Volume 430, Number 2, February I 2005
|Page(s)||549 - 560|
|Section||Interstellar and circumstellar matter|
|Published online||20 January 2005|
The interstellar CO/CO ratio in the solar neighbourhood: The ρ Ophiuchus cloud *
Joint Astronomy Centre, 660 N. A'ohoku Place, University Park, 96720 Hilo, Hawaii, USA e-mail: firstname.lastname@example.org
2 Radioastronomisches Institut, Univ. Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
3 Istituto di Radioastronomia, CNR, via Gobetti 101, 40129 Bologna, Italy
4 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Accepted: 15 September 2004
Observations of up to ten carbon monoxide (CO and isotopomers) transitions are presented to study the interstellar C18O/C17O ratio towards 21 positions in the nearby ( pc) low-mass star forming cloud ρ Oph. A map of the C18O –0 distribution of parts of the cloud is also shown. An average 12C18O/12C17O isotopomeric ratio of , reflecting the 18O/17O isotope ratio, is derived from Large Velocity Gradient (LVG) calculations. From LTE column densities we derive a ratio of . These calculations also show that the kinetic temperature decreases from about 30 K in the cloud envelope to about 10 K in the cloud cores. This decrease is accompanied by an increase of the average molecular hydrogen density from 104 cm-3 to 105 cm-3. Towards some lines of sight C18O optical depths reach values of order unity.
Key words: ISM: abundances / ISM: clouds / ISM: molecules / Galaxy: abundances / radio lines: ISM / ISM: individual objects: rho Oph
Based on observations collected with the Swedish/ESO Submillimeter Telescope (SEST) at the European Southern Observatory, Chile (ESO 62.I-0752). All spectra (some of which are shown in Fig. [see full text]) are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/549
© ESO, 2005
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