Interstellar molecules in the Large Magellanic Cloud

Details Interstellar molecules in the Large Magellanic Cloud Interstellar molecules in the Large Magellanic Cloud

Nov 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...291...89j&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 291, no. 1, p. 89-105

Astronomy and Astrophysics

Astrophysics

56

H Ii Regions, Interstellar Gas, Line Spectra, Magellanic Clouds, Molecular Clouds, Molecular Spectra, Star Formation, Brightness Distribution, Luminosity, Optical Thickness, Spatial Resolution, Telescopes

Scientific paper

We present the results of a mm-wave molecular line search in the Large Magellanic Cloud (LMC) obtained with the Swedish-ESO Submillimeter Telescope (SEST) in the area of the N 159 complex of HII regions. Isotopomers of CO, CS, SO, CN, HCN, HNC, HCO(+) and H2CO have been detected. Some of these molecules have been observed in two transitions allowing excitation temperature estimates, For (13)CO and (12)CO we find temperatures close to, or in excess of 10 K, while C(18)O, CS and HCO(+) show temperatures between 5 and 10 K. All observed isotopomer intensity ratios suggest small optical depths provided that Galactic isoptopic abundances ratios apply. From the C-12 and C-13-isotopomer data of CS and HCO(+), combined with the C(34)S line emission we estimate the gas phase C-12/C-13 abundance ratio to be 50+25, sub-20 in N 159. The 0-18/0-17 abundance ratio is observed to be a factor of about 2 lower than in the Galaxy, and another factor of 2 lower than in starburst galaxies. Using the OSO 20m telescope, we have mapped the Galactic molecular cloud associated with the H II region S 138 in the 3 mm transitions of most of the molecules detected in the LMC. These data have been used to investigate the effects of linear resolution on line ratios and fractional abundances. Based upon cloud luminosities, normalized to that of the CO(1-0) line, the detected lines in the LMC are weaker by a factor of 1.5 to 3 relative to those of the S 138 cloud. The exception is HCO(+), which shows stronger normalized emission in N 159. The normalized luminosities of the S 138 cloud are similar to those observed in a sample of strong CO galaxies. Fractional abundances of the detected molecules have been estimated using the conversion factor from CO emission to H2 column density. While the derived fractional abundances of S 138 generally agree to within a factor of 3 with those reported for Orion KL and TMC-1, all molecular concentrations observed in the LMC are typically a factor of 10 lower.

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