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High-Resolution Study of Molecular Clouds in the LMC with Short ALMA Baselines

Explore molecular clouds in the Large Magellanic Cloud using short ALMA baselines to understand cloud structure and abundances. Observations with ATCA reveal unique star-forming environments with implications for ALMA study design.

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High-Resolution Study of Molecular Clouds in the LMC with Short ALMA Baselines

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  1. Molecular Clouds in in the LMC at High Resolution:The Importance of Short ALMA Baselines T. Wong1,2,4, J. B. Whiteoak1, M. Hunt2, J. Ott1, Y.-N. Chin3 1CSIRO Australia Telescope National Facility 2School of Physics, University of New South Wales, Australia 3Tamkang University, Taiwan 4Contact: Tony.Wong@csiro.au

  2. Australia Telescope Compact Array • Six 22m antennas near Narrabri, NSW, five moveable on rail tracks. • National Facility open to proposers worldwide. • Operates in 5 frequency bands from 1-25 GHz. • 3mm (85-105+ GHz) upgrade in progress for 5 antennas (due late June). • Wide-bandwidth (2 GHz x 4 IF) correlator under development (mid-2006). Longitude 150° E, Latitude 30° S

  3. The ATCA Millimetre Upgrade • 3 elements currently have dual linear polarization 3mm receivers, 5 by July. • 2 observing bands: 84.9-87.3and88.5-91.3 GHz. Full coverage of 85-105 GHz expected by July, extension to 115 GHz planned. • A~0.35, Tsys~300 K (above atmosphere). (above) 3mm low-noise amplifiers based on InP MMIC technology (left) Both 3mm and 12mm systems are housed in a single dewar.

  4. N113 HII region SEST spectra (Chin et al. 1997) A unique nearby, low-metallicity star formation environment. Contours: CO at 2.6’ resolution from NANTEN (Mizuno et al.) Molecular Clouds in the LMC

  5. HCO+ (1-0) HCN (1-0) C2H (N=1-0) HNC (1-0) Mapping of N113 in 4 transitions We observed N113 in HCO+, HCN, HNC, C2H, and 87 GHz continuum. Observations conducted in 2003 July & August in two E-W configurations of 3 antennas (baselines 30-135 m). RMS noise ~30 mJy in a 2 km s-1channel. Reference pointing on SiO maser R Dor, phase calibration using PKS B0537-441 (25° away). Integrated intensity images for 4 lines. Contour levels: 0.5 Jy bm-1 km s-1 for top panels and 0.2 for bottom panels.

  6. Comparison in u-v plane Assuming a SEST gain of 25 Jy/K, the total HCO+ and HCN fluxes are 80 and 60 Jy km s-1 (Chin et al. 1997) respectively. Thus only ~15% of flux is detected on the shortest (30m) ATCA baseline. The relative sizes of the emission regions differ: HCO+ is more extended than HCN, which in turn is more extended than HNC. Possible explanations: HCO+ associated with an extended PDR, HNC/HCN enhanced in dense cores.

  7. Implications for ALMA DRSP Emission from abundant species like HCO+ and HCN is heavily resolved, even on a 30m baseline. Information on abundances & cloud structure will require observations from an array of smaller dishes (ACA). Lack of small-scale emission implies high-resolution observations will require much greater sensitivity. Note that ATCA’s field of view at 90 GHz (36”, see figure) is similar to ALMA’s at 230 GHz (~10 pc at LMC distance) —mosaicking clearly needed.

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