The ammonia emission was detected in about half of the sources in the
range of right ascension of . This is comparable with the
detection rate obtained in Paper I for the outer Galaxy
(37%)
but the number of sources in the present study is certainly insufficient
for statistical conclusions.
Comparing the detected and non-detected sources we should mention two
apparent differences between these groups:
(1) the non-detected sources are significantly more distant on the
average than the detected ones (the mean distances are
and
, respectively) and
(2) the detection rate for the sources with outflows is much higher
than for the sources without reported outflows (8 from 9 and 3 from 8,
respectively). However, the second item can easily be a consequence of
the first one because it should be more difficult to detect outflows in
more distant objects. Therefore, we cannot conclude whether the detection
rate is related to the outflow activity. Perhaps the differences can be
explained by the beam dilution effects. Further we shall concentrate
mostly on the detected objects.
The masses derived from
the column densities are remarkably close to virial masses
in most cases.
It shows in particular that our assumption of the ammonia abundance,
, is in general correct (at least for
the detected clouds). The
striking exception is S 88 B. The mass derived for this
source is an order of magnitude lower than the virial mass. This may
indicate that the ammonia abundance here is correspondingly lower than
in most of the other detected cores. The map of the ammonia emission in
S 88 B (Fig. 1 (click here)) shows a ``hole" at the IR source position
where we detected no ammonia at all. This resembles
the distribution of the CO emission which has a horseshoe-like structure
(White & Fridlund 1992) with a significantly reduced
intensity of the CO line in approximately the same region. However, it is
worth noting that the CS emission towards the IR source observed with a
similar beam size (though not mapped) is rather strong (Paper II).
For explanation of the apparent ammonia underabundance
it might be important that the IRAS point source associated with S 88 B
is very luminous (), second in our sample by the
luminosity. The most luminous source in the sample is S 100 (K 3-50)
and we detected no ammonia emission in this object (earlier Churchwell
et al. 1990) observed a weak ammonia emission here; the intensity was
lower than our detection limit). However, K 3-50 is a strong source of
molecular emission in the lines of other
high density tracers, e.g. HCN J=1-0 (Burov et al. 1988) and
CS J=7-6 (Plume et al. 1992).
For S 87 and S 255 the virial masses also exceed (by a factor of 2) the masses derived from the ammonia column densities. Both these sources have pronounced bipolar structure and can be really unbound. It is worth noting also that the estimates of the virial masses in the case of their highly elongated geometry should be made in a more refined way.
The difference between the ammonia distribution on the one hand and CS and dust distribution on the other hand in S 255 (which is also very luminous) can also be attributed to the ammonia underabundance in the neighbourhood of the central source.
Thus, we suspect that ammonia is underabundant in the vicinity of the most powerful IR sources. Of course, this is not a decisive conclusion because the number of the investigated objects is too limited and the beam is pretty large. Recently Davis & Dent (1993) suggested that ammonia is underabundant towards young stars being the central sources of molecular outflows but did not relate this effect with the IR luminosity. However, many of our sources demonstrate outflow activity but mainly the most luminous ones have signs of ammonia underabundance.
What can be the reason for the ammonia underabundance? Davis & Dent (1993) suggested that there is a mechanism which restricts ammonia formation in such places. In our opinion there are basically two ways to explain the possible ammonia underabundance:
(1) The time-dependent chemical models show that the ammonia abundance is relatively low at early times (e.g. Bergin et al. 1995; Taylor et al. 1996). Thus, the ammonia underabundance can be related to the evolutionary status of the clouds. However, there is no direct relation to the luminosity of the central source in this model.
(2) An enhanced radiation field in the vicinity of young stellar objects
increases the thickness of the layer
and
is destroyed primarily by a reaction with
(e.g.
Turner 1995). At the same time the species like CO, CN,
, etc. which are formed via
are still very abundant in these
regions in spite of the high photodestruction rates (Bergin et al.
1995; Jansen et al. 1995). Probably this difference
in their chemistry under the conditions of enhanced radiation field
is the most
plausible explanation for the relative ammonia underabundance.
About half of the mapped sources show elongated structures with a more or less significant velocity gradient along the major axis. The most prominent example is S 255. S 87 has a two-component structure with 2 distinct velocity components which overlap partly spatially. However, as it has been argued above ammonia probably does not reflect the dense gas distribution correctly especially in these cases. Anyway, there is a question what is the reason for this appearance. The most probable explanations are a rotating disk or an outflow. Barsony (1989) argued in favour of the latter model for S 87. We shall discuss the data on these sources in more detail in a separate paper.
Another important item in respect of the core kinematics is the variations of the velocity dispersion in the cores. The CS data for southern cores with an apparently spherical structure indicate that the velocity dispersion decreases outwards with radius (Zinchenko 1995). The present ammonia data are not so definite. In S 199, S 231, RNO 1B and BFS 48 the line widths increase significantly near the ammonia peaks. In S 87 the widths of the separate components also increase near their corresponding emission peaks. In the other cases there are either no regular variations (S 86) or gradients of the line widths along certain directions can be seen (S 76E, S 88B, S 255). But most of the cores in the present sample are far from the spherical symmetry.
Considering the kinetic temperature distribution in the cores we see
that in most cases it is centrally peaked reaching and drops
to
at the edges of the ammonia emitting regions.
These values refer to the 40
beam of the Effelsberg antenna
and do not exclude a presence of hotter compact clumps.
Probably,
in the outer layers of the clouds which are not traced by ammonia,
the temperature rises again as indicated by the fact that the CO
temperatures in many cases are higher than the kinetic temperatures
derived from ammonia data.
Finally, we have looked for hyperfine intensity anomalies in the (1, 1)
transition in our sources. In addition to S 87 where such anomalies have
been seen by Stutzki et al. (1984) we found similar deviations
in S 199 (Fig. 5 (click here)).
These anomalies are a rather
common phenomenon in warm clouds (Stutzki & Winnewisser 1985)
and have been interpreted in the framework of clumpy models with the
individual clumps having small line widths of
(see Stutzki & Winnewisser 1985 and references therein).
Figure 5: The (1, 1) line in S 199 showing
hyperfine intensity anomalies