values
In Paper I we showed that for a "normal'' AGB star the typical
value of 
ranges between
20 and 220 Jy/K, with a mean value 
(
is measured with
a 30 m-antenna). In fact, only objects with extreme values of parameters
can display 
values above 120.
Nevertheless, we found 37 objects out of 81 
with larger than 120. The distribution is shown in
Fig. 3 (click here). Non-detections are considered significant when
.
As in Paper I, we define two groups:
group 1 contains objects with 
and group 2 those with
(Table 3.1 (click here)).
Figure 3: Distribution of the northern
sources of
regions IIIa1 and IIIa2 as a function of .
The grey-filled histogram refers to sources detected before our program,
the "paved'' histogram adds our detections and the empty
one adds non-detections. In this case, 2
rms is
taken for 
| Group 1 | Group 2 | |
 | | |
| Total | 40 | 39 |
| lum. class I | 0 | 18 |
| lum. class III | 12 | 10 |
 M5 | 2 | 22 |
 M6 | 24 | 10 |
 | 13 | 31 |
 | 27 | 8 |
 | 11 | 25 |
 | 29 | 10 |
|
|
and spectral type, galactic latitude, and the IRAS variability index
var
The discussion below is based on the assumption that the IRAS S60
flux is reliable, i.e. that it does not suffer of interstellar contribution,
which would increase the ratio.
This could not be the case for a few objects, in particular those at low
galactic latitude. Nevertheless, as no correlation appears between
the ratio and
(Fig. 7 (click here)), such an effect must be negligible.
This is confirmed by the examination of the spectral energy distributions
from 1 to 
(Paper IV) which do not exhibit any excess
at .
Among the 37 sources with 
, 22 (59%) have
spectral types earlier than M5. The galactic distribution of
our main sample is shown in Fig. 4 (click here). The highest values of
are clearly concentrated at low galactic latitude (
). This is
characteristic of a young disk population. This link with the
initial mass of the star is confirmed by the
correlation between and the IRAS variability index,
as shown in Fig. 5 (click here). High values are
preferentially found in objects with little or no
known variability. Together with the average low galactic latitude,
these are common characteristics of supergiants.
Table 9 (click here) shows that the 18 sources from our main sample
identified as red supergiants (see Paper III) all have
larger than 160. In Paper I, we demonstrated that
supergiants are expected to have high values from
about 200 to 2000, mainly because of their high luminosity,
but also because of a relatively small photodissociation radius
. Among the identified supergiants, only 3 have been detected in CO:
17513-2313, 19307+1338 and 22512+6100 (the latter is only a tentative
detection). All display very high values of (306, 293 and 830,
respectively).
As shown in Paper I, the value of is a new tool to
distinguish AGB stars from infrared supergiants.
Indeed, both from theoretical
(Loup et al., in preparation) and observational points
of view, it is clear that an object with 
must be an
AGB star and hence have a
low mass progenitor. In our main sample, this leads to the identification
of 13 new AGB stars with no spectral type, and 9 for with spectral
types but without a known luminosity class.
However, we may not conclude that all the sources with a high
value are supergiants, as we find some counter-examples
in our sample, as discussed in Sect. 4.1.
Figure 4: Correlation of the ratio with the galactic
latitude. Objects detected in CO(1-0) are represented by
filled circles.
Objects not detected in CO(1-0) are represented by bars and 2 rms
is taken for . Star symbols represent upper limits on
CO emission, affected by interstellar contamination
Figure 5: ratio vs. IRAS variability index.
For the meaning of the symbols, see Fig. 4 (click here)
In the limited range of IRAS colours considered here,
there is no correlation of with either
or with
, as shown
in Figs. 6 (click here) and 7 (click here), respectively.
The C21 colour is considered as a good estimator of opacity, and so
we deduce that in this range the current mass-loss rate has no appreciable
effect (see details in Paper IV). This does not exclude an influence of the
mass-loss history.
Figure 6: vs. IRAS colour C21 for the main
sample (Tables 7 (click here) and 9 (click here)). The symbols are the same
as in Fig. 4 (click here)
Figure 7: ratio vs.
IRAS colour C32. The symbols are the same as in Fig. 4 (click here)
Figure 8: S
ratio vs. IRAS LRS type for the northern sample
of objects located in regions IIIa1 and IIIa2. This diagram is limited to
(see text)
We also looked for a possible correlation with the IRAS
Low Resolution Spectra (LRS) type (IRAS team, 1986).
For the range of infrared colours of our sample, objects
with LRS types 2n and, in particular, types 28-29
(indicating strong silicate emission), are dominant.
Some objects may be sufficiently optically thick to display self-absorption
of silicate features. In particular it is well-known that
some 4n objects are indeed oxygen-rich but a self-absorbed silicate
feature at 
can be
confused with SiC emission at 
.
Such objects with a LRS type 4n, but for which the chemical nature
is established thanks to OH masers, are excluded in Fig. 8 (click here), as
the LRS type is then meaningless. As far as objects of 2n class are
concerned, no clear correlation with the index n, representing the strength
of the silicate emission, seems to exist (see Fig. 8 (click here)). However,
there may be a slight predilection for high values of in sources
with low n 
.
Indeed, only two objects (20077-0625 and 20267+2105) out of nine with
weak silicate emission ( with 
) have a low value of
.