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1. Introduction

The early stages of stellar evolution are dominated by processes involving strong mass loss. The effect of this mass outflow on nearby molecular cloud material is evidenced principally by the presence, in the radio domain, of molecular outflows and, in the optical domain, by the presence of Herbig-Haro objects and highly collimated jets. Several lines of evidence indicate that molecular outflow is one of the earliest observable phases of the stellar evolution (e.g., Rodríguez 1990). Likewise, Eiroa et al. (1994a) and Persi et al. (1994) concluded that an important fraction of what are thought to be the youngest objects (the so-called Class 0 sources; André et al. 1993) are associated with Herbig-Haro objects, suggesting that not only the molecular outflows but also the optical ones start in the early stages of the star formation process.

One of the remaining open questions regarding the outflow phenomenon is that of the driving mechanism of molecular outflows. There is a growing belief that highly collimated (moving at high velocity; tex2html_wrap_inline3041 tex2html_wrap_inline3043) circumstellar (tex2html_wrap_inline3045) optical jets drive the less collimated (and of lower velocity; tex2html_wrap_inline3047 tex2html_wrap_inline3049) molecular outflows that extend to larger, interstellar scales (tex2html_wrap_inline3051). Detailed models have been developed in this line (see, e.g., Raga et al. 1993 and references therein). In these ``unified models'', a high velocity, collimated wind (which would correspond to the optically detected HH objects or jets) drives an envelope of slower, less collimated material (e.g., environmental material set into motion by viscous coupling), which is identified with the molecular outflow. Within this scenario, both optical and molecular outflows would coexist during the pre-main-sequence stages. Despite this coevality, depending on the evolutionary stage of a particular young stellar object, observations could appear dominated by either type of mass loss phenomenon. For the youngest objects, which are still deeply embedded in high density molecular material, circumstellar optical jets are expected to be highly extinguished and hardly detectable, while molecular outflows can be more prominent. As the object evolves, the ambient molecular gas is progressively being swept up by the outflow, and the drivingjet becomes more easily detectable at optical wavelengths. The decrease of the high density gas near the star is expected to be evidenced through a decrease in the line intensity of high-density tracers, such as the tex2html_wrap_inline3053 molecule.

Another important issue in the outflow study is the identification of the outflow exciting sources. These sources are commonly embedded in high-density gas, and located near the position of the emission maximum of high-density tracers like the tex2html_wrap_inline3055 lines, as shown by Anglada et al.\ (1989). This association, at a scale tex2html_wrap_inline3057, between the ammonia emission peak and the outflow exciting source does not contradict the fact that the ammonia emission could present a much smaller scale structure near the object (e.g., cavities), as revealed by very high angular resolution observations (see the discussion by Anglada et al. 1995). Thus, single-dish ammonia observations can be an useful tool to help to establish the position of an outflow exciting source, to confirm a given candidate or to discriminate between several candidates.

In order to further investigate these issues, we selected a sample of 15 star-forming regions with signs of outflow activity, and we mapped with the Haystack 37 m telescope the tex2html_wrap_inline3059 emission around the position of the suspected outflow exciting sources. In this paper we present the results of this study. In Sect. 2 we describe the observations, in Sect. 3 we discuss the sources individually, in Sect. 4 we discuss the global results of our study, and in Sect. 5 we give our conclusions.


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