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; 
) circumstellar (
) optical
jets drive the less collimated (and of lower velocity; 
)
molecular outflows that extend to larger, interstellar scales (
). 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 
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 
lines, as shown by Anglada et al.\
(1989). This association, at a scale 
, 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 
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.