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Molecular Clouds in in the LMC at High Resolution: The Importance of Short ALMA Baselines

Molecular Clouds in in the LMC at High Resolution: The Importance of Short ALMA Baselines. T. Wong 1,2,4 , J. B. Whiteoak 1 , M. Hunt 2 , J. Ott 1 , Y.-N. Chin 3 1 CSIRO Australia Telescope National Facility 2 School of Physics, University of New South Wales, Australia

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Molecular Clouds in in the LMC at High Resolution: The Importance of 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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