ATLASGAL – Ammonia observations towards the southern Galactic plane

Abstract

Context. 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 µm, ATLASGAL, has therefore been conducted to discover high-mass star-forming regions at different evolutionary phases. Aims. We derive properties such as velocities, rotational temperatures, column densities, and abundances of a large sample of southern ATLASGAL clumps in the fourth quadrant. Methods. Using the Parkes telescope, we observed the NH 3 (1,1) to (3,3) inversion transitions towards 354 dust clumps detected by ATLASGAL within a Galactic longitude range between 300• and 359• and a latitude within ±1.5• . For a subsample of 289 sources, the N 2 H + (1 − 0) line was measured with the Mopra telescope. Results. We measured a median NH 3 (1,1) line width of ∼ 2 km s −1 , rotational temperatures from 12 to 28 K with a mean of 18 K, and source-averaged NH 3 abundances from 1.6 × 10 −6 to 10 −8 . For a subsample with detected NH 3 (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 ∼ 1 is determined using the broader N 2 H + line width than is estimated from the NH 3 line width of ∼ 0.5 with a general trend of a decreasing virial parameter with increasing gas mass. We obtain a rising NH 3 (1,1)/N 2 H + line-width ratio with increasing rotational temperature. Conclusions. A comparison of NH 3 line parameters of ATLASGAL clumps to cores in nearby molecular clouds reveals smaller velocity dispersions in low-mass than high-mass star-forming regions and a warmer surrounding of ATLASGAL clumps than the surrounding of low-mass cores. The NH 3 (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.

ATLASGAL – relationship between dense star-forming clumps and interstellar masers

“…where σ 2 v is the velocity dispersion of the NH 3 inversion transition emission observations (e.g. Urquhart et al 2011;Wienen et al 2012Wienen et al , 2018 and the other parameters are as previously defined. The virial parameter is a measure of the balance between gravitational collapse and internal energy that can counteract this collapse.…”

ATLASGAL – molecular fingerprints of a sample of massive star-forming clumps★

“…Wienen et al (2018 subm) has investigated the NH 3 deuterium fraction using NH 2 D lines towards the same evolutionary sample investigated in this paper. Mopra NH 2 D observations are combined with NH 3 data reported by Wienen et al (2012Wienen et al ( , 2018 to obtain a subsample of 253 sources (36 per cent of our sample) that has been detected with a S/N ratio > 3. Comparing the distributions of the NH 2 D and NH 3 line-widths, they found a correlation between the two, indicating that the two tracers probe similar regions within a source.…”

Thermal balance and comparison of gas and dust properties of dense clumps in the Hi-GAL survey

“…We note that a catalogue of NH3 observations of the (1,1) to (3,3) inversion transitions of 354 dense ATLASGAL sources between 300 • < ℓ <359 • using the Parkes 64m telescope, has been recently released by Wienen et al (2018). Although that catalogue is not considered in our analysis since it was published after the present work was completed, the IV Galactic Quadrant is not well covered by the NH3 surveys previously described and therefore the association between Hi-GAL sources and those line observations could lead to complementary results to those presented in this work.…”