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2. Observations

2.1. Selection of the objects

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'' tex2html_wrap_inline2863
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.): tex2html_wrap_inline2865 at 1 kpc, where tex2html_wrap_inline2867 is estimated according to (Loup et al. 1993)
displaymath2861
with the IRAS fluxes tex2html_wrap_inline2869 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 tex2html_wrap_inline2871) AGB stars which satisfy the colour criteria. But supergiants will be selected deeper tex2html_wrap_inline2873, 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 tex2html_wrap_inline2875 contains 88 objects. It was however not systematically observed in CO.

  figure279
Figure 1: 12/25/60 tex2html_wrap_inline2877m IRAS colour-colour diagram of studied sources. The regions are those defined by van der Veen & Habing (1988), Omont et al. (1993). Objects with tex2html_wrap_inline2879 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

2.2. Determination of the positions

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 tex2html_wrap_inline2895 (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 tex2html_wrap_inline2907. 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''.)

2.3. CO observations

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

Table 1: Log of the observations

2.3.1. Observations at SEST

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 tex2html_wrap_inline2927. 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

Table 2: IRAM-30 m-telescope and SEST-15 m-telescope efficiency data. The conversion from antenna to main-beam temperature is done according to tex2html_wrap_inline2931 forward efficiency/beam efficiency. The efficiency values correspond to rather good observing conditions and are given in main beam scale (see IRAM Newsletter No. 5 (1992) and 18 (1994))

2.3.2. Observations at IRAM

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 tex2html_wrap_inline2953 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 tex2html_wrap_inline2963 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.

  figure377
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).

2.3.3. Results of CO observations

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).

2.4. OH observations

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 tex2html_wrap_inline2997. The velocity range where OH was searched was tex2html_wrap_inline2999 wide, centered on the star velocity if it was known from CO, or tex2html_wrap_inline3001 wide, centered on 0 if no velocity information was available.

The half-power beamwidth is tex2html_wrap_inline3003 in tex2html_wrap_inline3005 and 18' in tex2html_wrap_inline3009. The ratio of flux to antenna temperature is 1.1 Jy K-1 at 0tex2html_wrap_inline3013 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 tex2html_wrap_inline3019 tex2html_wrap_inline3021 tex2html_wrap_inline3023 tex2html_wrap_inline3025 tex2html_wrap_inline3027 tex2html_wrap_inline3029 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 -

Table 3: New OH observations made at the Nancay radiotelescope. Blue and red peak velocities and fluxes are given in tex2html_wrap_inline3015 and Jy, respectively. Search for H2O emission is compiled in Brand et al. (1993 and references therein)

2.5. Summary of the source properties

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, tex2html_wrap_inline3037 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 tex2html_wrap_inline2783. 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 tex2html_wrap_inline2963 and tex2html_wrap_inline2777 ratios.


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