6 Summary and discussion

We use complementary data observed at lower angular resolution with the KOSMA telescope to estimate the error beam pick-up in the maps of the IRAM key-project. The pick-up in the second and third Gaussian error beam of the IRAM 30m telescope accounts for a substantial fraction of the observed intensity, on average 31%$-50\%$ for the ¹²CO $J=2\rightarrow$ 1 maps and 10%$-35\%$ for the ¹³CO $J=2\rightarrow$ 1 maps. The error beam pick-up significantly modifies the spatial intensity distribution and the line profile, with the corrected maps showing a considerably enhanced contrast. For the map-averaged line profile, the fraction of the observed intensity attributed to the error beam pick-up is larger in the line wings than in the line core. For the individual spectra, however, the degree of modification strongly varies with position. While for some positions the line wing is essentially removed with the correction, they are found to be even more pronounced for other positions. These results demonstrate that a careful correction is important and that a correction by application of a simple scaling factor ("effective'' or "full beam efficiency'') is insufficient here.

For the ¹²CO $J=1\rightarrow$ 0 map of MCLD 123.5+24.9, the error beam pick-up is determined to be 16.5% (map average). The intensity distribution as well as the line profiles are modified only to a minor degree.

We apply a self-consistent error beam correction to the released data set, where we match the KOSMA observations to the corrected IRAM observations, smoothed to the angular resolution of the KOSMA observations. The accuracy of the thus corrected spectral line maps is 11% (CO $J=1\rightarrow$ 0) and 15% (CO $J=2\rightarrow$ 1), except for the ¹³CO $J=2\rightarrow$ 1 map of L1512 and MCLD 123.5+24.9, where we assign larger uncertainties (18% and 22%, respectively). The systematic errors in the intensity calibration of the key-project data set are dominated by the error beam pick-up of the KOSMA telescope (accounting for $\le$18% in the ¹²CO $J=2\rightarrow$ 1 maps) and the uncorrected first error beam of the IRAM 30 m (resulting in the larger uncertainties for the ¹³CO $J=2\rightarrow$ 1 map of L1512 and MCLD 123.5+24.9). Taking both effects into account, we find that the residual errors are in accordance with the accuracy estimated by GKW for the beam pattern parameters of the IRAM 30 m. Only for the ¹²CO $J=2\rightarrow$ 1 map of L134A, we find indications that the actual error beam pattern slightly deviates from the assumed axial symmetry.

This study focuses on the observations made in the framework of the key-project with the IRAM 30m telescope. However, we expect that the error beam pick-up potentially plays a significant role for observations made with other (sub-)mm telescopes, too. An error beam pattern similar to the pattern of the IRAM 30m has been found for a number of (sub-)mm telescopes (cf. Kaufmann et al. 1987; Hills & Richter 1992; Prestage 1993). The correction methods (cf. Bensch et al. 2001) and the conclusions presented in this paper are therefore not limited to observations made with the IRAM 30 m. This should be taken into account when planning large mapping projects, where a fraction of the time should be allocated for the additional observations required to correct for the error beam pick-up.

Acknowledgements

The authors wish to thank A. Greve and C. Kramer for discussions and providing the detailed numerical results of their error beam measurements of the IRAM 30m telescope. The KOSMA 3m radio telescope at Gornergrat-Süd Observatory is operated by the University of Cologne and supported by the Deutsche Forschungsgemeinschaft through grant SFB-301, as well as special funding from the Land Nordrhein-Westfalen. The receiver development was partly funded by the Bundesminister für Forschung and Technologie. The Observatory is administered by the Internationale Stiftung Hochalpine Forschungsstationen Jungfraujoch und Gornergrat, Bern. The authors have benefitted from a joint European grant (Procope, grant# 312-pro-bmbw-gg) during the years 1995 and 1996. This research made use of NASA's Astrophysics Data System Abstract Service.