5 Summary and conclusions

  We have presented results of the CS J=2-1 survey towards 55 northern non-stellar H₂O masers. We detected 47 CS cores associated probably with 50 masers. The CS emission for this sample is weaker on the average than for the similar southern sample studied by us earlier at SEST. CS maps as well as CO, CS and when available C³⁴S spectra have been presented for 26 best studied cores.

From the CS maps and optically thin C³⁴S emission we have derived the basic physical parameters of the cores: size, LTE mass, mean density, virial mass. Combining the present results with the previous SEST data we obtained statistical distributions of the core parameters. The CO brightness temperature for most cores lies in the range $\sim$15- 50 K with a peak at 20-30 K. The size of most cores is $L\sim$1.0-1.5 pc. The mean density lies in the range $n\sim$10³-10⁵ cm^-3 which is much lower than densities needed for CS excitation from multitransitional analysis. The slope of the mass spectrum for is $1.6\pm 0.3$. The ratio of the IR luminosity of associated IRAS point sources to mass peaks at $\sim$10 $L_\odot/M_\odot$.The CS line widths are highly supersonic ($\sim$1.5-9 kms^-1).

We have analysed the dependences of these parameters on galactocentric distance R. The mean density of the cores decreases with increasing R in agreement with an exponential law with a scale length of $\sim 3$ kpc in the interval $R\approx 7-14$ kpc. The IR luminosity to mass ratio changes probably in a similar way. The core size increases with R in accordance with the density decrease and constant mass.

The comparison of the CS and C³⁴S data shows almost no broadening of the CS lines due to optical depth effects. This can be explained probably by small scale clumpiness in the cores. The velocity difference between the CS cores and H₂O masers is close to zero on the average with the standard deviation of $\sim$7 kms^-1.

Acknowledgements

We are very grateful to Dr. Lars E.B. Johansson for his help with the observations and for obtaining important additional data, to Prof. Kalevi Mattila for critical reading of the manuscript and to an anonymous referee for valuable suggestions. I.Z. and L.P. thank the Helsinki University Observatory for the hospitality during the data reduction and the paper preparation stages. They were also supported in part by the INTAS grant 93-2168-ext and grant 96-02-16472 from the Russian Foundation for Basic Research. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.