EDP Sciences
Free access
Volume 365, Number 2, January II 2001
Page(s) 275 - 284
Section Instruments, observational techniques and data processing
DOI http://dx.doi.org/10.1051/0004-6361:20000475

A&A 365, 275-284 (2001)
DOI: 10.1051/0004-6361:20000475

The IRAM key-project: Small-scale structure of pre-star forming regions

III. Influence of and correction for the error beam pick-up
F. Bensch1 , J.-F. Panis2,3 , J. Stutzki1 , A. Heithausen1,4 and E. Falgarone3 .

1  I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
2  ASIAA, Academica Sinica, PO Box 1-87, Nankang, Taipei 115, Taiwan, PR China
3  Radioastronomie, CNRS URA 336, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
4  Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

(Received 27 January 2000 / Accepted 1 August 2000)

The goal of the IRAM key-project "Small-scale structure of pre-star forming regions"is to map three nearby ( $d\sim 150 {\rm pc}$), quiescent molecular clouds with a high angular and spectral resolution to study the molecular cloud structure down to the smallest linear scales currently accessible by single dish radio telescopes. Here, we give a detailed discussion of the influence and the correction of the error beam pick-up. A new set of corrected data is presented using the beam pattern parameters of the IRAM 30 m telescope recently published by Greve et al. (1998). For the correction we use observations made at lower angular resolution with the KOSMA 3 m telescope. Smeared to the resolution of the error beam, they provide an estimate of the pick-up therein. The error beam pick-up accounts for a significant fraction of the observed intensity in the key-project maps: on average 31% to 50% for the 12CO $J=2\rightarrow$ 1 maps, 10% to 35% for the 13CO $J=2\rightarrow$ 1 maps, and 16.5% in the 12CO $J=1\rightarrow$ 0 map of MCLD 123.5+24.9. In addition, the line profiles are significantly modified by the error beam pick-up. The large data set available with the IRAM key-project allows us to investigate the accuracy of the correction method and its limitations in detail. For the corrected maps, we determine the overall accuracy of the temperature scale to be better than 15% , except for the 13CO $J=2\rightarrow$ 1 map of MCLD123.5+24.9 and L1512 where we estimate an accuracy of 22% and 18% . The key-project maps presented here and the released maps published by Falgarone et al. (1998) (with the correction done using the previous beam pattern parameters determined by Garcia-Burillo et al. 1993) differ by no more than 10% and only in localized regions. Both data sets and the supplementary observations made with the KOSMA telescope are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/Abstract.html The "error beam problem"is not limited to observations obtained with the IRAM 30 m. Similar beam pattern are found for other large single dish (sub-)mm telescopes, although few quantitative studies exist. In particular, for observations of spatially extended sources, the error beam contribution has to be considered and corrected for if necessary.

Key words: methods: data analysis -techniques: miscellaneous -radio lines: ISM -line: profiles -interstellar medium (ISM): clouds -ISM: structure

Offprint request: F. Bensch, bensch@ph1.uni-koeln.de

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