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Astron. Astrophys. 336, 654-661 (1998)

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2. Observations

2.1. The sample

Well studied O-rich IRVs were selected on the basis of their IRAS-60 µm fluxes in order to detect them in circumstellar CO emission for the first time. Based on our experience with the SRVs (SRIV) only objects redder than -1.2 in the IRAS-colour [12]-[25] were observed in order to save observing time. This biased our sample towards mass losing objects of similar properties like `red' and `Mira'-SRVs. The names in the GCVS4 and the IRAS-PSC of the 31 observed stars are given in Table 1. The source coordinates were taken either from the Hipparcos Input Catalogue (Turon et al. 1994) or from SIMBAD where most of the objects have coordinates of subarcsecond accuracy.


Table 1. Observational results of O-rich Lb variables

2.2. Observational results

The observations in the CO (J=1-0) line were performed with the Swedish-ESO Submillimetre Telescope (SEST), La Silla, Chile, and the 20 m telescope at Onsala Space Observatory (OSO), Sweden. CO (J=2-1) line data come from SEST, the IRAM 30 m telescope, Pico de Veleta, Spain, and the James Clerk Maxwell Telescope (JCMT), Mauna Kea, Hawaii, while the CO (J=3-2) line was observed only with the JCMT. Table 1 summarizes the results. In total 20 stars were detected (all are new detections). In three cases detections are precluded because of strong interference from interstellar CO emission. In total there are 43 individual measurements because some stars were observed in more than one line. The first letter of the code gives the observatory (I RAM, J CMT, O SO, or S EST), the rest the transition observed. Another code reflects the "success" of detection (D etection, or N on-detection). An `i' indicates contamination by interstellar CO lines. More detailed information about the observations and all the individual spectra are found in Kerschbaum & Olofsson (in preparation).

When determening the line parameters we compared the results of fitting parabolas or 4th order polynomials, which generally fit the lines very well, with eye estimates of where the profiles go to zero intensity. Both approaches agreed well in most of the cases. We determined the zero intensity velocities (i.e., the velocities at the two edges of the line profile) from the best fits. The stellar velocity was then derived from the average of these velocities, and the gas expansion velocity of the envelope from half the difference between these velocities. Before doing this we removed the major baseline irregularities by fitting low order polynomials to the spectra. We estimate that both quantities are uncertain by about 1-2 km/s, but the uncertainty varies with the S/N-ratio. This means that the low expansion velocities may be uncertain by up to 30%, while for the highest expansion velocities the uncertainty decreases to about 10%. The stellar velocity is given with respect to the heliocentric ([FORMULA]) and the LSR frame [[FORMULA]; the Local Standard of Rest is defined using standard solar motion (B1950.0): [FORMULA] km[FORMULA]s, [FORMULA], [FORMULA]]. The peak main beam brightness temperature, [FORMULA], is obtained as an average of the line profile intensities in the velocity range [FORMULA]. The integrated intensity, [FORMULA], is obtained by integrating the line intensities over the velocity range [FORMULA]. Once again the uncertainty in both quantities varies with the S/N-ratio, but we estimate that it is on average about 20%, and in the worst cases it may reach 50%. To this should be added an estimated uncertainty in the absolute calibration of about 20%. In no case do we expect that these uncertainties in the line profile characteristics will have any significant effect on the conclusions drawn in this paper. For a non-detection an upper limit to I is estimated by measuring the peak-to-peak noise ([FORMULA]) of the spectra with a velocity resolution reduced to 15 km[FORMULA]s and calculating [FORMULA]. The last column of Table 1 gives a quality ranking ranging from 5 (useless, nondetection) to 1 (very good). A spectrum must have quality 3 or better to be used in the analysis. Similar visual inspection was carried out also for the literature data that we used.

Some examples of spectra are shown in Fig. 1. The velocity scale is given in the heliocentric system. Within the limitations of the S/N-ratio all of the line profiles in our sample lie in the range of the expected, i.e., between a rectangular (spatially unresolved, optically thin emission) and a parabolic (spatially unresolved, optically thick emission) line shape.

[FIGURE] Fig. 1. Examples of CO spectra of O-rich Lb variables. The intensity scale is in main beam brightness temperature

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998