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Published February 10, 2010 | Published
Journal Article Open

First Detection of Ammonia in the Large Magellanic Cloud: The Kinetic Temperature of Dense Molecular Cores in N 159 W

Abstract

The first detection of ammonia (NH_3) is reported from the Magellanic Clouds. Using the Australia Telescope Compact Array, we present a targeted search for the (J, K) = (1,1) and (2,2) inversion lines toward seven prominent star-forming regions in the Large Magellanic Cloud (LMC). Both lines are detected in the massive star-forming region N 159 W, which is located in the peculiar molecular ridge south of 30 Doradus, a site of extreme star formation strongly influenced by an interaction with the Milky Way halo. Using the ammonia lines, we derive a kinetic temperature of ~16 K, which is 2-3 times below the previously derived dust temperature. The ammonia column density, averaged over ~17", is ~6 × 10^(12) cm^(–2) (<1.5 × 10^(13) cm^(–2) over 9" in the other six sources) and we derive an ammonia abundance of ~4 × 10^(–10) with respect to molecular hydrogen. This fractional abundance is 1.5-5 orders of magnitude below those observed in Galactic star-forming regions. The nitrogen abundance in the LMC (~10% solar) and the high UV flux, which can photo-dissociate the particularly fragile NH_3 molecule, both must contribute to the low fractional NH3 abundance, and we likely only see the molecule in an ensemble of the densest, best shielded cores of the LMC.

Additional Information

© 2010 American Astronomical Society. Print publication: Issue 1 (2010 February 10); received 2009 April 23; accepted for publication 2009 November 30; published 2010 January 15. This work has been supported by the National Radio Astronomy Observatory (NRAO) which is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. We thank the referee for his/her suggestions to improve the paper. Facility: ATCA

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