ratio
In Table 9 (click here), seven M giants
(00428+6854-04575 +1251
-17376-3021-17454-
3024-18556+0811-19422+3506
-20010+3011) and 2 K giants
(18079-1810-18450
-0922) show above 120.
As shown in Paper I, a luminosity close to the theoretical AGB
luminosity tip 
and/or a high expansion velocity
can lead to values between 120 and 220, even for giants.
Such objects should however be relatively rare. This may be the
case for 4 M giants. There remains puzzling objects: 3 M giants
(17376-3021, 19422+3506 and 20010+3011), the 2
K giants and 2 sources presumed to be AGB stars, 02407+3602 (TV Per)
and 20000+4954 (Z Cyg).
TV Per has a a low 
flux (7 Jy) and is at high galactic latitude
, likely being
out of the Galactic disk where all the supergiants
are located;
20000+4954 (Z Cyg) is discussed as a peculiar object in
Sect. 6.
One of these objects, IRAS 20010+3011 (V718 Cyg), is
identified as an SRb variable star
(Cameron & Nassau 1956).
For the objects with values out of the range expected for
AGB stars, one can consider some new factors. Observations of chromospheres
in K giants are relatively common. This hypothesis is examined in
Sect. 4.2. Additional causes, such as a low 12C abundance or the influence of
mass-loss history, are exposed in Sects. 4.3 and 4.5, respectively.
The presence of a chromosphere in supergiants is widely accepted.
The luminosity of supergiants combined with other parameters can explain
large values, but a low CO abundance is not excluded.
In particular, in the case of 
Ori (which has bluer colours
than those of our sample), for which the carbon abundance
problem has been largely addressed (see e.g.
Huggins et al. 1994), it has
been shown that the CO abundance is very
low, but the dust condensation is
also reduced and the
emission lowered. Consequently,
has a "normal'' supergiant value (1500).
The presence of a chromosphere in AGB stars might raise by
producing UV radiation which photodissociate CO.
However, it has not been clearly established that all AGB stars,
especially O-rich ones, have chromospheres, and
even if present, the chromospheres of M giants would be very thin
(
)
(Eaton & Johnson 1988).
Pasquini & Brocato (1992) have shown that chromospheric activity
in M giants on the RGB is linked to stellar mass via
with 
and 
.
F'k is the CaII K line flux, which traces the chromospheric
activity. This is consistent
with the fact that, in view of the range of values,
photodestruction would occur preferentially in massive objects. But
this activity remains dependant on the evolution stage.
Another possible source of UV radiation is the interstellar medium, and in particular OB associations, where many supergiants are located. This UV field is particularly intense in the Galactic Plane. Whatever the origin of the UV field, the efficiency of photodissociation depends on the structure of the circumstellar envelope. As emphasized by Bertoldi & Draine (1996) for molecular clouds, photodissociation occurs in a transition layer between a relatively dense cloud and a tenuous medium, so that the clumpier the medium, the more efficient the photodissociation. This could be applicable to the interface between the atomic and molecular media in a circumstellar envelope. On the basis of observational evidence, Skinner & Whitmore (1988) asserted that M supergiants lose mass in the form of blobs. This would generate an envelope with a clumpy structure, comparable to that of a molecular cloud. The increased surface area of the interface could make photodissociation more efficient than if the envelope was smooth and unclumped. A photo-induced chemistry may dominate in these clumps (see e.g. Howe et al. 1994) and carbon could be present in forms other than CO. This theory is less meaningful for most AGB stars, since their envelopes are generally spherically symmetric and probably not very clumpy, at least at relatively large scale.
For massive stars 
),
hot-bottom burning occurs
if the temperature is sufficiently high
.
12C is then partially
converted into 13C and 14N, via the CNO cycle
(see e.g. Renzini &
Voli 1981; Leisy & Dennefeld 1995).
The star will then be relatively 12C-poor. On the contrary,
if the temperature is relatively low, the envelope will be enriched in
12C produced during the 
dredge-up.
Considering recent investigations, we suggest that a high
in an AGB star could reflect a low abundance of 12C, much of
which could have been converted into 13C and 14N.
This hypothesis could be checked by searching for 13CO. However
observing 13CO might be difficult, considering the weak intensity
of 12CO.
The study of the
ratio is limited to the objects
observed and detected during the 3rd and 4th runs (Table 1 (click here)). In
particular, it is biased towards the least massive objects, as only 3
supergiants were detected. It should also be kept in mind that the uncertainty
in both the pointing accuracy and the source position might give a lower
measured intensity of
the (2-1) emission and thus would lead to an underestimate of
.
However, the observed values of the
ratio are
consistent with what is usually observed for such objects.
It appears from the data in Table 13 (click here) and Fig. 9 (click here) that
tends to increase with
.
Since the supergiants are under-represented, it is very difficult to
characterize the effect of luminosity
class on the
ratio. Nevertheless, the few cases studied earlier, including Ori
(e.g. Heske et al. 1989), show a trend towards high values of this ratio.
Such a large spread in
might be attributed to
different processes of CO excitation. Indeed,
(see e.g. Groenewegen et al.
1996), a "normal'' AGB star, with a large mass-loss rate
and an expansion velocity close to
, has
close to 2.
Many authors (Kahane & Jura 1994;
Sahai 1990) agree
that is a good estimator of
excitation conditions (kinetic temperature, opacity).
Kahane & Jura (1994) observed values of
ranging from 2.5 to 6.0, from the coldest to the warmest envelopes, with an
average around 3.5. Then, the highest values
of
(up to 
) in our sample might reveal different excitation processes
for objects with high values of , i.e., supergiants and/or
massive AGB stars.
Figure 9: (
) vs. () plotted for the
detections made at IRAM (runs 3 and 4)
A superwind phase has already been invoked to explain a CO emission deficiency
in colder objects (region IIIb2; Heske et al. 1990). This also acts
on the ratio, by increasing it, because the
outer layer, main contributor to the CO(1-0) emission, becomes negligible
(see details in Delfosse et al. 1996).
Such a phase has been invoked by many authors to fit observations of massive
AGB stars.
This is consistent with the fact that the highest values are
observed in a massive population. However, one has to confirm that this
phase can occur in objects such as those of our sample.
Figure 10: CO(1-0) and CO(2-1) spectra of the objects
detected during the
2nd run (SEST)