1995A&A...301..236W

For a sample of 1357 IRAS sources with FIR colours of young stellar objects we have compared the occurrence and properties of H₂O maser- and ¹²CO(1-0) emission with the observed and derived IRAS parameters. We find that the distribution of differences between the CO and H₂O velocities is a Gaussian with a FWHM of about 11km/s. The mean velocity difference is nearly zero. The velocity of the peak H₂O emission is not at a preferred location within the velocity interval where emission is detected. The H₂O maser detection rate increases with both the FIR luminosity and the ¹²CO(1-0) line width: below log(L_FIR/L_☉)=3.5, less than 10% of the sources show maser emission, whereas H₂O is detected towards almost all of the sources above log(L_FIR/L_☉)=4.5. For the sources with H₂O maser emission, the average CO line width (FWHM about 5.0km/s) is approximately twice as large as for those without detected H₂O emission, an effect which is independent of the sources' FIR luminosity. However a significant fraction of sources with no detected H₂O emission still have line widths larger than 1.9km/s, the median value for the quiescent gas not associated with the IRAS sources. The maximum H₂O maser luminosity at each FIR luminosity is proportional to L_FIR: L_H2_O/L_☉=10^–7.4(L_FIR/L_☉). In IRAS colour-colour plots, sources can be distinguished in maser-like and non-maser-like sources according to their CO line widths and FIR luminosity. At a given L_FIR the maximum T^*_A found for ¹²CO(1-0) decreases with distance from the Sun, likely due to beam filling effects. Few sources with maser emission have T^*_A below 7.5K, when observed with the IRAM 30-m telescope. There is no difference in the maximum T^*_A of sources with maser-like and non-maser-like IRAS colours. We have derived the intrinsic luminosity- and mass distribution of sources with (non)-maser-like IRAS colours, correcting for incompleteness effects at lower luminosities. The slope of the IMF for sources with maser-like colours and mass larger than 10M_☉ is found to be -2.8±0.3.