A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

Abstract

Context. Infrared dark clouds (IRDCs) provide a useful testbed in which to investigate the genuine initial conditions and early stages of massive-star formation. Aims. We attempt to characterise the chemical properties of a sample of 35 massive clumps of IRDCs through multi-molecular line observations. We also search for possible evolutionary trends among the derived chemical parameters.Methods. The clumps are studied using the MALT90 (Millimetre Astronomy Legacy Team 90 GHz) line survey data obtained with the Mopra 22 m telescope. The survey covers 16 different transitions near 90 GHz. The spectral-line data are used in concert with our previous LABOCA (Large APEX BOlometer CAmera) 870 μm dust emission data. Results. Eleven MALT90 transitions are detected towards the clumps at least at the 3σ level. Most of the detected species (SiO, C 2 H, HNCO, HCN, HCO + , HNC, HC 3 N, and N 2 H + ) show spatially extended emission towards many of the sources. Most of the fractional abundances of the molecules with respect to H 2 are found to be comparable to those determined in other recent similar studies of IRDC clumps. We found that the abundances of SiO, HNCO, and HCO + are higher in IR-bright clumps than in IR-dark sources, reflecting a possible evolutionary trend. A hint of this trend is also seen for HNC and HC 3 N. An opposite trend is seen for the C 2 H and N 2 H + abundances. Moreover, a positive correlation is found between the abundances of HCO + and HNC, and between those of HNC and HCN. The HCN and HNC abundances also appear to increase as a function of the N 2 H + abundance. The HNC/HCN and N 2 H + /HNC abundance ratios are derived to be near unity on average, while that of HC 3 N/HCN is ∼10%. The N 2 H + /HNC ratio appears to increase as the clump evolves, while the HNC/HCO + ratio shows the opposite behaviour. Conclusions. The detected SiO emission is probably caused by shocks driven by outflows in most cases, although shocks resulting from the cloud formation process could also play a role. Shock-origin for the HNCO, HC 3 N, and CH 3 CN emission is also plausible. The average HNC/HCN ratio is in good agreement with those seen in other IRDCs, but gas temperature measurements would be neeeded to study its temperature dependence. Our results support the finding that C 2 H can trace the cold gas, and not just the photodissociation regions. The HC 3 N/HCN ratio appears to be comparable to the values seen in other types of objects, such as T Tauri disks and comets.

A molecular line study towards massive extended green object clumps in the southern sky: chemical properties

“…Our column density of N 2 H + ranges from 9.0 × 10 12 to 1.8 × 10 14 cm −2 , with an average of 6.3 × 10 13 cm −2 . This value is consistent with that derived by Miettinen (2014) and is also very similar to that of Sakai et al (2008). The top panel of Figure 5 shows column densities of N 2 H + versus those of H 13 CO + .…”

“…Sanhueza et al (2012) found no clear evolutionary trends for C 2 H column densities and abundances in their sample of IRDCs. On the other hand, Miettinen (2014) found that both the mean and median values of the C 2 H abundance are higher towards IR-dark clumps than towards IR-bright clumps, indicating C 2 H tend to trace the cold gas. More observations should be carried out to study C 2 H in massive star formation regions.…”

“…Although several studies have been carried out to investigate whether the evolutionary stages defined by Chambers et al (2009) chemically distinguishable (e.g. Vasyunina et al 2011;Sanhueza et al 2012;Hoq et al 2013;Miettinen 2014), chemical properties of massive star formation regions are still far less explored than their low-mass counterparts. Some observations (e.g.…”

Infrared dust bubble CS51 and its interaction with the surrounding interstellar medium

“…These indicate that Clump 4 is possibly at a later evolutionary stage as compared Clump 3 (Miettinen 2014;Yu & Wang 2015;Yu & Xu 2016). The radio peaks A and B fall within Clump 4 with B being close to the peak in the Herschel column density map.…”

“…Of the 16 molecular-line transitions covered in the MALT90 survey, only five molecular species (C2H, HCN, HCO + , HNC, and N2H + ) are detected towards these two clumps. The details of the transitions taken from Foster et al (2011) and Miettinen (2014) are listed in Table 7. Both the papers, especially the latter, give a nice review on the environment in which these molecules exist and the physical conditions required for their formation.…”