Astron. Astrophys. Suppl. Ser. 133, 337-352

Studies of dense molecular cores in regions of massive star formation

VII. Core properties on the galactic scale

I. Zinchenko^1,2 - L. Pirogov¹ - M. Toriseva²

Send offprint request: I. Zinchenko
Correspondence to: zin@appl.sci-nnov.ru

1 - Institute of Applied Physics of the Russian Academy of Sciences, 46 Uljanov str., 603600 Nizhny Novgorod, Russia
2 - Helsinki University Observatory, Tähtitorninmäki, P.O. Box 14, FIN-00014 University of Helsinki, Finland

Received March 25; accepted June 18, 1998

Abstract:

We surveyed 55 northern non-stellar H₂O masers in the CS J=2-1 line with the 20-m Onsala radio telescope and 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. Most of the detected cores were mapped in CS. The CS peaks were observed also in the C³⁴S J=2-1 and in the CO J=1-0 lines. We present CS maps as well as CO, CS and, when available, C³⁴S spectra for the 26 best studied cores.

From the CS maps and optically thin C³⁴S emission we derive the basic physical parameters of the cores: size, LTE mass, mean density, virial mass. Combining the present results with the previous SEST data we obtain statistical distributions of the core parameters. The CO brightness temperature distribution for most cores ranges from $\sim$15 K to $\sim$50 K with a peak at 20 - 30 K. The typical sizes of the cores are $L\sim 1.0-1.5$ pc. The mean density lies in the range $n\sim 10^3-10^5$ 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 analyze the dependences of these parameters on galactocentric distance R. The mean density of the cores drops with increasing R in the interval $R\approx 7-14$ kpc. It is consistent with an exponential law with a scale length of $\sim 3$ 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 probably explained 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.

Key words: stars: formation -- ISM: clouds -- ISM: molecules -- radio lines: interstellar