Molecular Line Emission Towards High-Mass Clumps: The MALT90 Catalogue

Abstract

The Millimetre Astronomy Legacy Team 90 GHz survey aims to characterise the physical and chemical evolution of high-mass clumps. Recently completed, it mapped 90 GHz line emission towards 3 246 high-mass clumps identified from the ATLASGAL 870 μm Galactic plane survey. By utilising the broad frequency coverage of the Mopra telescope's spectrometer, maps in 16 different emission lines were simultaneously obtained. Here, we describe the first catalogue of the detected line emission, generated by Gaussian profile fitting to spectra extracted towards each clumps' 870 μm dust continuum peak. Synthetic spectra show that the catalogue has a completeness of >95%, a probability of a false-positive detection of <0.3%, and a relative uncertainty in the measured quantities of <20% over the range of detection criteria. The detection rates are highest for the (1-0) transitions of HCO + , HNC, N 2 H + , and HCN (∼77-89%). Almost all clumps (∼95%) are detected in at least one of the molecular transitions, just over half of the clumps (∼53%) are detected in four or more of the transitions, while only one clump is detected in 13 transitions. We find several striking trends in the ensemble of properties for the different molecular transitions when plotted as a function of the clumps' evolutionary state as estimated from Spitzer mid-IR images, including (1) HNC is relatively brighter in colder, less evolved clumps than those that show active star formation, (2) N 2 H + is relatively brighter in the earlier stages, (3) that the observed optical depth decreases as the clumps evolve, and (4) the optically thickest HCO + emission shows a 'blue-red asymmetry' indicating overall collapse that monotonically decreases as the clumps evolve. This catalogue represents the largest compiled database of line emission towards high-mass clumps and is a valuable data set for detailed studies of these objects.

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

“…(vi) We have elaborated on the molecular-line intensity ratios presented by the MALT90 team, and identify the same general evolutionary trends identified by Rathborne et al (2016). We find four of the nine ratios therein to be fully distinct between protostellar, YSO, and HII stages, with the other five showing lesser degrees of distinction, suggesting some utility of these ratios as chemical clocks.…”

“…As for Fig. 20, but for the 5 line ratios discussed by Rathborne et al (2016) that are not included in our top 13 most significant ratios. & Langer 1997; Lee et al 2003;Bergin & Tafalla 2007) as CO is released from dust grains, which destroys N 2 H + in the gas phase.…”

The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone

“…Collapse occurs quickest on the highest density, lowest virial ratio scales. It is known in the literature that the column density profiles of the Brick and bricklet clouds are centrally concentrated and that the volume density of the Brick increases toward the center (Walker et al 2015;Rathborne et al 2016). The collapse will therefore occur quickest at the center, on scales smaller than the SWAG beam, which will wash out potential small-scale effects.…”

Chemical Evolution of N2H+ in Six Massive Star-forming Regions

“…These two sources are two dense clouds we selected from Rathborne et al (2016). Even though they are not listed as bubbles by Churchwell et al (2006;, we can see distinct radio emissions from the Sydney University Molonglo Sky Survey (SUMSS) (843 MHz; Mauch et al 2003), indicating they are also two classical HII regions.…”

“…The source sample of this paper involves two filamentary clouds identified by Li et al (2016), two bubbles from Churchwell et al (2006;, and two dense clouds from Rathborne et al (2016). The basic information of them is shown in table 1.…”