The structure of massive star-forming clumps

Abstract

Among many other critical questions in astrophysics, the high-mass star formation process is a matter of heated debate; so far a comprehensive understanding is lacking. Star formation takes place in cold molecular clouds. It also has been long known that massive stars form almost exclusively in star clusters. Galactic-scale surveys from millimeter to infrared wavelengths in the last decades have identified numerous massive molecular clumps (~1 pc in size, > 500 M_sun in mass), which are believed to be the nurseries for massive proto-clusters. Studying the physical structure and the evolution of these massive clumps is therefore an important prerequisite for understanding the process of massive star formation. In this thesis, we report intermediate spatial resolution (~0.1 pc) observations towards selected samples of massive clumps, aiming at revealing their underlying physical structures, including fragmentation properties, as well as density and temperature profiles and their variations, along the evolutionary track of massive proto-cluster formation. Throughout the thesis, we discuss dust continuum and molecular line data of the clumps, relying on various radiative transfer modeling methods to derive their physical properties, with an attempt to achieve stringent estimates. In Chapter 2, using ground-based 350 micron continuum imaging, we analyse observations of >200 massive clumps and obtain high-angular resolution (10 arcsec) dust temperature and hydrogen column density maps based on an image combination technique and iterative spectral energy distribution fitting procedure. We reveal the clump fragmentation properties at intermediate scales (~0.1 pc), and show that a large clump mass reservoir is essential for the mass accumulation into massive cores. In Chapters 3 and 4, with wide-band interferometric and single-dish spectroscopic observations, we utilize multiple molecular lines sensitive to a wide range of physical conditions to obtain a continuous mapping of the temperature and hierarchical density structures of a sample of massive clumps, which spans an evolutionary sequence from sources in the earliest infrared-dark stage to active OB cluster forming regions. In Chapter 3, we study the evolutionary variations of radial profiles of the temperature, density, linewidth, and dynamical state of massive clumps. We find that the temperature structure of massive clumps on scales of 0.1 pc in general follows predictions of simple central heating models with notable deviations that can be attributed to a particular source geometry, indicating significant central merging of protostars within massive clumps, which strongly shapes the temperature structure at >0.1 pc. The gas density profile becomes steeper with clump evolution, which, together with the variations of the radial profile of the virial parameter, suggests a gravo-turbulent picture of clump gas dynamics and evolution. By adopting single-dish multi-wavelength dust continuum which trace the density of the bulk gas, and molecular line densitometers which selectively probe the dense gas regimes, we reveal the hierarchical gas structures and the gas condensation process of massive clumps. The different radial profiles of the dense gas fraction may indicate different modes of the star formation process, which is either dominated by global contraction or exhibiting significant secondary fragmentation and structure formation around the global potential center. We corroborate the clump evolutionary sequence by comparing the abundance ratios of several molecules that have been suggested as evolutionary indicators. This line of work of Chapter 3 is extended to more massive clumps in Chapter 4, in which a detailed study is presented of 5 early-stage massive clumps residing in W43-main, an extreme Galactic OB cluster forming complex representative of a highly turbulent cloud environment. In addition to revealing the temperature and density structures of these clumps, we further investigate the relation between cloud-scale dynamics, specifically the cloud collision activities, and the formation and fragmentation of early-stage cores and the enhancement of more processed dense gas.