration may not be recognized as such. In addition to the flux density variations in individual emission components, the H2O maser output as a whole is found to exhibit a periodic long-term variation in several sources. This may be a consequence of periodic variations in the wind/jets from the exciting YSO.
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