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Received 15 July 1998 / Accepted 10 December 1998
We have observationally studied the effect of metallicity and far-ultraviolet (FUV) radiation on the physical conditions and the molecular abundances in interstellar clouds in the Small and the Large Magellanic Clouds (SMC and LMC, respectively). Spectral line emission from a number of molecules was observed in a sequence of clouds with positions in and between the 30 Doradus (30Dor) and the southern part of the N159 region in the LMC, and in one cloud (N27, also denoted LIRS 49) located in the SMC bar. Physical conditions and molecular abundances were estimated from the observational data by excitation and radiative transfer calculations. A comparison of the molecular abundances in clouds in the SMC, the LMC, and the Galaxy is presented. We also report the first detection of hydrogen sulphide (ortho-H2S) in an extragalactic source, detections of methanol (CH3OH) in thermal emission and methyl acetylene (CH3CCH), and a tentative detection of thio-formaldehyde (H2CS) in N159W.
The abundances (relative to H2) of molecular species (except CO) in the LMC sources and in N27 are estimated to be typically 510-10, and 110-10, respectively. These values apply to the gas volume defined by the CO line emission. Relative to Galactic clouds, the abundances in N159W (our reference cloud) are five to twenty times lower. In two of the clouds: N27 and the centremost cloud in 30Dor (30Dor-10), the derived abundances deviate significantly from those in the other clouds in our sample, by being on the average six and eight times lower, respectively. In N27, the most likely explanation is the lower metallicity in the SMC, whereas the underabundance in 30Dor-10 is probably mainly caused by a more rapid photodissociation due to the more intense FUV radiation in this area. An alternative explanation for the underabundances in both N27 and 30Dor-10 would be a higher H/H2 ratio inside these molecular clouds. The ethynyl radical (C2H), with an estimated average abundance of 510-9 in seven clouds in the LMC and 310-9 in two clouds in the SMC, is the most abundant observed trace molecule after the CO isotopomers. Qualitatively, the high C2H abundance can be explained as reflecting the C+-rich and FUV photon-rich environment, i.e., a chemistry characteristic for photon-dominated regions.
For N27 we have, using HCO+ and H13CO+ data, estimated the gas-phase 12C/13C ratio to be 40-90, a range that encompasses the values found in N159W and in Galactic disc clouds.
In all clouds in our sample, the number density estimates from an excitation analysis of CS, SO, HCO+, HCN and H2CO are in the range (1-100) cm-3. CO data gives a lower limit of a few cm-3. However, the average densities (estimated from the virial mass) are significantly lower, typically a few102 cm-3, suggesting that the clouds (as probed by trace molecules) are very clumpy with volume-filling factors 1. In N159W, where our data-base is by far most extensive, the number density and the kinetic temperature of molecular hydrogen in the dense part of the gas are estimated to be (1-10) cm-3 and 2510 K, respectively. The corresponding numbers in N27 are, although based on less data than in N159W, (5-50)104 cm-3 and 155 K. Thus, the metallicity difference between the LMC and the SMC does not seem to affect the density and the temperature of the gas dramatically.
In the SMC, the CO(J=1-0)/HCO+(J=1-0) line intensity ratio follows the same trend with respect to the star-formation activity as in the LMC: a lower ratio is found in clouds with a more vigorous star-formation activity. A similar trend is also exhibited by the CO(J=1-0)/C2H(N=1-0) line intensity ratio in the LMC.
Key words: ISM: molecules galaxies: abundances galaxies: ISM galaxies: Magellanic Clouds radio lines: galaxies radio lines: ISM
* Based on observations using the Swedish-ESO Submillimetre Telescope (SEST) at the European Southern Observatory (ESO), La Silla, Chile.
Present address: Observatory, P.O. Box 14, FIN-000 14 University of Helsinki, Finland
Send offprint requests to: A. Heikkilä
Online publication: March 29, 1999