Our main sample consists of all oxygen-rich sources in regions
IIIa1 and IIIa2 (Fig. 1 (click here)) from the list given by
Omont et al.
(1993). It contains all "northern''
O-rich IRAS
sources with a limit on the total mid- and far-infrared flux measured
by IRAS (see Equats. (1) and (2) in Omont et al.):
at 1 kpc,
where
is estimated according to
(Loup et al. 1993)
with the IRAS fluxes given in Jy.
The oxygen richness is recognized mostly from the strong silicate emission
band in the IRAS LRS spectra and in a few cases from known OH maser emission
(see also Lewis 1995). This main sample was systematically observed in CO.
The limitation on IRAS fluxes of our sample guarantees a
relative completeness for nearby (distance ) AGB
stars which satisfy the colour criteria. But supergiants will be selected
deeper
, as they are more luminous. An important
proportion of bright oxygen-rich IRAS sources is thus made of massive stars.
The similarly selected sample in the sourthern part of the sky
contains 88 objects.
It was however not systematically observed in CO.
Figure 1: 12/25/60 m IRAS colour-colour diagram of studied sources.
The regions are those defined by
van der Veen & Habing (1988),
Omont et al. (1993).
Objects with
ratio larger and smaller than 100 Jy/K
are represented by empty and full circles, respectively.
All objects presented in the summary tables
(see Appendix: Tables 7 (click here) to 11 (click here)) are plotted here
The half power beam widths of the IRAM 30-m telescope are 22''
for the 3 mm SIS receiver used to observe the CO(1-0) line and
(theoretically 10.4'',
without taking into account a typical
pointing jitter)
for the 1.3 mm G1 SIS receiver used to observe the CO(2-1) line. As the
position
uncertainties of the IRAS Point Source Catalog (PSC) can reach 10 to
20'' along the major axis of the error
ellipse, we had to determine positions with a higher accuracy in order to
have reliable CO measurements, in particular in the cases of non-detection or
weak emission. To obtain better coordinates, we used
the "XY machine'' built by R. Vitry and F. Sevre (both at IAP), to measure
the source positions on the prints of the Palomar Observatory Sky Survey
(POSS). We used the SAO reference catalog for
southern objects and the Agk3 catalog for northern objects. This
gives a position accuracy
better than 3'', but this method misses objects that are too
dust-obscured to be detected in the visible, i.e., with a R magnitude
fainter than
.
This was not a problem for most of our sources; only
a few of the coldest ones were not identified on the visible prints.
(Remark: Since then, the PPM catalogue has been implemented on this machine. The accuracy of the measured positions is now about 1''.)
A log of the observations is described in Table 1 (click here). We performed three runs of observations with the IRAM-30 m-telescope in 1988, 1993 and 1994 (runs 1, 3 and 4 in Table 1 (click here)), and two at the SEST telescope in 1992 and 1993 (run 2 in Table 1 (click here)). In total we observed 70 sources of our main sample. CO was previously detected in 17 objects by other observers. For the southern sample, only 5 objects have been observed (Table 10 (click here)) and 10 were previously detected, mainly by Nyman et al. (1992). For various reasons, we also observed a few miscellaneous sources with different infrared colours which we add to the results. The total numbers of new detections and significant non-detections (see Sect. 3.1) are summarized in Table 1 (click here).
Period | telescope | observed lines | code | new detections | new detections | significant non-det. |
(main sample) | (misc. objects) | (main sample) | ||||
09/1988 | IRAM-30 m | CO(2-1) | 1 | 1 | 4 | |
03/1992, 03/1993 | SEST | CO(1-0) | 2 | 2 | 3 | 1 |
04/1993 | IRAM-30 m | CO(1-0), CO(2-1) | 3 | 14 | 4 | 2 |
07-08/1994 | IRAM-30 m | CO(1-0), CO(2-1) | 4 | 24 | 3 | 20 |
|
Observations have been done with the 15 m SEST telescope at ESO in
March 1992 and March 1993 (run 2 in Table 1 (click here)).
We observed the CO(1-0)
line only. Telescope and system parameters are given in Table 2 (click here).
As with the IRAM antenna, we observed in dual beam switch mode. The
beam separation was 11'. The velocity resolution was
.
These CO(1-0) observations were typically a factor 8 less
sensisitive than IRAM observations, because the SEST antenna
is twice smaller in diameter, and the SEST Schottky receiver was about
twice less sensitive than the IRAM SiS receiver. This explains why
we made fewer detections with the SEST than at IRAM.
runs | Line | Receiver | Half power | Main Beam | Forward |
beam width | Efficiency | Efficiency | |||
IRAM, runs 3,4 | CO(1-0) | 3 mm SIS | 22'' | 0.68 | 0.92 |
CO(2-1) | 1.3 mm G1 SIS | 10.4'' | 0.39 | 0.86 | |
SEST, run 2 | CO(1-0) | Schottky | 45'' | 0.70 | ... |
IRAM, run 1 | CO(2-1) | 1.3 mm G1 SIS | 11.6'' | 0.45 | 0.90 |
|
For the 3rd and the 4th runs, both the CO(1-0) and
CO(2-1) lines
were observed
simultaneously with the 3 mm and 1 mm SIS receivers. Their characteristics
are given in Table 2 (click here). The
velocity resolution was 2.6 and
for the CO(1-0) and CO(2-1) lines, respectively.
The observations were made in position switching mode with the
wobbling secondary mirror, which provides very flat baselines. The calibration
was done by successive observations of a cold load, a room temperature load
and the sky.
The first observations, made in 1988, concerned mostly a
few supergiants, and were limited to the CO(2-1)
line. Its detection can be
achieved with a shorter integration time than the CO(1-0) line,
as its intensity is higher.
However, the data analysis is more difficult, as the
ratio varies within a class of objects
(see Sect. 4.4).
The present study of CO emission is mainly based on the CO(1-0) line. Its measurement is more reliable than that of CO(2-1). As the beam is nearly twice as wide, pointing errors have less influence on the measured intensity. It also means that the values given for the CO(2-1) line intensity are under-estimated in many cases.
A problem often encountered is interstellar contamination (see Fig. 2 (click here) and Heske et al. 1990). The presence of interstellar emission affects the quality of the detection or of the upper limit on the CO emission. Since the data were obtained in position switching mode, we tried to reduce the size of the offset to eliminate contamination. As often as possible, we used an offset of 30''. In some cases we had to reduce it to 20'' or even 15''. Assuming a point source (i.e., a Dirac distribution) and a gaussian beam, the subtraction of the intensity at the OFF position with small offsets induces a loss of 10% and 28% of the signal for the 20'' and 15'' offsets, respectively, for the CO(1-0) observations at IRAM (half power beam width of 22''). For an offset of 30'', the loss is 0.5%, and is considered as null. The affected measurements have been corrected by these factors. On a bright source unresolved by the IRAM-30 m-telescope, IRAS 22272+5435, a change in the offset from 30'' to 20'' induced a signal loss of 13%. This is reasonably consistent with our predictions. We then corrected the fluxes of the objects observed in such a way according to the calculated losses given above.
Figure 2: Example of interstellar contamination: IRAS 20015+3019. OH has
been unsuccessfully searched for circumstellar lines, so no velocity is
known for this object
We rejected some observations when the contamination was too important to give a useful upper limit. An example is shown in Fig. 2 (click here). A list of these objects is given in Table 12 (click here) (Appendix A). Observations of such objects should be made with the special procedure described by Heske et al. (1990), which is, however, more time consuming. Some of our detections suffered from interstellar contamination (see e.g. the spectra of 18009-2019, 18550+0130, 18554+0231... in Fig. 12 (click here)). This may lead to slight under-estimations of CO(1-0) intensity. These objects are indicated by a star (*) in Table 9 (click here).
The numbers of detections and non-detections are summarized in Table 1 (click here) for each observing run. The results and parameters of our CO observations are listed in Tables 13 (click here) to 17 (click here). Detected sources from our main sample are listed in Table 13 (click here), other detected sources in Table 14 (click here), tentatively detected sources in Table 15 (click here), significant non-detections (see Sect. 3.1) from our main sample in Table 16 (click here), and other significant non-detections in Table 17 (click here). In these tables, the expansion velocity is defined as half the full line width at zero power, and temperatures at the peak of the line are given in the main beam scale. The spectra of the new detections are shown in Figs. 10 (click here) to 12 (click here).
Many sources of our main sample have been previously searched for 1612 MHz
OH maser. We performed some observations with the Nancay
radiotelescope, in order to improve the statistics. The observations were
done in January and February 1995. The total observing time per source
(ON+OFF) was one hour. One can then reach an rms of .
The
velocity range where OH was searched was
wide, centered on the star
velocity if it was known from CO, or
wide, centered on 0 if no
velocity information was available.
The half-power beamwidth is
in
and 18' in
. The ratio of flux to antenna temperature
is 1.1 Jy K-1 at 0
declination.
We obtained 6 new detections and 11 significant non-detections.
The results are presented in Table 3 (click here)
and are analyzed in Sect. 5.
Tables 7 (click here) to 11 (click here) indicate for all the sources studied
here whether they have been detected in OH or not, or not searched for OH
emission.
IRAS name | ![]() | ![]() | ![]() |
![]() | ![]() | ![]() | rms | H2O |
02153+5711 | 0.05 K | -- | ||||||
03572+5509 | 0.05 K | |||||||
07054-1039 | 0.05 K | + | ||||||
17361+5746 | 0.05 K | |||||||
18386-0624 | 50.0 | 94.1 | 9.17 | 4.97 | 72.1 | 22.1 | ||
18450-0922 | -28.5 | 0.22 | ||||||
18522+0021 | 0.05 K | - | ||||||
18531+0016 | 0.05 K | + | ||||||
19231+3555 | -34.8 | -13.9 | 0.85 | 1.85 | 24.4 | 10.5 | - | |
19371+2855 | 0.05 K | + | ||||||
19374+0550 | -32.3 | -0.9 | 0.59 | 1.35 | -16.6 | 15.7 | ||
20015+3019 | 0.05 K | |||||||
20194+3646 | 0.05 K | |||||||
22345+5809 | -29.1 | -21.5 | 0.33 | 0.38 | -25.3 | 3.8 | - | |
22546+6115 | 0.05 K | - | ||||||
23000+5932 | -32.0 | -24.4 | 0.15 | 0.22 | -28.2 | 3.8 | ||
23278+6000 | 0.05 K | - | ||||||
|
We summarize the properties in Tables 7 (click here) to 11 (click here). Our main
sample consists of the sources of Tables 7 (click here) and 9 (click here).
In Table 9 (click here), sources are classified as a function of spectral
type. First we list first supergiants, second identified giants, third
objects with indications of spectral type without luminosity class,
and finally sources with no information on spectral type.
We give associations when
available, with a priority on visible ones. IRAS parameters (variability
index, colours, flux, LRS type) are indicated. We give the
coordinates
used during the observations and, when we measured them ourselves, the
distance to the IRAS position. Finally, we give the galactic latitude,
spectral type and OH detection, together with the value of
.
Table 12 (click here) lists the sources that are too strongly contaminated by
interstellar features to be analyzed.
Tables 13 (click here) to 17 (click here) summarize CO observation parameters:
LSR and expansion velocities, main beam temperature CO intensity if the
source is detected, upper limit if not, and
and
ratios.