1 Introduction

A bipolar outflow is a physical phenomenon which accompanies the process of star formation. This is the earliest phase of stellar evolution seen by observers. At the present time bipolar outflows are some of the most widely studied phenomena in astrophysics; the number of bipolar outflows detected thus far is more than 200. Their properties and characteristics have been described in detailed surveys, e.g. Bally & Lane (1991), Bachiller & Gómez-González (1992). The most complete catalogues of bipolar outflows were made by Fukui (1989), Xiang & Turner (1995), and Wu et al. (1996). Numerous observations show convincingly that flowing matter usually has a bipolar structure. Such flows have characteristics similar to those of young objects of all masses and luminosities.

On the other hand, molecular maser activity often accompanies the process of star formation and is also a widespread phenomenon. Methanol masers as well as OH and H₂O masers, are often located close to young stellar objects, ultracompact HII regions, IR sources, HH objects, Bok globules, and other unusual objects, associated with regions of star formation. The influence of bipolar outflows on methanol maser activity has not been adequately studied, but some publications suggest a connection between methanol masers and bipolar outflows (Plambeck & Menten 1990; Haschick et al. 1990; Kalenskii et al. 1992). It has also been shown that the presence of bipolar outflows increases the abundance of methanol (Bachiller et al. 1995).

Some well studied regions of star formation have powerful bipolar outflows, but do not manifest methanol activity. In others, where both bipolar outflows and methanol masers coexist, the role of the central star's mass, age, the power of the flow, and other characteristics in maser excitation is unknown.

It is possible that methanol masers are formed in a gas disk surrounding a protostellar object, and interferometric observations show that methanol masers are associated with the centre of bipolar outflows (see e.g. Pratap & Menten 1992).

It is possible also that methanol masers are produced at a shock front, which results from the interaction between flow and molecular cloud (Johnston et al. 1992).

In both cases maser condensations are likely to be deeper imbedded in regions of very dense gas.

In this paper we present the results of a J=2-1 CS and C³⁴S survey of 158 sources (bipolar outflows and methanol masers). CS lines are indicators of very dense gas. These lines are excited by a density above the critical level $n_{\rm crit}=10^{4}-10^{7}$ cm^-3 (Snell et al. 1982; Snell et al. 1984; Mundy et al. 1986).

The present survey was carried out in order to derive some physical parameters (e.g. $N_{\rm CS}$ and $\tau$) and compare densities in a large number of bipolar outflows and of Class I and Class II methanol masers (Menten 1987; Batrla et al. 1987), both associated with and unrelated to bipolar outflows.

1.1 Important surveys in CS lines

Observations of CS lines in HII regions and dark clouds have been done in the following works:

Turner et al. (1973) surveyed interstellar objects in the (1-0) and (2-1) transitions of the CS and C³⁴S lines. The CS line was detected in 19 out of 26 sources. C³⁴S was detected in 5 out of the positive 19 CS sources: W3(OH), Ori A, Sgr B2, W51, and DR21(OH).

Liszt & Linke (1975) surveyed five molecular clouds associated with HII regions in the CS(1-0), (2-1), and (3-2) lines.

Linke & Goldsmith (1980) surveyed 32 molecular clouds in the CS(2-1) and (1-0) lines.

Plume et al. (1992) surveyed 179 regions of star formation in the CS(7-6) line, selected in accordance with the criterion for the existence of H₂O masers close to the star forming regions. The CS(7-6) emission line was detected in 104 sources.

Zinchenko et al. (1994) surveyed the CS(2-1) and C³⁴S(2-1) lines in 11 molecular cloud cores, associated with regions of star formation.

Wolf-Chase et al. (1995) surveyed 10 outflows and 30 IRAS sources in the Mon OB1 dark cloud in the CS(2-1), (5-4), and (7-6) lines.

Juvela (1996) surveyed 33 southern molecular cores associated with H₂O masers in several CS and C³⁴S lines. The clouds have been mapped mainly in the CS(2-1), CS(5-4), and C³⁴S(2-1) lines. All central positions of the clouds have been observed in both isotopomers in the transitions J=3-2 and 5-4 as well as in C³⁴S(2-1). Some selected clouds were also mapped in CS(3-2). CS(7-6) was observed in 14 and C³⁴S(7-6) in seven clouds.

Bronfman et al. (1996) carried out a CS(2-1) survey of IRAS point sources in the Galactic plane. Sources were selected according to a colour criterion. In 843 sources out of 1427 the CS(2-1) line was observed successfully.

Anglada et al. (1996) surveyed 172 regions of star formation, which areassociated with H₂O masers. Observations were carried out to compare the width of lines detected in the directions of H₂O masers and in the other directions.

Plume et al. (1997) surveyed 150 regions of massive star formation in the CS(5-4), (3-2), and (2-1) transitions and in the same transitions of C³⁴S selected by the presence of an H₂O maser.

A few star formation regions were observed in the CS(2-1) and (1-0) lines by other authors (Bally 1982; Zhou et al. 1991; Ohashi et al. 1991; Pastor et al. 1991; Churchwell et al. 1992). In spite of numerous CS surveys there is no one which was devoted to comparison of properties of bipolar outflows and methanol masers.

 

Table 1: Line parameters determined from CS(2-1) and C³⁴S(2-1) observations

 

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