4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Poster

Poster session

Are the Infrared Dark Clouds Really Quiescent?


  • Dr. Siyi FENG

Primary authors


Context: Dense, cold regions where protostellar objects have not yet formed (high mass starless cores, HMSCs) are poorly characterised. However, observations of regions in this phase, before the violent feedback from massive star formation events takes hold, are crucial to characterise the initial conditions for high-mass star formation (HMSF).

Aims: The goal of this work is to investigate the initial conditions of HMSF by studying the dynamics and chemistry within HMSCs.

Method: We present the continuum map obtained from Submillimeter Array (SMA) interferometry at 1.1 mm, and show the line surveys at 1mm/3mm of four infrared dark clouds (IRDCs, G28.34S, IRDC 18530, IRDC 18306 and IRDC 18308) obtained from IRAM 30 m single-dish observations. Strong (sub)mm continuum emissions and low-J line emissions are shown toward 70 $\mu$m extinction features.

Results: (1) At a spatial resolution of 2" ($\sim 10^4$ AU at an average distance of 4 kpc), the 1.1 mm SMA observations resolve each source into several fragments. The mass of each fragment is on average $\rm >10\,M_{\odot}$, which exceeds the predicted thermal Jeans mass of the whole clump by a factor of up to 60, thus indicates that thermal pressure does not dominate the fragmentation process. In contrast, linewidths of 30 m lines imply that non-thermal motion support the fragment. (2) Non-detection of high-J transitions as well as hyperfine structure fitting of certain species ($\rm N_2H^+~(J=1\rightarrow0)$, $\rm C_2H~(N=1\rightarrow0)$, $\rm HCN~(J=1\rightarrow0)$, and $\rm H^{13}CN~(J=1\rightarrow0)$) indicate the cold and young evolutionary stage of our sources. Moreover, obvious detection of SiO and the asymmetric $\rm HCO^{+}$ line profile in G28.34\,S indicate a potential protostellar object and probable infall motion. (3) With a large number of N-bearing species, anti-correlated spatial distributions between $\rm N_2H^+$, $\rm NH_2D$, and CO, and the existence of carbon rings and molecular ions, our sources exhibit similar chemical features as low mass prestellar cores. However, comparing the molecular abundances, HCN/HNC ratio, CO depletion factor, and ionization fraction, we found HMSCs are slightly chemically more evolved than low mass prestellar cores.

Conclusions: This small sample study of HMSCs illustrates that thermal Jeans instability alone is not sufficient to support the fragmentation of the cold ($\rm T<15$ K), dense ($\rm >10^5~cm^{-3}$) HMSCs. Chemically, they are not completely quiescent, and are more evolved than their low-mass counterparts.