ATLASGAL - Ammonia observations towards the southern Galactic Plane

Abstract

The initial conditions of molecular clumps in which high-mass stars form are poorly understood. In particular, a more detailed study of the earliest evolutionary phases is needed. The APEX Telescope Large Area Survey of the whole inner Galactic disk at 870 micron, ATLASGAL, has been conducted to discover high-mass star-forming regions at different evolutionary phases. Using the Parkes telescope, we observed the NH3 (1,1) to (3,3) inversion transitions towards 354 ATLASGAL clumps in the fourth quadrant. For a subsample of 289 sources, the N2H+ (1-0) line was measured with the Mopra telescope. We measured a median NH3(1,1) line width of about 2 km/s and rotational temperatures from 12 to 28 K with a mean of 18 K. For a subsample with detected NH3 (2,2) hyperfine components, we found that the commonly used method to compute the (2,2) optical depth from the (1,1) optical depth and the (2,2) to (1,1) main beam brightness temperature ratio leads to an underestimation of the rotational temperature and column density. A larger median virial parameter of about 1 is determined using the broader N2H+ line width than is estimated from the NH3 line width of about 0.5 with a general trend of a decreasing virial parameter with increasing gas mass. We found a warmer surrounding of ATLASGAL clumps than the surrounding of low-mass cores and smaller velocity dispersions in low-mass than high-mass star-forming regions. The NH3 (1,1) inversion transition of 49% of the sources shows hyperfine structure anomalies. The intensity ratio of the outer hyperfine structure lines with a median of 1.27+/-0.03 and a standard deviation of 0.45 is significantly higher than 1, while the intensity ratios of the inner satellites with a median of 0.9+/-0.02 and standard deviation of 0.3 and the sum of the inner and outer hyperfine components with a median of 1.06+/-0.02 and standard deviation of 0.37 are closer to 1.

arXiv comment20 pages, 20 figures

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